Unit 7: Glaciers, Ice, and Permafrost
Welcome to Unit 7
Glaciers, Ice, & Permafrost—Yosemite, Glacier, & Bear Meadows
Although not quite as large as minivans, musk oxen have better acceleration and cornering. This picture shows musk oxen thundering across the tundra of east Greenland. If you had been in central Pennsylvania's Bear Meadows with a camera 20,000 years ago, you might have taken this picture--tundra and musk oxen very similar to these existed in Pennsylvania and adjacent states back then, while an ice sheet even bigger than modern Greenland's loomed just to the north. How do we know that the ice came and went, and what caused the changes? Look both ways for musk oxen or minivans, depending on where and when you are, and let's go see.
What to do for Unit 7?
You will have one week to complete Unit 7. See the course calendar for specific due dates.
As you work your way through the online materials for Unit 7, you will encounter a video lecture, several vTrips, some animated diagrams (called GeoMations and GeoClips), additional reading assignments, a practice quiz, a "RockOn" quiz, and a "StudentsSpeak" Survey. The chart below provides an overview of the requirements for this unit.
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REQUIREMENTS |
SUBMITTED FOR GRADING? |
|---|---|
| Read/view all of the Instructional Materials | No, but you will be tested on all of the material found in the Unit 7 Instructional Materials. |
| Submit Exercise #3: The Age of Nittany Valley. | Yes, this is the third of 6 Exercises and is worth 5% of your total grade. |
| Begin Exercise #4: Which Way is Up? | Yes, this is the fourth of 6 Exercises and is worth 5% of your total grade. |
| Take the Unit 7 "RockOn" quiz. | Yes, this is the seventh of 12 end-of-unit RockOn quizzes and is worth 4.5% of your total grade. |
| Complete the "StudentsSpeak #8" survey. | Yes, this is the eighth of 12 weekly surveys and is worth 1% of your total grade. |
Questions?
If you have any questions, please feel free to email "All Teachers" and "All Teaching Assistants" through Canvas conversations.
Keep Reading!
On the following pages, you will find all of the information you need to successfully complete Unit 7, including the online textbook, a video lecture, several vTrips and animations, and an overview presentation
Students who register for this Penn State course gain access to assignment and instructor feedback, and earn academic credit. Information about registering for this course is available from the Office of the University Registrar.
Main Topics, Unit 7
When we try to pick out anything by itself, we find it hitched to everything else in the Universe.
— John Muir, My First Summer in the Sierra, 1911, p. 110
Overview of the main topics you will encounter in Unit 7.
Ice Is Nice: Yosemite, Glacier, Rocky Mountain, Bear Meadows, and NE Greenland
- A glacier is a pile of ice and snow that flows.
- It forms if snowfall exceeds melting by enough, for long enough, to make a big enough pile.
- A glacier takes water (as ice) and sediment from the accumulation zone (where snow accumulates faster than it melts) to the ablation zone (where melting, also called ablation, exceeds snow accumulation) or to calve icebergs.
- A glacier flows in the downhill direction of the upper surface (where ice meets air), even if that means the bottom flows uphill.
- Think of pancake batter flowing on a waffle iron.
Slip Sliding Away
- A glacier moves by deformation within the ice, and if the bed is warmed to the freezing point, by sliding over the glacier's bed or deforming the sediment there.
- Most deformation in the ice of a glacier is deep, but the top of a glacier moves fastest because it rides along on the deeper layers.
- The ice of a glacier deforms under stress because the ice is almost hot enough to melt.
- Glaciers erode by plucking rocks loose, sand-papering the bed, and through the actions of subglacial streams.
- Glaciers with thawed beds, especially those with surface meltwater reaching the bed, change the landscape more rapidly than is typical for landscapes shaped by streams, wind or mass movement.
Ages of Ice
- Recent (about 20,000-year-old), features known to be made by glaciers and no other processes are observed now across broad areas of the Earth where glaciers do not occur, suggesting that we have had an ice age or ice ages in the past.
- The hypothesis of past ice ages predicts that the land now should be rising where the ice was, and sinking just beyond where the ice was, and these are indeed observed.
- The hypothesis of past ice ages also predicts that sea level was lower when the ice was big, and indeed we observe dead shallow-water corals of that age in growth position in deep water on the sides of oceanic islands, flooded river valleys, etc.
- The success of the ice-age hypothesis in predicting things that have since been observed, and the failure of other hypotheses to do so, give us high confidence that ice ages did occur.
Ice-Age Records
- Isotopically lighter water evaporates more easily.
- An ice sheet is formed from water that evaporated, mainly from the ocean, and then snowed, so during an ice age the water remaining in the ocean should be isotopically heavier than observed today.
- And isotopically heavier water gives isotopically heavier shells
- The history of the isotopic composition of shells recovered from cores of mud from the ocean floor shows that the ice grew and shrank, with the biggest ice every 100,000 years, and smaller wiggles in ice size about 41,000 and 19,000 years apart.
- These exact timings were predicted by Milankovitch decades before they were observed, because they are the timings of features in Earth's orbit that control the distribution of sunshine on the planet.
- Ice has grown globally when the far north was getting relatively little sunshine, especially in midsummer.
- The rest of the world has cooled when ice grew in the north, even though parts of the world were getting extra sunshine, and the world has warmed when ice melted in the north, even when some places were getting less sunshine.
- This seemingly bizarre behavior occurred because the changing ice and other features of the climate changed atmospheric CO2, which rose when ice melted and fell when ice grew, and the CO2 controlled global temperatures.
Bear Meadows
- Ice sheets today cover about 10% of the land area; at the height of ice age the ice sheets covered about 30% of modern land; central PA was just beyond edge of Canadian ice.
- The high parts of Rocky Mountain and the coastal parts of the NE Greenland National Parks are among the places that have permafrost—soil at some depth is frozen year-round.
- Permafrost regions are especially affected by freeze-thaw processes that break rocks, and by rapid downhill soil creep (during the summer, the meltwater can't drain downward through the frozen soil beneath, so the soil gets very soggy and creeps easily).
- These characteristics of permafrost regions contribute to formation of distinctive features, which are observed in places such as central Pennsylvania that were just south of the ice-age ice sheets but which are not forming there today.
- This shows that central Pennsylvania, and other such regions just south of the ice-age ice sheets, were really cold during the ice age.
Glacier Tracks
- Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock.
- Abrasion smooths the upglacier sides of bedrock bumps, while the glacier plucks blocks loose from the downglacier sides of bumps.
- Glaciers erode valleys to give them “U”-shaped cross-sections, often with the floors of valleys coming in from the side left "hanging" above the floor of the main valley; streams make “V”-shaped valleys without hanging valleys.
- Glaciers gnaw bowls called cirques into mountains.
- Glaciers deposit till, which contains rock pieces of all different sizes.
- Melting glacier ice also feeds streams that deposit outwash (because it is washed out of the glacier); the pieces in outwash are sorted by size (mostly sand here, mostly gravel there).
- Till and outwash often form ridges called moraines that outline the glacier.
Textbook 7.1: Yosemite
What Glaciers Do, Erosion and Yosemite
When your tour guide, Dr. Alley, was a much younger man (the year he graduated from high school, 1976), he traveled with his sister Sharon and cousin Chuck on a camping tour of the great national parks of the American west (in Chuck’s 1962 Ford Galaxy 500 land boat). At Yosemite, they hiked from the valley up to Glacier Point. The trail switch-backs up the granite cliffs, opening increasingly spectacular panoramas across the great valley of the Merced River. The view from Glacier Point, across the side of Half Dome, and the thundering Vernal and Nevada Falls, is well worth the climb. It was here that John Muir helped convince President Theodore Roosevelt of the need for a National Park Service to care for the National Parks, which were protected by law but not by rangers for some decades after the parks were established.
The hikers were a bit disheartened by the crowd at Glacier Point—the view is also accessible by the Glacier Point Road. While they sat and lunched, a tour bus pulled in. Most of the passengers headed for the gift shop, but three settled at a picnic table while a fourth strolled over to the railing to see the scenery for a few moments before joining the others at the picnic table. One of the quick-sitters asked “Anything out there?” To which the ‘energetic’ one replied “Nah, just a bunch of rocks. Let’s go check out the gift shop.” At last report, the gift shop had been removed. Regardless, it must be a sad person indeed who would not walk 50 feet to see the glory of Yosemite.
To anyone with open eyes, Yosemite Valley—the “Incomparable Valley”—is well worth inspection. It is carved from the granites and similar rocks of the high Sierra Nevada of California. Once, this granite was magma (melted rock below the surface), far beneath an earlier mountain range. The magma may have fed subduction-zone volcanoes much like those of the Cascades, which continue to the north of the Sierra. However, stratovolcanoes along this part of California have died as the East Pacific Rise spreading center ran into the trench along the west coast, forming the San Andreas Fault but ending subduction. Such a fate eventually awaits the Cascades volcanoes, some millions of years in the future.
The Sierra Nevada was raised and tilted along the great fault to its east, and looks down on Death Valley and the rest of the Great Basin. Earthquake activity, and faults cutting recent sediments, show that the mountain range is still being lifted above the still-dropping Great Basin.
The tough granite of the Sierra Nevada is more resistant to weathering and erosion than are most rocks, but granite does eventually break down, and some streams have managed to exploit weaknesses and cut deep channels through the range. These streams include the Tuolomne River, which carved the mighty Hetch Hetchy valley, now dammed so that a valley the equal of Yosemite is lost under water. The Merced River, which runs through Yosemite Valley, also cut into the range.
The stage was then set for the ice ages. Glaciers gathered on the high peaks, flowed into the valleys, and began to change the landscape.
Which Way Did It Flow?
A glacier is a mass of snow or ice that deforms and moves. Glaciers form wherever snowfall exceeds melting over enough years to make a pile big enough to flow. In places with extremely high snowfall, this can occur where average temperatures are near or even slightly above freezing, such as on the mountains of the Olympic Peninsula. In dry places, glaciers may be absent even if average temperatures are well below freezing. Such places have frozen ground instead, called permafrost (because the frost is permanent).
A pile of pancake batter spreads across a griddle, moving away from where the upper surface of the pile is highest. In the same way, a glacier moves from where its upper surface is highest to where its upper surface is lowest. In the figure below, the pressure at point A (the weight of the material above point A) is larger than at point B, because there is more ice above A than above B. The higher pressure at A gives a net push from A to B. Whether the diagram shows pancake batter, or the ice sheet on Antarctica, this push causes the material to deform and flow.
If you make a pile of pancake batter on a waffle iron, some of the batter may flow along the low grooves and then move up to cover the bumps, but the flow will always move away from the place where the upper surface of the pile is highest. In the same way, ice can flow up a hill in the bedrock if the flow is going in the “down” direction of the upper surface. For example, pieces of Canada are strewn across northeast Pennsylvania, and were brought across Lake Ontario and New York by the ice-age ice sheet. Ice at the bottom of that glacier climbed out of the low spot that now is the lake basin, driven by the upper surface of the ice sloping down from Canada to the U.S.
The total imbalance in pushes is larger for thicker ice than for thin. A very thin ice mass will not deform fast enough for the motion to be measured, and so is not considered to be a glacier. Typically, ice thicker than about 50 m (150 feet) will deform and flow, making a glacier.
Look at the second part of the figure. Glaciers move in one or more ways. All glaciers deform internally, like your slow pancake batter spreading on the griddle. A vertical hole drilled in a glacier will deform as shown in the figure. The stresses are largest, causing most-intense deformation (the permanent bending of the hole shown in the figure), in the deepest ice. The upper ice rides along on the deeper ice, so the velocity is fastest at the surface. Some glaciers are at the melting point at the bottom, warmed by heat flowing out of the Earth beneath. These melted-bed glaciers may move over the material beneath them, either by sliding over those materials (shown in the figure) or, if the materials are soft sediment, by deforming those sediments in a sort of slow landslide (not shown).
Recall that rivers adjust to move sediment and water from one place to another. So do glaciers. The water is supplied, frozen, in the accumulation zone, where snowfall exceeds melting, causing snow and ice to accumulate. The frozen water flows to the ablation zone, where melting exceeds snowfall (ablation means wearing away), or else flows to where icebergs break off (called calving) and drift away to melt elsewhere. For ice sheets covering continents or for smaller ice caps covering plateaus or mountain tops, the ice forms a dome and spreads out in all directions. For glaciers on the sides of mountains, the ice flows down the mountain—the upper and lower surfaces of the ice slope in the same direction.
When we talk about the advance and retreat of a glacier, we are referring to the position of its terminus, where the glacier ends by melting or calving. A glacier is advancing when it is getting longer, and retreating when it is getting shorter. Notice that ice almost always continues flowing from the accumulation zone to the terminus whether the glacier is advancing or retreating—retreat occurs when ice loss by melting or calving is faster than new ice is supplied, and advance occurs when ice is supplied more rapidly than it is removed by melting or calving.
Permanent deformation—flow—within ice may seem strange—after all, ice is a solid. But, as for the soft rock of the asthenosphere down in the mantle, or the soft chocolate bar in a hot pocket, or the red-hot horseshoe in the blacksmith’s shop, ice in all glaciers on Earth is nearly warm enough to melt, and so can flow slowly. As a general rule, materials heated more than halfway from absolute zero to their melting point can flow slowly, and flow becomes easier the closer the temperature is to the melting point. For ice, the coldest yearly average temperature on Earth is about eight-tenths of the way from absolute zero to the melting point, so ice at the Earth’s surface is “hot” and is able to flow. For more on this, and on the occurrence of crevasses as well as flow, see the Enrichment.
Glacier Tracks
A glacier frozen to the rock beneath does not erode much. However, thawed-bed glaciers, especially those with surface meltwater streams draining to their beds through holes (something like cave passages, although formed in different ways), can erode much more rapidly than streams or wind erode. Consider for a moment the Great Lakes of the U.S. and Canada. The lakes were carved by glaciers. The bedrock beneath Lakes Superior and Michigan is well below sea level, and was carved that deep by glaciers, not rivers! Today, rivers carry sediment into the Great Lakes, slowly filling them up. We will see later that over the last million years, times when glaciers were eroding have alternated with times when streams were filling the lakes back up with sediment, and the streams have had more filling-up time than the glaciers had eroding time. And yet, there are the lakes, not the least bit full of sediment. Evidently, the glaciers have been much better at their “job” than the streams have been. The same can be said for many other places. It is not too extreme to say that the regions glaciated 20,000 years ago and free of ice today still preserve a glacial landscape.
Ice moving over bedrock “plucks” rocks free, and then uses those rocks to abrade or “sandpaper” the bedrock, scratching and polishing it. As ice flows over a bedrock bump, the side the ice reaches first is abraded smooth while the other side is plucked rough. Subglacial streams sweep away the loose pieces, and may cut into the rock as well.
Plucked and abraded rocks show clearly that glaciers were present, but so do big features, as seen in Yosemite and elsewhere. If you could make a cut across a typical stream valley in the mountains, you would see that it usually is shaped like the letter “V”. The narrow stream cuts downward, and then mass-movement processes remove material from the walls, giving a “V”. (Where a V-shaped stream enters an ocean to make a delta, the outward and upward growth of the delta over time may eventually fill the bottom of the V with mud to make a flood plain, as we saw with the Mississippi, but initially, when a stream is cutting down, it makes a V.) However, glaciers are quite wide, and can erode across a broad region. Glaciated valleys exhibit a characteristic “U” shape. Yosemite Valley, with its near-vertical walls and near-horizontal floor, is a classic “U”, not a “V”.
The steeper a stream is, the faster it erodes. If a main river cuts down rapidly, then the side streams that flow into it will become very steep, and will cut downward very rapidly. In this way, even a small side stream can “keep up” with the main stream as it erodes downward, and stream processes usually produce “rapids,” rather than waterfalls where the side streams must plunge over cliffs to reach the main stream. Glaciers are different. A main glacier often fills its valley, the ice burying most or all of the rock. The ice from a side glacier then does not drop steeply down into the main glacier because there is no drop. So the side glacier is not steeper than the main glacier. The main glacier has more ice and rock and water than the side glacier, and so the main glacier erodes down more rapidly. When the ice melts, a “hanging valley” remains—a small stream that replaces the small side glacier must plunge over a glacially carved cliff and then flow across the bottom of the “U”-shaped valley to reach the main stream. Eventually, the side stream will wear away the waterfall. But today in Yosemite, numerous streams emerge from small “U”-shaped hanging valleys to cascade down the glacially carved cliffs—the landscape is pretty much what the glaciers left. (Piles of rocks at the bottoms of waterfalls show that the streams are indeed changing things, but slowly.)
Glaciers make many other erosional features. At the head of a glacier, where freezing and thawing can break rock that is hauled away by the glacier, a bowl can be carved into the side of a mountain. If bowls chew into a mountain from opposite sides until they meet, a knife-edged ridge is left—the Garden Wall of the continental divide in Glacier National Park, which we’ll meet in the next chapter. Where three or more bowls intersect from different sides, a pinnacle of rock is left, such as the Matterhorn of Switzerland. Mountaineers have dubbed the bowls cirques, the ridges aretes, and the pillars horns, and geologists continue to use these terms.
Glaciers also leave distinctive deposits. Streams, waves and wind all sort rocks by size, leaving too-big ones behind and carrying away smaller ones. Glaciers don’t care how big the rocks are that the ice carries, so a deposit put down directly from ice may have the tiniest clay particles mixed in with house-sized boulders. Such a deposit is called glacial till. Till plus glacial outwash (sediment washed out of a glacier by meltwater) may be piled up together in a ridge that outlines the glacier, called a moraine.
Pennsylvania has a Moraine State Park, which features glacial moraines. Cape Cod is a moraine, and a moraine is draped across Long Island, showing some of the places where glaciers from the ice age ended.
Textbook 7.2: Glacier National Park
Glaciers and Glacier National Park
Glacier National Park is the southern half of the Glacier-Waterton Lakes International Peace Park, extending north-south across the Canadian-U.S. border and east-west across the great Front Range of the Rockies. Glacier is wolves and grizzly bears, mountain goats balanced on cliffs, moose munching on water plants, beargrass and avalanche lilies. Going-to-the-Sun Road winds past Going-to-the-Sun Mountain, among the best-named features of the park system. The continental divide snakes along the Garden Wall, in many places a knife-edge ridge left as glaciers gnawed into the backbone of the continent from the east and the west. Long, narrow lakes lie along the valleys, which sometimes host lines of lakes strung like beads on the string of the connecting river. (Such glacier-carved strings of lakes are called paternoster lakes, after a resemblance to the beads of a Catholic rosary.
Glacier National Park had roughly 150 active glaciers a century ago, but that is down to 25 or so very small ones, and many of them may be essentially dead now, as modern warmth melts many away (see the changes shown by the older and more-recent photos below). Glacier National Park thus is now more noted for the tracks of past glaciers than for the activity of present ones. But, we suspect that “Ex-Glacier National Park” would not have made the Great Northern Railway happy when they were promoting tourism in Glacier National Park (via the Great Northern Railway, of course). When the last glacier has melted away, perhaps within a few decades (we'll return to this with the Arctic National Wildlife Refuge, near the end of the course), there are no plans to change the name of the park.
Evidence of Ice Ages
Today, permanent ice covers about one-tenth of the land on Earth, mostly in Antarctica and Greenland, with a little ice in mountainous regions. We saw at Yosemite that glacier erosion and deposition produce features that differ from those produced by mass movement, rivers, wind or coasts. Geologically recent examples of those features, from roughly 20,000 years ago, are spread across almost one-third of the modern land surface—in places such as Wisconsin, and northern Pennsylvania, and Yosemite, and Glacier, and many others, the mark of the ice is unmistakable. The 10,000 lakes of Minnesota, the Great Lakes, the gentle moraines of Illinois, and many more features reveal a landscape that is glacially dominated. Such features in Europe first motivated the hypothesis that ice ages have occurred.
This ice-age hypothesis makes many predictions, which allow testing. In times before modern geology, the glacial deposits were called “drift” because they were thought to have drifted into place in icebergs during Noah’s flood. Other people have suggested that the deposits were splashed into position by a giant meteorite that hit Hudson Bay, and still other hypotheses have been advanced. But, the ice-age hypothesis makes predictions that differ from the Noah’s-flood hypothesis or the meteorite hypothesis in many ways. (The biggest difference is that icebergs and meteorites simply do not make features that even vaguely resemble those actually observed, but let’s look at some other differences.)
If huge ice really existed, its great weight must have pushed down the land beneath—recall that the deep rocks are hot and soft, with a “water-bed” cover of stiffer rocks on top. If the ice age peaked only about 20,000 years ago, the slow flow of the soft, deep rocks should mean that the land would still be rising after the melting of the ice, while land around the former ice would be sinking as the soft, deep rocks return to their pre-ice-age positions. The global-flood hypothesis and the meteorite hypothesis do not predict such a bulls-eye pattern of rising and sinking centered on the regions with features known to be made by glaciers today—the flood would have spread evenly across the land, and so would not have concentrated its weight in one place, and the sudden blast of the meteorite would not have left its weight long enough to push the slow-flowing deep rocks far. Measurements by GPS and other techniques show just the pattern expected from the ice-age hypothesis, a pattern that was not predicted and cannot be explained by the other hypotheses.
The water for huge ice sheets would have been supplied by evaporation from the oceans, with the water getting stuck in the ice rather than returning rapidly to the sea in streams. Hence, if ice ages occurred recently, there should be evidence of lower sea levels at the time the ice was big. No such prediction comes from the meteorite or big-flood hypotheses (the meteorite might have made a wave but otherwise would not have affected sea level; the big flood would have raised sea level). Again, the ice-age prediction is borne out by the evidence, and the predictions of the other hypotheses are wrong. For example, some corals grow only in shallow waters where there is much sunlight. Dead samples of such corals from about 20,000 years ago can be found where they grew, down the sides of islands and now under more than 300 feet of ocean water. Other evidence also points to lower sea level in the recent past—the Chesapeake Bay, for example, is a river valley that was drowned by rising waters.
How Many Ice Ages?—An Ocean of Clues
So, much evidence shows that ice ages occurred. Piled tills separated by soils demonstrate that the ice has come and gone many times. But how many times? On land, the glacial record is somewhat confused—often, an advancing glacier will erode the evidence of a previous one. A pile of four tills separated by soils may record four advances, or forty, with some of the record having been eroded away. In many places in the deep oceans, sediment has been piling up without erosion for millions of years. If there were a marker of glaciation in the marine sediments, we could tell how many glaciations have occurred. If there were a way to date these sediments, we could tell when the glaciation happened. Fortunately, we can identify glaciations using shells in marine sediments, and we can date them. Identifying glaciers from shells is covered here, and learning the age of the shells is coming in the next few units.
Water in the oceans is not all the same—roughly one molecule in 500 has an extra neutron or two in one or more of the oxygen or hydrogen atoms. Such “heavy” water is still water, but weighs a little extra. (If you don’t remember isotopes, go back and have a quick look at the introduction to chemistry near the start of the course.) Not surprisingly, light molecules evaporate more easily than heavy molecules. Water vapor, rain, and snow thus are slightly “lighter” than the ocean; that is, the ratio of light water molecules to heavy ones is larger in vapor, rain, and snow than in the ocean from which the vapor, rain, and snow came.
When sea level drops during an ice age - as water vapor is changed to snow and then to ice sheets, the oceans have lost a lot of water. More light water than heavy water has been lost from the oceans, so the oceans are left a little bit isotopically heavier than normal. When ice melts, that light water from the ice sheets is returned to the ocean and makes it lighter.
These changes are very small. If we round off the numbers a little, we can say that in the modern ocean, 1 of each 500 water molecules is heavy, which is the same as saying that 1000 of each 500,000 water molecules are heavy. When the ice sheets were big, had you weighed a whole lot of molecules in the ocean, you would have found that about 1001 of each 500,000 water molecules were heavy. This is a tiny change, the water was still water, but sophisticated modern instruments are so good that such a change is very easy to measure. (And yes, the instruments actually measure the weight of waaaaaaay more than 500,000 molecules, to obtain good statistics.)
Many plants and animals that grow in the ocean build shells of calcium carbonate (the stuff of limestone) or of silica, both of which contain oxygen. These shells record the isotopic composition of the water in which they grow because the oxygen in the carbonate or silica is obtained from the water. Critters growing during big-ice times grow shells in which the oxygen is isotopically slightly heavy, and critters growing during small-ice times grow isotopically light shells. When the critters die, their shells pile up in layers on the sea floor with the youngest ones on top. A core collected from these sediments is a history of the ice volume on Earth. Just date the core, pull out the shells, analyze them isotopically, and there is the answer. With enough care, knowledge, and instrumentation, dedicated workers can obtain consistent, reproducible data that tell a wonderful, clear story. (There are a few additional details, but the main story is this simple.)
Over the most recent 800,000 years, ice has generally grown for about 90,000 years, shrunk for 10,000 years, grown for 90,000 years, shrunk for 10,000 years, etc. Superimposed on this are smaller wiggles, with a spacing of about 19,000 years and 41,000 years.
The Cold of Space
More remarkable, these cycles were predicted, and not until decades after the prediction did technology become good enough to test the prediction and show that it worked. During the 1920s and 1930s, a Serbian mathematician named Milutin Milankovitch calculated how the sunshine received at different places and seasons on the Earth has changed over long times. As the sun, moon, Jupiter and other planets tug on the Earth, the orbit changes a bit. Earth wobbles with a 19,000-year periodicity, the north pole tilts a little more and then a little less with a 41,000-year periodicity, and the orbit changes from more-nearly round to more squashed or elliptical and back with a 100,000-year periodicity. With modern computers, these changes are relatively easy to calculate for many millions of years; for Milankovitch, the calculation was the labor of a lifetime. (He did it very well, though, even correctly noting that the 19,000-year periodicity really goes from 19,000 to 23,000 years and back, a pattern that is indeed observed in the data testing his prediction!)
These orbital wiggles have little effect on the total sunshine received by the planet, but they do move the sunshine from north to south, poles to equator, or summer to winter in various ways. For example, today the northern hemisphere is farther from the sun in northern summer than in northern winter. (Remember that summer is controlled by the tilt of the planet’s spin axis relative to the plane in which the planet orbits, not by the distance from the sun!) In the few millennia centered on 9000 years ago, the northern hemisphere had slightly warmer summers and cooler winters than recently, because the Earth was closer to the sun during northern summers and farther from the sun during northern winters than today. (Meanwhile, the south had slightly cooler summers and warmer winters than recently, because the Earth was closer to the sun during southern winters and farther from the sun during southern summers than today.) The intense summer sunshine in the north 9000 years ago made mountain glaciers smaller then. As the summer sunshine decreased in the north, those glaciers expanded slowly for several thousand years, culminating in the Little Ice Age of the 1600s to 1800s; strong melting of glaciers since then is probably mostly the result of human-caused warming. (We will discuss this later in the course.)
Summer in the northern hemisphere appears to be key to controlling ice ages, probably because the northern hemisphere is mostly land and can grow big ice sheets, but the southern hemisphere is mostly water, already has ice on Antarctica, and so can’t change its land ice much more. In the north, even during warm winters the highlands around Hudson Bay are cold enough to have snow rather than rain. Survival of this snow requires cool, short summers. As summers have cooled around Hudson Bay, ice has grown; as summers have warmed, ice has melted. The way the various cycles interacted led to larger or smaller changes, and thus to the ice ages we know.
You may guess that this is slightly oversimplified so far. For example, during times when Canada has received reduced summer sunshine, allowing ice to grow, the southern hemisphere or the tropics often were receiving extra sunshine, yet they cooled during many of those times. How Canada told the glaciers of Patagonia and Antarctica to grow was for a long time a great puzzle. The answer involves the global warming from atmospheric carbon dioxide. The growth and shrinkage of the vast ice sheets, the changes in sea level, and other changes had the effect of shifting some carbon dioxide (CO2) from the air into the deep ocean during ice ages, and bringing the CO2 back out to the air during warm times. The orbits affected the ice, which affected currents and sea level and plants and other things, which affected CO2. But, as we will discuss later in the semester, CO2 in the air tends to warm the Earth's surface no matter how that CO2 got into the air. And, the changing CO2 explains why, when the ice was growing, places getting more sunshine still got colder, and why, when the ice was shrinking, places getting less sun still got warmer.
Climate records show many other types of changes. Very large, rapid changes have been caused by sudden surges of ice sheets, and by jumps in the way the ocean circulates. We do not understand these faster changes well enough to know whether they could happen again, although we're cautiously optimistic that we won't have any for a while. Naturally, the Earth’s orbit right now is in an intermediate state, and we should be looking forward to another 20,000 years or more with little change before we begin the slide into a new ice age. (See the Enrichment for a little more on this.) However, humans almost certainly are now more important to the climate than are such slow changes, as we will see later.
Textbook 7.3: Trail Ridge Road and the Seven Mountains
Permafrost and Periglaciation
Meanwhile, what of things back in central Pennsylvania and in the many other places that were not quite reached by the ice-age ice? As you might imagine, with the world cold enough to grow ice to cover New York, Chicago, Minneapolis, and Seattle, as well as much of Europe, the climate was colder everywhere then than it has been more recently.
If you climb the ridges of central Pennsylvania, perhaps up in the Seven Mountains just southeast of State College (go up Bear Meadows Road past the ski area, for a start), you may notice several interesting things geologically. Beneath the hemlocks and rhododendron, the soils and streams and hillslopes have more in common with the high meadows of Trail Ridge Road in Rocky Mountain National Park, or with the coast of Greenland, than they do with the modern climate of State College. Trail Ridge Road crosses tundra, where small, hardy plants grow atop permafrost. Although the uppermost soil along Trail Ridge Road thaws during the brief summers, and the deep Earth is thawed by the heat of the Earth, the materials between are frozen year-round in permanent frost. (These areas are also called “periglacial,” because they may occur around the glacier, or on its perimeter.)
Consider the following features of the Seven Mountains.
- Many of the headwaters streams have the braided pattern that forms when lots of big rocks are supplied rapidly. Large rocks are evident across the beds of these braided streams. But, the sediment is not “active.” Trees grow on the bars. Even huge floods, such as the winter flood of 1996, do not move the rocks beneath them. Something in the past delivered much coarse sediment to the streams, and then that delivery stopped. Meanwhile, along Trail Ridge Road today, freeze-thaw processes in the tundra break loose large blocks of rock that can be moved to streams.
- The highest points on the ridges of the Seven Mountains are composed of resistant sandstone bedrock, but near the surface, the rock has been broken into huge blocks, of the sort that are worked loose by freeze-thaw activity. On flat places such as Big Flat, these blocks sometimes are patterned, with higher and lower, coarser and finer regions a few yards (or meters) across. (These features were described by geologists during times when logging and fire had removed the thick vegetation; the features are very hard to see and almost impossible to photograph today, but can be found during careful bush-whacking.) Meanwhile, similar features occur along Trail Ridge Road and in other permafrost regions, where expansion-contraction processes during the seasonal freezing and thawing of the upper layer sort and stir the rocks and soil into such patterns.
- Stripes or fields of sandstone blocks extend from the ridges down across other rock types to the streams. The blocks are often aligned, as they would be in a creeping soil mass. The coarsest blocks typically are on top with finer material beneath, and patterns such as those on Big Flat may be present but elongated as if they were creeping downhill. Yet the blocks are not now creeping downhill; trees grow on top, and have not been knocked over or bent by landsliding or soil creep. Meanwhile, when the top of permafrost melts on Trail Ridge Road or elsewhere, the water cannot drain out through the rocks beneath because the spaces between those rocks are plugged by ice. The water is trapped in the thawed layer, which then is capable of creeping on very gradual slopes. “Solifluction” or soil-flow lobes thus are common extending downhill.
- Bear Meadows is a young feature, probably formed during the coldest part of the most recent ice age, and probably dammed by a debris-flow or soil-flow lobe extending down from the ridge above it. Bear Meadows is one of the few natural wetlands of any size in central Pennsylvania. The meadows provide a favorite blueberry-picking spot for people—and bears—in the region. The plants of Bear Meadows are quite interesting and varied. Examination of a core pulled from the mud that fills the bog shows that the bottom is almost free of organic material—just silt. Above that, pollen and other remains of cold-weather plants appear, dating to the first bit of warming from the ice age, followed by a progression to warmer-weather types and on to the modern, productive bog. A nearly barren tundra of the Trail Ridge Road type, with a solifluction lobe that dammed a stream, followed by warming, would have produced the sediments we see.
- Other permafrost indicators can be found near State College, including some down in the valleys.
The conclusion is nearly inescapable—Trail Ridge Road in Rocky Mountain today is an excellent picture of what the Seven Mountains looked like during the ice age. Permafrost is common across much of northern Canada and Siberia and around the coast of Greenland, and in high-altitude regions. Permafrost poses grave problems for construction—the heat of a building can melt permafrost beneath, causing uneven settling that breaks the building. Permafrost also records the climate changes that have come to central Pennsylvania and other regions.
An Important Aside: Is This Story-Telling or Science? (Hint: Science...)
Perhaps more meaningful than the conclusion of past Pennsylvania permafrost is the underlying reasoning. Some people today, including important government officials, claim that “historical” geology is not really science, does not use the scientific method, does not produce scientific results, and so should be ignored. (Many commentators have noted that these government officials typically dislike policies that might be motivated by the science, and are probably really more interested in complaining about the policies than the research. But, let's look at the research.)
Consider how the process works. Go up to Bear Meadows, start up toward the ridge above, and look around carefully. You see that big rocks are present, of a type that is quite different from the bedrock directly beneath.
Many hypotheses are possible to explain this observation—space aliens dropped the big rocks; or bulldozers pushed the rocks into place; or, the rocks came screaming down from uphill in a giant landslide; or, they came creeping down slowly; or, … you could think of others. Each hypothesis leads to predictions. If a bulldozer pushed the big rocks in, we should find the bulldozer tracks, and we should be able to trace back in historical records to who was driving the bulldozer, and why. The first settlers, who arrived before bulldozers were invented, should have found hillslopes free of big rocks. If the big rocks came from uphill, we should be able to find a source of such rocks uphill. Landslides start with big falls or slumps from particular places, so a landslide should have a big scar at its head, whereas creep slowly collects rocks as they are worked loose and carries them along, lining them up as they go.
So, you look for evidence that supports or refutes each of your hypotheses. The early settlers complained about the big rocks, old cabins are built on the big rocks, so the bulldozer hypothesis won’t work. There is no evidence for a landslide scar anywhere, despite evidence for lots of different “stripes” of big rocks extending downhill from a ridgetop source where bedrock of the same type as the big rocks sticks out. You quickly come to the realization that the rocks look like a soil-creep deposit extending down the ridge crests; the predictions from each of your other hypotheses fail, but each of the predictions from the soil-creep hypothesis is supported by additional data that you collect for testing purposes.
Then, you note that the material is not now creeping—roads and trails are not being slowly buried by big rocks today, the trees are not knocked over, etc. Tree roots hold many of the rocks in place and prevent motion. So you look for a time in the past when tree roots were not holding the rocks in place. You collect more information—the big rocks are on top of smaller rocks and soil, not on the bottom, the big rocks are often standing on edge, the rocks show patterning of coarse and fine, etc. Other geologists are scanning the whole planet, laboring over centuries, and among the many things these geologists report are the conditions of creeping hillslopes in the tropics, the deserts, the temperate zones, and the poles. You talk to other geologists, devote a decade of your life to careful study, and eventually learn that the things you see on the slopes of the Seven Mountains resemble features of permafrost, and not features of any other modern setting.
But, if you are correct and these are permafrost features, there should be other evidence of cold conditions in the past, at the time that these features were active. So you take a core in the bog, and find that the bog started in a very cold time (the deepest pollen you find is from plants that today are found only on the tundra), and the bog seems to be dammed by one of the soil-flow lobes, linking the soil-flow lobes to the time of the tundra cold. (It is true that no one has used a backhoe to take the dam apart to look for a space-alien-constructed dilithium-crystal foundation, so maybe the space-alien hypothesis has not been completely falsified and the science could be improved; but, there comes a point of diminishing returns….)
Next, you ask whether this makes sense. You have tentatively concluded that the hillslopes of Pennsylvania record cold conditions at a particular time in the past. Is there a reason why cold should have been here at that time? Well, just to the north, glaciers were pushing up moraines at the same time. And astronomers making orbital calculations find that the high northern latitudes were receiving about 10% less sunshine than today during that glacial age. Climate modelers who test whether such a drop in sunshine would be sufficient to grow glaciers and make conditions very cold find that cold indeed makes sense, especially when the modelers include the effects of the drop in atmospheric CO2 levels that was triggered by the change in sunshine and that is recorded in ice-core bubbles from the time.
Now, a modern geologist who tells the “story” of this chapter—Pennsylvania hikers twist their ankles on permafrost deposits—actually has a lot more evidence than the little sketch provided here. The libraries of information collected by centuries of Earth scientists are woven together in a sophisticated, carefully tested, highly reliable whole. This great tapestry of knowledge still has gaps, dropped stitches and moth-bitten places, and the ragged edge where knowledge runs out into the unknown that so excites us as scientists. But the science of the tapestry is well-woven and exceptionally strong. We can only hope that the misguided attacks on this science come from ignorance and not malice, because ignorance is more easily changed.
Virtual Field Trips
Join Dr. Alley and his team as they take you on "virtual tours" of National Parks and other locations that illustrate some of the key ideas and concepts being covered in Unit 7.
TECH NOTE
Click on the first thumbnail below to begin the slideshow. To proceed to the next image, move the mouse over the picture until the "next" and "previous" buttons appear ON the image or simply use the arrow keys.
Virtual Field Trip #1: Yosemite National Park
Image 1: Yosemite valley under cloudy skies. Forest in the foreground and mountains in the background. Bridalveil Falls on right in distance. Tracking the Great Glaciers from Yosemite to Greenland and Alaska. Yosemite truly is an incomparable valley. Bridalveil Falls, on the right, is a hanging valley; its small glacier did not cut downward as rapidly as the main glacier. All photos by R. Alley.
Image 2: Yosemite valley under cloudy skies. Bridalveil Falls in the foreground to the right. Closer view of Bridalveil Falls, right. The U-shape of the valley of Bridalveil Creek shows that it was glaciated.
Image 3: Close-up of Bridalveil Falls, Yosemite. Rock cliffs to each side of falls. Green shrubs just above the falls and forest below. Bridalveil Falls, Yosemite National Park. Bridalveil Creek has just started to cut a notch into the U-shaped cross section of the former glacial valley. The rapids at the bottom of the waterfall is running on rocks dumped there by the creek; eventually, the waterfall will be completely transformed into a rapids unless another ice age brings glaciers to re-form the waterfall.
Image 4: Close-up of Lower Yosemite Falls under blue sky, Yosemite National Park. Rock cliffs to each side and large evergreen in right foreground. Lower Yosemite Falls, Yosemite National Park. The numerous waterfalls of the park exist because the ice-age glaciers eroded the tremendous cliffs of the valley.
Image 5: Yosemite Valley, California. The deep, U-shaped valley was carved by glaciers. Half Dome above Yosemite Valley. A glacier flowing to the lower right deepened and widened the valley; see the next slide for a modern example.
Image 6: A valley in east Greenland that is about the same size and shape as Yosemite, but still contains the glacier that is carving it. Glacier draining Greenland Ice Sheet into head of Scoresby Sund, NE Greenland National Park. The scale is similar to that in the previous picture. Yosemite’s glaciers ended on land; this one calves icebergs into the fjord.
Image 7: Ice sheet, NE Greenland National Park. Wave-shaped folds are visible, showing flow of ice. Bright blue meltwater lake visible, top left. Near ice-sheet edge, NE Greenland National Park. The folds show that ice flows. Blue at top is a meltwater lake; such lakes may drain to the bed.
Image 8: Corridoren Glacier, Greenland. Very visible medial moraines, dark and light wavy stripes. Corridoren Glacier, Greenland. The stripes are medial moraines, rock debris picked up from ridges where two tributary glaciers join.
Image 9: Tributary glaciers joining in NE Greenland National Park. Arrows point to accumulation area, lakes in ablation zone, and medial moraine. Several tributary glaciers joining; flow is to right. NE Greenland National Park. Accumulation area in cirque (red arrow), lakes in ablation zone (green arrow) and medial moraine (blue arrow) are visible.
Image 10: Glacier in east Greenland, with high-elevation accumulation zone, low-elevation ablation zone, and older, lower elevation moraines. Accumulation zone in cirques (top), ablation zone (bottom), 150-year-old moraine (red), subtle, 11,500-year-old moraines (blue), NE Greenland Natl. Park.
Image 11: Close-up of glacier in south Greenland. Arrow points to debris-bearing basal ice. Water in foreground. Debris-bearing basal ice (red arrow) of a glacier in south Greenland. Rocks in such ice sandpaper, or abrade, the bedrock beneath as the ice moves.
Image 12: Close-up of a marmot on granite that has been abraded by debris-bearing glaciers, highlands of Yosemite National Park. The granite behind George the marmot has been abraded by debris-bearing glaciers, in the highlands of Yosemite National Park.
Image 13: Close-up of a Rock Ptarmigan on glacially striated granite, east Greenland. Arrows point to striae which are faint lines on rock. Rock ptarmigan on glacially striated granite (striae are faint lines on rock; a few of many are shown by blue arrows), east Greenland.
Image 14: Glacially striated and polished bedrock under blue sky. East Greenland. Glacially striated and polished bedrock, east Greenland. Ice flowed up the cliff from the lower right.
Image 15: Glacially abraded and plucked rock in fjord wall S. Greenland. Arrow shows ice came from left. “S” marks scratched area, “P” plucked area. Glacially abraded and plucked rock in fjord wall, S. Greenland. The ice came from the left, as indicated by the arrow, scratching/abrading (S) some places and plucking (P) others. Picture is about 10 feet across.
Image 16: Several small snow avalanches into a cirque surrounded by layered rock peaks, east Greenland. Small snow avalanches into a cirque, east Greenland. The layered rocks are flood basalts from opening of the Atlantic Ocean.
Image 17: Horn, Stauning Alps, NE Greenland National Park. Several cirques intersect and leave a towering peak. Horn, Stauning Alps, NE Greenland National Park. Several cirques have intersected to leave this towering peak.
Image 18: U-shaped valley in Tracy Arm Wilderness Area, Alaska. Thick cloud cover, highway in foreground. U-shaped valley from glacial erosion, Tracy Arm Wilderness Area, Alaska.
Image 19: Aerial view of moraines around retreating glaciers, Aple Fjord, NE Greenland National Park. Moraines around retreating glaciers, Alpe Fjord, NE Greenland Natl. Park.
Image 20: Aerial view of moraines around retreating glaciers, NE Greenland National Park. Moraines around retreating glaciers, NE Greenland Natl. Park.
Image 21: Deposits of Bjornbo Glacier, NE Greenland National Park. Rock pieces in foreground and mountains in background. Deposits of Bjornbo Glacier, NE Greenland Natl. Park. Ice was here about 1850. Glaciers carry pieces of different sizes, which make till when deposited.
Virtual Field Trip #2: Glacier, Glaciers and Glaciation: The Ice Really Was Bigger, Glacier National Park
Image 1: Glacier-free cirque under blue sky in Glacier National Park, Montana. Bear Grass blooming in left foreground. Bear Grass (really a lily, not a grass) blooming in a glacier-carved but now glacier-free cirque along Going-to-the-Sun Road, Glacier National Park, Montana. All photos by R. Alley, except for some of the Alaska pictures, which are by either R. Alley, C. Alley, J. Alley or K. Alley, and we’re not sure on some of them because we kept trading cameras.
Image 2: Glacier-carved scenery, Logan Pass, Glacier National Park. Snow-spotted mountains under cloudy skies, water in foreground. Glacier-carved scenery, Logan Pass, Glacier National Park.
Image 3: Comeron Falls, Waterton Lakes, Alberta, Canada. Water falls from different heights and at different angles. Cameron Falls, Waterton Lakes, Alberta, Canada (Glacier-Waterton Lakes International Peace Park). The rocks were folded by mountain-building, and the waterfall follows the bent layers.
Image 4: Female bighorn sheep at driver’s side window of car. Glacier National Park. Female bighorn sheep, Glacier National Park. In search of salt, she was going from car to car, licking steering wheels where perspiring hands had left deposits.
Image 5: Male bighorn sheep among boulders, greenery in background. Canada, near Glacier National Park. Bighorn sheep. This ram was over the border in Canada, but surely has relations in Glacier.
Image 6: Eight mountain goats on mountain side in Glacier National Park. Mountain goats are aptly named. Glacier National Park.
Image 7: Glacier National Park. Valley with green mountainsides and two “paternoster” lakes in view. Glacially carved “paternoster” lakes, Glacier National Park.
Image 8: Cliff in Glacier National Park, that was eroded when it was the side of a glacier that since melted away. Glacially truncated cliff. The ridge on the left was cut off by a glacier that reached at least as high as the sunlit peak, and that flowed over the point where the photographer stands. Glacier National Park.
Image 9: Surface of Greenland Ice sheet with view of melterwater stream entering blue lake at left center. Meltwater stream entering lake, surface of the Greenland Ice Sheet. At higher elevation, the ice-sheet surface is just snow. Such ice sheets now cover 1/10 of the Earth’s land, but during the last ice age covered almost 3 times more.
Image 10: Eight caribou on the dulled white surface of the Greenland ice sheet. Caribou on the surface of the Greenland ice sheet, here about 1 mile from the edge, avoiding mosquitoes. Healed crevasses are evident. This is in the ablation zone, and meltwater plus wind-blown dust have dulled the white snow.
Image 11: Margerie Glacier, Glacier Bay National Park. Blue ice visible in foreground, background glaciers grey. Bald Eagle sits on peak in center. Bald eagle (arrow) on top of Margerie Glacier, Glacier Bay National Park, Alaska. Glaciers can be large. Glacier ice is blue, as seen here, for the same reasons that water is blue (preferential absorption of red by water molecules).
Image 12: Bald Eagle sitting atop a flag pole in Sitka, Alaska, near Glacier Bay National park. Bald eagle, Sitka, Alaska, near Glacier Bay National Park, in case you wanted to know what the bald eagle in the previous picture looks like. Mostly, this is an excuse to stick in a cool picture.
Image 13: Northwest Fjord, east Greenland. Iceberg under blue sky and surrounded by blue water. Float-plane propeller right foreground. Iceberg behind float-plane propeller, Northwest Fjord, east Greenland. The berg reaches about 400 feet above the water, and is close to one-half-mile long.
Image 14: Harbor seal on small iceberg, Glacier Bay national Park, Alaska. Harbor seal on very small iceberg, Glacier Bay National Park, Alaska. Glaciers lose mass either by melting, or by calving icebergs.
Image 15: Cloudy skies and double rainbows over a bright white iceberg in Scoresby Sound, NE Greenland National Park. Rainbow above iceberg, Scoresby Sound, NE Greenland National Park. Icebergs are highly relevant to the study of glaciers and ice ages, but we’re not above sticking in pictures primarily because they’re pretty.
Image 16: A Fulmar in flight over a sea of ice, NE Greenland National Park. Fulmar in front of sea ice, NE Greenland National Park. Sea ice is frozen ocean water, usually less than 10-20 feet thick. Icebergs calve from glaciers formed from snowfall, and can be more than 1000 feet thick and the size of small states.
Image 17: Marble Island, Glacier Bay National Park, Alaska, covered with sea lions. Marble Island, in Glacier Bay National Park, Alaska, was scoured smooth by glaciers, and is now home to numerous sea lions.
Image 18: A bald eagle soars among a large number of sea gulls and ravens at South Marble Island, Glacier Bay National Park, Alaska. An immature bald eagle (arrowed) concerns the gulls and ravens of South Marble Island, a glacially scoured rock in Glacier Bay National Park, Alaska.
Image 19: A humpback whale breaching in the fjords of Glacier Bay National Park, Alaska. The deep, glacially carved fjords of Glacier Bay National Park and surrounding Alaska are home to humpback whales and other charismatic macrofauna (big, cute critters).
Image 20: Small island in Kong Oskar Fjord,Greenland, with raised beaches that formed as the island rose from the ocean. Raised beaches form a bulls-eye on this small island in east Greenland. Melting of ice sheets raised global sea level at the end of the ice age, but some regions that had been depressed under the former ice sheets rebounded faster than the sea rose, raising beaches out of the water.
Image 21: Satellite image of Chesapeake Bay, an old river valley that was flooded by the rising sea as the ice-age sheets melted. Satellite image of Chesapeake Bay. Geological study of the Bay and its surroundings confirms what you can see by inspection: the bay is a drowned river valley, indicating either that sea-level rose or the land fell fairly recently (mud is filling the bay; if the change happened a long time ago, the bay would be filled). Similar features along many coasts, including those being raised tectonically, show that sea level rose rather than the land falling.
Virtual Field Trip #3: Bear Meadows
Image 1: Snow spotted Rocky Mountains, greens trees in foreground. What do Rocky Mountain, Bear Meadows, and Greenland have in common? Hint: It’s cold up there on top of Rocky Mountain… All photos by R. Alley, maps from USGS and NOAA
Image 2: Three side-by-side close-ups of flowers. On the left, spotted coral-root orchid. Center, Glacier lily. Right , blue columbine. First, some Rocky Mountain wildflowers. left: Spotted coral-root orchid, middle: Glacier lily, right: Blue columbine.
Image 3: Maps of ice-age ice, which covered almost 1/3 of modern land areas, and modern ice, which covers only about 10% of the land area. http://www.ncdc.noaa.gov/paleo/slides/slideset/11/11_177_slide.html National Geophysical Data Center, NOAA, Mark McCaffrey The last ice age was most intense between about 24,000 and 18,000 years ago. This image re-creates the northern hemisphere about 18,000 years ago (left) and today (right). Notice that broad areas now submerged were exposed during the ice age by the lower sea level (e.g., green areas west of Alaska), and that both land ice (white) and summer sea ice (gray) were more extensive then. Southern-hemisphere changes were smaller. Also notice that the ice came close to Pennsylvania.
Image 4: Map showing that ice-age ice extended into the U.S. from Canada. http://www-atlas.usgs.gov/articles/geology/features/glaciallimit.html This map from the USGS uses distinct colors for different geological units. The white line shows how far south ice-age ice came from Canada. The “Driftless Area” in Wisconsin (white outlines and arrow) was missed by ice, which was channeled south along Lake Michigan and west along Lake Superior. Numerous flooded river valleys including the Chesapeake Bay (yellow arrow) are evident. Bear Meadows (pink arrow) and surroundings were beyond the Canadian ice, but must have been very cold when ice was close.
Image 5: Bear Meadows, State College, PA. Water and water grasses in the foreground. Forest in the background. Bear Meadows, above State College, PA, occurs where natural wetlands are rare. Sediment cores indicate that the bog formed during the ice age.
Image 6: Picture from east Greenland of a “striped” hillside because of downhill creep of rocks and plants formed on permafrost. Permafrost enhances mass movement (downhill creep). Direct measurements have documented motion of roughly 1 inch per year. Such downhill motion can move a lot of rocks over time, and might dam a stream occasionally. Shown here is a permafrost hillside roughly 1/2 mile across, in east Greenland, with rocks creeping downhill toward you.
Image 7: Large band of rocks, some with patches of moss, surrounded by fallen leaves. Trunk of small tree is visible to the left. This band of large blocks has moved down to just above Bear Meadows from the ridgetop, most of a mile away. (Is it any wonder that Pennsylvania hikers need good boots to avoid twisted ankles?) Trees growing on some of the rocks show that motion is not occurring now.
Image 8: Patch of stones in Greenland, sorted by size into a unique pattern, as the result of permafrost. Permafrost often sorts stones by size, making fascinating patterns, as here in Greenland. Dr. Alley’s boot toe (bottom center) for scale.
Image 9: Sorted stone circle in central PA, surrounded by fallen leaves. Unfortunately, the best sorted stone circles in central PA are now obscured by leaves. Weak examples are shown here, with coarse clasts circling the finer centers, which are marked by the white “X” symbols in the lower left and right.
Image 10: Side-by-side images of large bands of rocks. Bear Meadows on the left and Greenland on the right. The tendency for moving hillsides in permafrost to align clasts tipped on edge can be seen in Greenland (right), and is evident above Bear Meadows (left).
Image 11: Side-by-side images of large bands of rocks. Bear Meadows on the left and Greenland on the right. Similarities in size and orientation of blocks in flows that have moved downhill are evident in Greenland (right) and above Bear Meadows (left).
Image 12: Three permafrost hillsides. Top, Rocking Mountain National Park. Bottom left, Juniata River, PA. Bottom right, Devil’s Lake State Park, WI. Top: Rocky Mountain National Park – still moving downhill. Bottom Left: Pennsylvania (Frankstown Branch, Juniata River) Bottom Right: Wisconsin, Devil’s lake State Park/Ice Age Trail Below all three images: Present and Past Permafrost Hillsides
Virtual Field Trip #4: Are Glaciers Really Changing?
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McCarty Glacier, AK, June 21, 2004, photo by B.F. Molnia.
Image 1: Muir Glacier, Alaska, 2004. Blue water in foreground, glacier in background, large amount of rock exposed. Are glaciers changing? Yes. Over the last century, and over the last few decades, essentially every glacier on the planet has gotten smaller. Over shorter times, the “fun things” glaciers do (surges, kinematic waves, etc.) have caused some to grow while others shrank, but overall shrinkage is strong. Here are some photo pairs, historical and more recent, assembled by Bruce Molnia of the United States Geological Survey, and archived at the National Snow and Ice Data Center. The lenses used are not always identical, but the photos are taken as nearly as possible from the same spot, and you should be able to see the changes clearly.
Image 2: Muir Glacier, Alaska in 1941. Glacier covers most of photo, no water visible. Muir Glacier, Alaska, August 13, 1941, photo by B.F. Molnia
Image 3: Muir Glacier, Alaska, 2004. Blue water in foreground, glacier in background, large amount of rock exposed. Muir Glacier, Alaska, August 31, 2004, photo by W.O. Field
Image 4: Holgate Glacier, AK, 1909. Glacier covers much of rock, water in foreground. Holgate Glacier, AK, July 24, 1909, photo by U.S. Grant
Image 5: Holgate, AK, 2004. Small amount of glacier visible, large portions of rock exposed, water in foreground. Holgate Glacier, AK, Aug. 13, 2004, photo by B.F. Molnia
Image 6: McCarty Glacier, AK, 1909. Large glacier in center, water in foreground, mountain visible to left in background. McCarty Glacier, AK, July 30, 1909, photo by U.S. Grant
Image 7: McCarty Glacier, AK, 2004. No glacier visible. Water in foreground, mountains in background. McCarty Glacier, AK, Aug. 11, 2004, photo by B.F. Molnia
Image 8: McCarty Glacier, AK, 1906. Rocky water’s edge and water in foreground. Glacier in background, in front of snow capped mountains. McCarty Glacier, AK, Aug. ??, 1906, photo by C.W. Wright
Image 9: McCarty Glacier, AK, 2004. Rocks in foreground, no water, melting glacier in background in front of snow capped mountains. McCarty Glacier, AK, June 21, 2004, photo by B.F. Molnia
Word Document of Unit 7 V-trips
GeoClips
This week, we feature two GeoClips, both featuring Dr. Alley. As before, we hope you enjoy these, and find them to be useful complements to the readings, class notes, and slide shows of Unit 7.
The Bear Meadows National Natural Landmark, just over the ridge from Penn State’s University Park campus, was recognized by the National Park Service in 1966 as a site that “possesses exceptional value as an illustration of the nation’s natural heritage.” Although many guide books somehow have decided that Bear Meadows is 10,000 years old, the Meadows are clearly much older, having formed during the last ice age. Here, take a walk just above the Meadows, and learn why Pennsylvania hikers, like those in the high country of the Rocky Mountains, are wise to wear sturdy shoes. Then, see what this has to do with the Formation of the Meadows—they really are related.
Rivers of Rocks and Permafrost
So why do hikers in Central Pennsylvania carry so many ace bandages? And the answer is that there's rocks on top of everything. All the trails in Central Pennsylvania are covered with rocks that are sitting up on a edge like this on top of the dirt. Why do the rocks get on top of the soil? And that story's sort of interesting. If you ever have a cat and you buy a bag of kitty litter, and you shake the bag and then you open it, you'll find the big pieces are on top.
You may find this in cereal boxes too that you'll get the big pieces floating to the top. And that's linked to a very simple geometric fact which is that little pieces can fall under big ones. And big ones cannot fall under little ones. If you want to find things like this that are happening today you won't find them here. These trees are not being rolled over by rocks that are moving. Our trees are perfectly happy here.
To find places where things like this are really moving today, you go to the top of Trail Ridge Road in Rocky Mountain National Park. You go to the North slope of Alaska. And there the ground is permanently frozen at some depth. And the rocks are slowly creeping down in the summer on top of that, lining up and turning up as the freezing and thawing move things around.
Here if you thaw the snow, it just soaks down through the rocks. It goes through the spaces. It goes down the river and it's fine. If it's frozen underneath, it can't soak down. And so you get soft mud that's full of water. It can't get rid of its water. It's sitting on top of slippery ice. What's it do? It slides downhill slowly. And so you go to the North slope of Alaska. You go to the top of Rocky Mountain. And all the hillsides are moving. And they're tipping the rocks up on edge and they're lining the rocks up in the direction they're going. And they're making things that look just like this without the trees. And so what we see here is a route of the Ice Age.
The Formation of Bear Meadows
So here we are at Bear Meadows, perhaps the biggest and best natural wetland in Central Pennsylvania. Natural wetlands, lakes, bogs, are fairly rare in Central Pennsylvania. And that's because nothing has been making them recently. And nature fills them up. Rocks wash in in streams. Trees fall and leave's fall. And wetlands fill up. So when you see a wetland, you have to say geology made this fairly recently. Or humans made it. And this one's natural.
If we were to go out into this bog and stick a pipe down in the mud about 20 feet and pull it up and split it open, the mud on top has sticks and leaves and twigs of things that live here today. At the bottom it has a remnants of things that live on the North slope of Alaska today. It has evidence of tundra. This formed during the Ice Age. Below that's rocks.
And so it's rocks and then Ice Age and then stuff that lives there today. So this formed when the climate was different. And it formed by those beautiful rivers the rocks that we were looking at just up the hill. When this was tundra, when this was the North slope of Alaska or the top of Rocky Mountain, the hillsides were creeping down in these great rivers of rocks. And one of those dammed the stream. And that made a lake. And since then the lake has been filling in to give us this beautiful wetland that's full of good things all year.
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Here are some optional animations you might also want to explore! (No, these won't be on the quiz!)
Glacier Physics
(An extensive collection of animations on this subject)
Glacial Landforms Resulting from Erosion and Deposition
(An extensive collection of animations on this subject)
Examples of Deglaciation
(An extensive collection of animations on this subject)
Video Lecture
The Unit 7 lecture features Dr. Sridhar Anandakrishnan and is 64 minutes long.
Good morning. Welcome to Geosc 10, Geology Of The National Parks. Today we're going to be talking about my very favorite subject in geology, glaciers. My name is Sridhar Anandakrishnan, and I'll be your guide through some of the most amazing scenery on the planet. And some of the most important geology that we have today, in my opinion. We're going to be talking about glaciers, and ice sheets, and sea level.
And sea level impacts you. You might never see a glacier in your life, but I guarantee that you know about how sea level affects beaches and coastline communities, and your life is going to be affected by that.
So, let's go and first have a quick look at some of these lovely places, and then we'll come back to the presentation.
Today's tearing down mountains. Glaciers are wonderful at destroying mountains. They're one of the best ways we have to both destroy mountains and to make them beautiful. If you didn't have glaciers, these mountains would look very, very different than what they do today.
This is Yosemite National Park in California. It's just up from San Francisco. You can fly into San Francisco, and drive, and you'll be up there in Yosemite in three hours, four hours, something like that. It's one of the most famous places because of people like John Muir, and the amazing photography that has come out there. Some of the history of the place. So, it's something that is almost legendary in conservation circles and in mountain climbing.
This is Bridalveil Falls over on the right over there. And the classic shape of that valley, going up from it, that U-shape for it tells us that this was glaciated. And we'll find out why that U-shape tells us it was glaciated.
And you just look at it way in the back, there. You have Half Dome. And there's probably people that are crawling up the side of it. It's one of the classic climbs of the world.
Here's a close up shot of that Bridalveil and showing that sort of rounded valley heading up from the falls.
Here's Lower Yosemite Falls. There's all these waterfalls all through there. We're on the lee side, the Pacific side of the Sierra Nevada mountains.
Remember last time we talked about, in the Redwood National Parks section, how we talked about when the winds, these wet, cool winds come along, and they start to rise up the mountain, and they get colder and colder, the air gets colder and colder, and then all the water gets squeezed out. Well, here's where it gets squeezed out. It gets squeezed out in Yosemite National Park. And so you've got tons of water pounding down through there. And then, you have these beautiful valleys, and then all the water comes cascading down the sides.
Here's another picture of Half Dome. The glacier was coming right down that valley. You can just imagine 20,000 years ago, if you were standing where this photographer was standing, you'd just see that whole valley filled up with ice all the way to the back, and that ice would be flowing towards you. And as it flowed along, it would be carving out that glacier, making it a deeper and deeper, year after year. And that's why you get these vertical walls.
This is a photograph not in Yosemite, but we've gone off to Greenland at this point. This is Scoresbysund. This is perhaps what Yosemite looked like. You can see there's a similarity. You have this big mass of ice coming down. You've got these huge walls going straight up. 20,000 years ago, instead of this minuscule amount of ice that we have in here— it's an enormous amount of ice, but 20,000 years ago, that ice would have been half a mile or a mile higher up. It would filled up that whole valley.
This is near the edge of the ice sheet. There's a blue pond at the surface. The ice here is perhaps half a mile thick. So, if you were to sit there, and take your shovel, and start digging, digging, digging, you'd have to dig for half a mile before you got to rock. And you can see those big folds where the ice is flowing down. A bunch of cracks and crevasses. If you're trying to travel across that, you could fall very easily into those. Absolutely dramatic, gorgeous.
This is [UNINTELLIGIBLE] glacier in Greenland. And you can see it flowing. Those big stripes coming down towards you, those are individual glaciers that all have come down and merged together. And you can still see those bands where they came together, and then as a valley curves, it comes down.
Here we have a bunch of tributary glaciers where the green arrow and the red arrow are that have come together, and these amazing, huge crevasse. I wouldn't want to be walking across that glacier. You'd just be falling into those holes all the time.
Another photograph of the glacier. Here we have some evidence, just the first beginnings of a clue that glaciers can change their size. The blue arrows are pointing at where the glacier used to sit just a little while ago. About 150 or 200 years ago, that glacier used to be at a different place. And we can tell that because of the color of the rock, and the marks on the rock. And then, the glacier has retreated back from there in the last few hundred years.
This is looking at the side of a glacier, and you have all of these debris bands in the bottom, and that tells us that glaciers aren't just these clean, beautiful, white, ice and snow masses. They also have all of this rock and mud and debris along the bottom. And so, here's a second clue that glaciers can modify the landscape. They can rip out rocks and carry them away, and here's evidence that they do that. And they're very good at doing that.
All the places in this country that have lakes— Minnesota, the land of 10,000 Lakes. The finger lakes in New York. The Great Lakes themselves, Lake Michigan, Erie, Superior. All of them are there because there was an enormous glacier—an ice sheet, really— that sat on top of North America, that filled all of Canada and flowed down into the northern tier of the US, and as it flowed down, it just churned out these big lakes and ripped them up.
Here are some of the critters that live in Yosemite National Park. And here are some more. These are not in Yosemite. This is in Greenland. Then, you can see these striations were the glacier came by. But now, the glacier's gone, and there are ptarmigan wondering across that landscape.
Here's another example of what glaciers can do. Glaciers flow across this, and they just polish it. It's like taking sandpaper and rubbing it for hundreds and thousands of years across this rock. You smooth it and striate it.
Just beautiful. Snow avalanches coming down. Just some of the most dramatic scenery. I've got a confession to make. I'm not really a geologist. I'm an engineer. I started out my life as an engineer, and I worked as an engineer. And then, I got a job to go to Antarctica and work on an engineering project. And I said, man, I got to keep doing this, and I became a geologist, just so I could go and work on glaciers, and understand them, and continue to go to these absolutely beautiful places.
Look at this. Look at that ridge running up there. If there hadn't been a glacier there, you wouldn't have that ridge. The glacier came along and literally chewed away at the side that mountain. And there was another one on the other side of the ridge that chewed away at it, and the two of them formed this sharp ridge that runs up the side of them. And then, where there's two or three of them that come together, you get these sharp horns that go up into these very, very steep peaks. You wouldn't have it unless you had glaciers doing that. This is some of the most beautiful scenery that I've ever seen.
This is in Alaska. Another one of those nice, U-shaped valleys where a glacier had come pounding down that towards us. It's all gone now. Where did it go? Why did it go? That's what we're going to be talking about today.
Here's another one. I can just sit here all day and just show you pictures, and I'd be perfectly happy. Unfortunately, we're going to have to go and talk about why this is important, and we'll do that next. Let me just run real briefly to Antarctica, and we'll see some pictures there. And then, we'll go to the real stuff.
So, this is Antarctica. The reason we want to go here is that Antarctica is the best analog to what Yosemite might have looked like 20,000 years ago. There's still huge glaciers in Antarctica. This is a mountain range that's 10,000 feet high. And that glacier almost fills it right to the top. Yosemite National Park, the mountains there are 6,000 feet high, and the glacier once did almost fill that valley. Same way here, and these glaciers do fill their valley. And so, we can study these glaciers and learn a lot about what Yosemite might have looked like 10,000 years ago or 20,000 years ago.
Here's [? Ketlet's ?] glacier that's coming down towards us. You can see those flow stripes. And as I said, those are enormous mountains, but there's only a little bit of them, maybe 2,000 or 3,000 feet, that are sticking up out of the snow. All the rest of that valley— and you can imagine if you ripped off that glacier, you'd have this two mile deep valley that would be sitting there, and you'd be standing there looking up these two mile high walls.
Here's a valley. This is a dry valley. It's in Antarctica. For fairly complex reasons, the glaciers can't get into this valley. They did at one time. They carved it out, and now they've retreated, and they have these frozen lakes in the bottom of them. But here, you have one of those two mile high valleys. We're flying over it in an airplane and looking down at it. And way in the back, way, way in the back, you have the main Antarctic ice sheet.
Here's the Transantarctic Mountains. You have these beautiful, layered sedimentary structures a long time ago. These were much lower, and then they were raised up over the last 100 million years or 150 million years. And now they stand 10,000, 12,000 14,000 feet high. And then, right behind them, you can see that white area there is the East Antarctic ice sheet flowing down towards us.
Here's one of those glaciers trying to get through the Transantarctic Mountains, coming down and flowing right around through all the rocks, and trying to erode it, and trying to deepen those valleys.
This is Mount Erebus. This is a volcano in Antarctica. You can see the steam rising up out of the top of it. There's a big lava pool up at the top. There's lots of people that go up there and study it. It's 13,000 feet high. And all the sides of it are ice covered. It's so cold in Antarctica that even though it's this huge mountain that is a volcano, and there's this bubbling lava pool at the top of it, you still have this mass of ice that's covering the whole thing.
This is Mcmurdo Station in Antarctica. This is where we do our research. We fly in here. You can see it looks kind of like a mining town. It's a very gritty place. There are no trees there. There's no vegetation there. The soil is all frozen, so you can't dig underneath it to put all your utility lines, and sewer lines, and electric lines. All of those run above.
So, it's just a very gritty place. But you've got to have that to do your research, to have the airplanes fly around, and so on. This is not one of the most beautiful places in the world, but hey.
This is a view looking out from Mcmurdo, out to the ocean. The ocean is frozen there. It's so cold that the ocean simply freezes over. In some years, it never opens up. Some years, it just remains frozen all through the summer. And then the winter comes, and it freezes, and then the ice just thickens. And that might happen for two or three years in a row, but usually that area does open out.
This is one of those distressing photographs. You got to have fuel to fly airplanes. These are big fuel tanks. And then right in the background, you have this absolutely beautiful shot of Mount Erebus rising up above the town.
There's some critters that live in Antarctica. The most famous, of course, are penguins. I've been down there many times, and I still am delighted every time I see one, because they're just such charming little fellows. And here we are, enjoying watching one of them who is looking at us as well.
And here's a close up. This is an Adalie penguin. They're about a foot and a half or so tall, and just as curious and fearless as anything. They have no predators on land, so when they're walking around on the snow here, they are not bothered if anybody walks around.
And he's off. We're not quite sure where he's off to, he or she. And off into the distance.
This is a penguin rookery. The penguins are displaying. They're sitting there sticking their head up. And those little circles of rocks— I don't know if you can see them— those are their nests. There are no twigs here. There's no place to put their eggs, except the only way to protect them is in these little circles of rocks. And so those rocks become really valuable. And they'll steal them from each other, and they'll get into these titanic battles. When the one steals it, they'll go and fight each other, and they'll bounce against each other. And it's quite serious to them. It's quite amusing us.
This is a view of Erebus stretching up above us, and in the foreground, you have a hut from one of the early explorers from 1910. This is Shackleton's hut. This is what he lived in through the winter before he went on his epic journey to try and get to the South Pole. He didn't succeed, but it was quite a story. There's a close up of his hut and one of my colleagues waiting to go into it.
This is a glacier. We're looking at a glacier. It's coming down towards us. The front of it is breaking off and falling down, but you can imagine— you have to imagine, because we're down below— that it just heads up and up and up the side of Mount Erebus for almost 10 miles. It's a huge glacier.
And here's some people for scale. So, there's these people walking around at the base of it. And that's not even all of it. Just as much as there is above, there's probably three times as much as that below the ground, below what we're looking at, below where those people are standing. The scale of these things is so majestic, it's hard to imagine.
There's a close up of it. You can see all the cracks in the face, and where it's going to break off, and the next chunk is going to fall off towards us.
Here's another picture of a glacier flowing down around the curve and coming towards us.
And to give you a scale of that glacier, off in the middle of the screen, there are two huts. Those are actually fairly big houses. They'd be a house that you might see around town over here. A nice, single story Cape Cod or something like that. These aren't Cape Cods, but that's about the scale of it. So, there's a couple of huts over there, and they're just dwarfed by the edge of the glacier, which you can see to the right of them.
And here's one of these glaciers coming down, and all destroyed, and cracked, and broken, and crevassed. And you can't travel over it, which is why it's nice to have helicopters and airplanes.
And here's another shot of it from the air. And a close up of all the huge seracs and crevasses. And another shot.
So, we're going to end this slide show. Here's one helicopters that we use to fly around. And I think I might have a couple more pictures here. Mount Erebus again, and Castle Rock in the foreground.
Even I don't want to end the slide show here, but —and here's how we move around on the surface with these snowmobiles in the storm. This was a pretty bad storm. You can see the wind blowing across the surface. And this is what we live in. Some tents. That's the tent that we spend two or three months in Antarctica, sleeping in and doing our work from. Putting up a bigger tent. It's cold down there. You've got to wear your parka, pull up your hood.
So, we've had a little tour of some lovely places around the planet. And now, we're going to find out what it all means. Why are there glaciers? How do they flow? How do we know that they came and went? Why was Yosemite filled with two miles of snow a long time ago, and it isn't today? Those are the sorts of things that we're going to be talking about. What glaciers are. Erosion by glaciers. How a glacier makes a hole. How did it make Lake Superior? How did a glacier make the finger lakes? How did a glacier make the 10,000 Lakes of Minnesota? Those kinds of things.
And then ice ages. What evidence is there for them, and why do they happen? Why did it get cold, and why did it get warm?
A glacier is very simple. It's a mass of ice and snow that deforms and moves. By deformation, I mean it simply changes its shape. It goes from here to there. It flows across the surface of the landscape. That's all it is. And it does so under the force of gravity. Glaciers flow when snow falls on the surface of the land and it doesn't melt, so it accumulates. If it's cold enough that the snowfall doesn't all melt and disappear, as it does around here, year after year —every year it snows here, too, but we don't have any glaciers. And the reason we don't is all that snow disappears in the summertime.
But if you go to a place where it's cold year round, that snow remains. And then, there's more snow that falls on it the following year, and more snow that falls on it the following year. And eventually, if the ice gets thick enough, then you form a glacier.
Where is it cold? It's cold at the North Pole, and it's cold at the South Pole. Why? If you remember from last time, we talked about that. The sun is shining on the Earth. At the equator, all the sun's rays just slam straight into the equator. And so, all that energy goes right onto the equator. Up at the poles, that same amount of energy is smeared across a larger area because of the curvature of the Earth. And so, it's colder at the polls, warm at the equator. If it's cold at the poles, then that snowfall doesn't melt.
If you go up high mountains, we talked about that last time. As you go higher and higher up, it gets colder and colder. And you get to a point where it's so cold that the snow doesn't melt in the winter time. That's where we could form a glacier.
Or if it just snows so much and the summers are relatively short so we can't melt all that snow in the summer time, then you could form a glacier. For example, in the Olympic Mountains, they just get so much snowfall that even though it's a relatively warm place, these aren't huge mountains, you can still form a glacier.
You have glaciers at the equator. You have glaciers on Mount Kilimanjaro in Africa. And the reason for that is, they're high, and they get a fair amount of snowfall.
If it's cold but you don't get a lot of snowfall, then the ground just freezes and you get what's called permafrost.
Glaciers move and deform because of gravity. Gravity is one of the key forces in geology. I've talked to you about it over and over again, many, many times. We talked about it with mass movement last time. We talked about it with convection cells earlier on. And here it is. It's coming at us again. It's how a glacier moves and deforms.
When you make a pile of anything, the highest spot in that pile will put more weight at the bottom than the lowest spot, or a lower spot, on that pile. So, there's more force under the highest spot and less force under the lowest spot. And like everybody else, glaciers respond to forcing, and they want to move from where there's high force to where there is lower force. And so, the glacier simply moves off to the left.
I'm going to go to the drawing tablet now, and we're going to sort of summarize this part of it.
Here's the ocean. Everything always starts with the ocean. Here's land. And we're going to put some trees on land. And we're going to put some houses on here. And then, we're going to have it snow. We get water that evaporates from the ocean. It gets blown up onto land, and it starts to snow, and we'll show snowfall as these little X's, and down they come. It's snowing on the land.
And if it is cold enough, that snow will make a pile. And if it's cold enough, and the snow doesn't melt, it'll make a bigger pile. And a bigger pile. And a bigger pile, year after year. Until the weight of that pile gets large enough that the glacier wants to start— I'm going to remove these little snowfall marks, because it doesn't snow inside a glacier. It only snows on the outside. So, we'll get rid of some of these things.
So we've got a pile of snow and ice. And if we make that pile big enough, it will start to flow off to the sides. And the reason for it is that the force here is high. Why? Because it's got lots and lots of ice sitting on top of it. Lots of ice.
And the force here is low. Why? Not so much ice on top.
And so, the ice flows from where the force is high to where the force is low. It flows off to the side like this, and it flows off to the side like that.
This sketch has gotten a little messy. I'm going to start it all over again. We'll put the ocean in. We'll put the land in. And we'll put in our glacier. Now we've built this big pile of ice, and it's flowing off to the side. And when it gets to the edge, it will melt. And the water will simply run back into the ocean.
So why doesn't that pile disappear? It's flowing off the side. It's melting. It's going off. And the reason it doesn't disappear is it's still snowing on top. If you remove material off on the edge, if you add material up on top, and you flow from the one to the other, you can keep that glacier at its same size. If you get it just right, if you melt just the same amount as you add on top, and you flow that from the one spot to the other, and you do this continuously over and over again, that mass of ice isn't going to change its size. It's going to stay the same year after year.
This is known as the accumulation zone. That's where snow accumulates. Simple enough.
This is known as the oblation zone. Oblation is a fancy word for removal. To oblate is to remove. The oblation zone is where you remove ice and snow.
And then, the red line indicates flow from the one to the other.
We got to finish this picture out with one more thing, which is evaporation of ocean water followed by transport inland. And then that ocean water deposits as snowfall.
So do you see that cycle? You get evaporation in the ocean. It comes inland. It falls onto the accumulation zone of the glacier. It flows down to the oblation zone. It melts, and it goes back to the ocean. And if you get it just right, the glacier doesn't change its size and the ocean doesn't change its size.
And if the temperature is constant, then the system will come to equilibrium. It will come to a spot where you are removing just as much as you're taking in. And year after year, the glaciers will stay the same. If you change the temperature, if you start to warm things up or to cool them down, then that balance will shift. And that's what we'll talk about.
So this is how a glacier works. It flows from the accumulation zone to the oblation zone, and it does so under the force of gravity.
Glaciers always move from the high to low spot. The high on the surface of the glacier, what do I mean by that? Here we have that mass of ice and snow with the rock underneath it. The rock is relatively flat. The glacier will always flow from where it is high to where it is low.
Not the rock underneath. The rock underneath, you can see, is pretty flat. And the analogy is if you take a bowl or plate, and you start ladling molasses onto it, or you start to ladle pancake batter onto it. As you make a pile, and you keep ladling material into the middle of your plate, what happens? It isn't if your pile just goes straight up in the sky as you ladle onto it. Your molasses just doesn't build up and up and up. It flows off to the side. And which way does it flow? It flows away from where that pile of molasses is high to where that pile of molasses is low. That's all that glaciers do. They flow from where they are high to where they are low.
And in fact, they can even flow— I'm going to go to a new page. Now, we're going to have a glacier sitting in a bowl. The glacier will still flow in that direction even though the rock underneath slopes up. Doesn't matter. Even though that rock slopes up, the glacier will still flow in this direction because the top of the glacier is higher there.
Now, there's a limit to this. If you make the rock really, really steep, then at some point the glacier won't be able to climb that hill. But you have to get pretty steep. You have to get 10 times as steep as the slope at the top of the glacier for the glacier to change direction and head down.
So, this is flow of glaciers. And the balance, the hydrologic balance of evaporation and accumulation and oblation.
We're going to go back to the presentation, and then we'll come back to this drawing tablet in a minute.
Whoops.
Glaciers move from where the surface is high to where their surface is low. "Their" surface. Not the surface of the rock that they're riding over, but their surface themselves. They can even move uphill, like pancake batter flows up the sides of a bowl when you pour it into a bowl. The pancake doesn't all congeal right in the bottom of the bowl. It can actually lap up the sides of the bowl and eventually drip over the edges if you keep pouring more and more into it. The North American glaciers came up into Pennsylvania, and they flowed uphill into the mountains of Pennsylvania because of this process.
And here we have a cross section of a glacier. And you have flow of it from the left, in this case, where it says "Lake Ontario," to the right, where it says "Pennsylvania." Even though Pennsylvania's higher up than Lake Ontario— State College is higher than the bottom of Lake Ontario— when the ice filled it, the ice actually flowed, quote, "uphill." The rock heads uphill, but the glacier itself, the top of it, was sloped from Canada down towards the US. The glacier was higher in Canada than it was in the US, and so it flowed from the high spot of the glacier to the low spot.
When glaciers flow and deform, they don't just move as a mass. They don't just simply have this big fat thing, and the whole thing moves together. They actually deform and change their shape. And that's what's shown in the bottom graph, and I'm going to go back to the tablet and illustrate that.
We'll use that same picture that we had on the presentations. We have the glacier that looks like this. And the bottom of it looks something like that. This is Lake Ontario, and this is Pennsylvania.
If you were to drill a hole down through this glacier, a nice, straight hole— and people do that all the time. They do it to sample what's underneath. They do it to see how thick the glacier is. They do it to sample the ice itself. So, there's lots of reasons for doing it. You drill this hole in the ice, and you come back a year later or two years later, and you measure what that whole looks like. It won't be straight anymore.
If you come back in a year, that hole will look something like that. The top moved a lot. And by "lot," I mean 100 feet, 500 feet. Maybe as much as 1,000 feet after a year, maybe as much as 5,000 feet after a year. So glaciers don't move enormously fast. In a year, they flow anywhere from a few feet to a few thousand feet, but they don't move as fast as you or I could walk, for example.
So, the top has moved a lot. The bottom has moved a little. And there is this curve in between where different places inside of the glacier have moved different amounts.
So, the top is always moved the most. If I were to stand here on the top, I would always move forward, ahead of a marker at the bottom. But it isn't a straight line. It isn't a flat line. It's this sort of complicated curve. And this is a big area of research. People want to know what shape will that hole look like after a year or after 10 years, because it tells us a lot of a glacier flow. But this is a very simple thing, is you have deformation within the glacier that will change the shape of the glacier.
Now, in addition to that, you also have sliding. of the glacier over the base, over the rock. So, glaciers can slide as well. They might slide a little, they might slide a lot. It really depends on how much water there is down there.
And you can have water underneath glaciers. Even though glaciers are ice and snow, and they're very cold at the top, they can be very warm at the bottom because of the heat that's coming up from inside the Earth. And where is that heat coming from? You know, radioactive decay. Remember that. The inside of the Earth is hot. That heat is coming up. It's coming up all around us. If you look down, you'll see heat come up through your feet. You won't be able to measure it. There isn't a lot of it. But there's enough that the bottom of the glacier is warm.
So, you get sliding of the glacier over the base, and that's where the erosion goes on. As a glacier slides along, it rips up the rock, and it destroys the rock. It pulls it up and carries it away, and you have erosion of rock as the glacier slides along.
Let's go back to the PowerPoint.
Glaciers with lots of water at the bottom are good at eroding. If you get lots of water down there, then you get this sliding mechanism. Instead of simply having deformation within the ice, you get sliding at the bottom, and then the glacier can pull up bits of rock and carry them away. "Plucking" is when the glacier literally breaks loose small rocks. You have this big mass of ice. There'll be some small crack in the rock, and it will break off or pluck a piece of rock away and literally carry it off.
"Abrading" is when the glacier drags those small rocks. So, it's plucked one of these small rocks, and it's carrying it away. And as it's carrying it away, that rock acts like sandpaper. It actually scrapes away at the rock that's still underneath the glacier, and then will break off small bits of other rock that will also then be carried away and washed away because of all this water that's down there.
Any water flow under there really helps. If you've ever done any woodworking, you know that your sandpaper will get clogged after a while. You got to clean it off. This is the same thing. So, as you rub at this, if there's water underneath the glacier, that water will simply wash away all the loose material, and then you'll have this clean surface to rub some more. The water will wash away the loose material, rub some more, and the glacier will dig down and down and down, and deeper into the rock underneath.
Here's a picture illustrating that. Both you have plucking on the one side, abrasion on the other side. If you get one of these sort of sloped rocks underneath there, then you can have both of those processes going on. The ice is flowing over these, and it's plucking and abrading. And over time, it will carry that material away, and it will keep digging down deeper and deeper.
And here's an example from the Alps where the glacier would have flowed from right to left across there, and that face we're looking at is where all of the rocks were plucked away and gone. And the glacier, of course, is gone, but this is what the bottom of the glacier would have looked like when it was there. It's smooth on one side where the glacier came from. It's rough on the other side. It's known as a roche moutonnee or rock sheep, that looks a little like a sheep, sort of a rounded thing with this fuzzy edge on the side of it.
The abrasion can be seen. Anywhere there's been glaciers, you get these smooth, polished surfaces with these long, straight lines on them. And the long, straight lines always point in the direction that the glacier was heading. And those long, straight lines are simply from those rocks that it plucked. As they get dragged along the remaining rock, it just leaves these long striations and stripes on there.
The net result of doing all this work of abrading, and plucking, and going on and on, is to build mountains. When the glacier was there, it was just tearing away at the mountains, and it leaves these huge valleys, these deep valleys. And because glaciers are very wide and broad, the valleys that they leave behind are very wide and broad and rounded. And that's why a classic glaciated landscape, like the photographs we saw in the beginning of Yosemite, have this rounding to them.
As you go higher up the valley, and you get to the top, you get to where these glaciers have been chewing away at the side, and you get these very sharp ridges between them. And then if two of these glaciers come together or three of them come together, you get these very sharp-sided features that are diagnostic of glaciated landscapes. U-shaped valleys, hanging valleys, rounded bowls, and sharp ridges and sharp mountains. These are all the things that you'll see if you go up to Glacier National Park, for example.
Glaciers are really, really good at eroding. The finger lakes are there because the glaciers. The Great Lakes are there because of glaciers. The 10,000 Lakes of Minnesota are there because of glaciers.
Streams make a very different landscape. A stream can cut down really sharply where it is, but streams are generally not a mile wide. Streams are usually a few 10s of feet wide, or 100 feet wide, or something like that. You can go out, you can look at your stream that's out there. And they're really good at eroding, too. And they're really good at cutting down. But they're not good at cutting down across broad territories. They'll cut down right here, and then more rocks will fall in and they'll cut down, and more rocks will fall in. And so you get these V-shaped valleys when a stream has been there for a long time.
Any time you drive around here, and you look at the shape of these valleys, and they have these really sharp Vs to them, you know that a stream's been going through there. You go to Glacier National Park, you get these nice, rounded, broad U-shaped valleys.
And here's an example of one of those. Nice, beautiful, rounded valley, broad across the bottom. Now there's a stream running down the middle of it. And if you leave that situation alone, and you let that stream run in the middle of it for 50,000 years, that stream will cut down, and cut down, and it will turn it into a nice V. But because the glacier's only been gone for a few thousand years, it's still rounded and U-shaped.
Here's one of those lakes that's left behind from the glacier. And here's another fjord. This is an arm of the ocean that's come in. The glacier used to flow down through there. The glacier went away and the ocean came running back in.
More, just beautiful, alpine scenery and glaciers. Here's one of those really sharp features where the glacier's eaten away at the side of it and left these sharp ridges.
This is the Matterhorn. Very, very famous mountain in Europe on the French-Swiss border. And the reason it is three-sided like that— there's two that you can see and one more on the side, on the other side— is there were three glaciers on the three sides that were eating away at the side of it. And they kept eating away and making it steeper and steeper, and where those three came together, you get a horn. Beautiful place.
We're going to go back to the drawing tablet now and start to talk about ice ages.
We're back. We'll do this again. We'll make our glacier and land. Our ocean and land. And we'll put a big old pile of ice and snow. And if it gets big enough, we call it an ice sheet. It's no longer a glacier. It really acts like a glacier, but we call it something special, just because it's so big, if it covers a continent.
The size of these is enormous. 2,000 miles, 3,000 miles. All of Canada, all of Antarctica, all of Europe. Something like that. That's how big these ice sheets can be.
Let's make this ice sheet go away. Let's just melt it all. Let's just take a thermostat knob and just heat up the planet. Where is that water going to go? It's going to go to the ocean, right? And when it gets to the ocean, it's going to raise sea level. So, remove ice means you raise sea level. It's as simple as that.
And that has happened over and over again over the last million years. It's actually happened all through the history of this planet, but it has happened very regularly for the last million years that every 100,000 years, the ice grows, sea level drops, the ice disappears, sea level rises. It just happens again and again and again. And when the planet gets cold, there's enough ice that builds up that sea level can drop by more than 300 feet. So, there's a lot of ice that gets built up on Canada, Europe, Antarctica, Greenland, when it gets cold on this planet.
Why does it get cold? Why do we have these cycles? Why is it that every 100,000 years it gets cold, the ice grows, sea level drops, and then it gets warm, the ice shrinks, sea level rises? Back and forth, back and forth. So why ice ages? And really, what evidence for ice ages?
Let's look at the evidence first. We'll go back to the ocean. Ocean with big ice. So, we've got a big old ice sheet. All of North America's covered with ice. Europe is covered with ice. The Antarctic ice sheet is huge. Greenland is huge.
This is where the ocean would sit. It would be at some level. Sea level of the ocean. If I had built a house on the beach over there, I could look out and watch the ocean water lapping up against my house.
This is my ocean level with small ice. I make the ice sheets go away in North America. There's no ice in North America. There's no ice in Greenland. There's no ice in Europe. And sea level's going to rise.
What happens is that in this process of making big ice, you change the chemical composition of the ocean. So, let's take a look at that. I'm going to back up here a second, and we'll come back to this.
In the process of making big ice, you change the chemical composition of the ocean. And the critters that live in the ocean notice that. And when they die and their shells fall to the bottom of the ocean, their shells record that. So, all I have to do is go and find a shell from the time of big ice, and I'll be able to tell how much the ocean had dropped.
What happens is— I'm going to go to the next page over here— in the process of building big ice, we have to evaporate water. You remember that. You got to take water out of the ocean and dump it on these big ice sheets.
It turns out that there are two types of oxygen. Isotopes. Remember what an isotope was? Homer Simpson's favorite team, the Isotopes, the Springfield Topes. These are when you have the nucleus of an atom has a slightly different number of neutrons. It's still oxygen, because that's determined by the number protons. But if you have a different number neutrons, it acts the same way, it's still oxygen, but it just has a slightly different weight to it.
The lighter isotope evaporates more easily. So the ratio of light to heavy changes if we build, quote, "big ice."
So, we build this big old ice sheet. And the way we build it is by preferentially grabbing these light isotopes and dumping them on the ice sheet. So, we grab all of the light isotopes— not all of them, but more of them. Preferentially, grab the light isotopes and dump them onto Antarctica, and Greenland, and Canada. And so, you're left with more of the heavy ones in the ocean.
And the critters notice that. The critters notice that in their shells. Their shells are now built up with a different ratio of light to heavy isotopes, and we can measure that. We can find a shell from 30,000 years ago, pull it out, measure the ratio of light to heavy isotopes, and tell, oh, look. The only way this could have happened is if we pulled out lots and lots of the light isotopes from the ocean. We can take that same critter from today, pick up its shell, take it to the lab, measure its ratio, and know that, oh, there's lots and lots of light isotopes in the ocean today.
So, this is the evidence for it. But why?
Everything in this section of the class, tearing down mountains, is driven by the heat of the sun. Remember that. And glaciers very their size because of variations in sunlight. The amount of sun that hits the Earth changes. Not a lot, but enough over time that you end up with variations in sunlight. It gets cooler, it gets warmer.
I'm going to go back to the slides because there's a nicer picture of it, and I won't be able to sketch those variations as easily as the more professional illustration. We're just going to jump over these.
The heat of the sun drives all of this. The amount of sunlight varies according to three things. The shape of the orbit, and I'll show a picture in a second. You know how the Earth goes around the sun. And it does so in an elliptical orbit. And that ellipse gets more squashed, and more round, and more squashed, and more round every 100,000 years. It goes back and forth. Just wobbling back and forth, like that.
And as it does that wobble, as it does that going from more squashed to more rounded, the amount of sunlight that hits the Earth changes.
The amount of tilt to the Earth's axis. You all know the Earth's axis is tilted. That changes every 40,000 years. It goes back and forth, back and forth. And as it does that, the amount of sunlight that hits the Earth changes.
And finally, the direction of the Earth's axis slowly spins around like a top. And that changes about every 20,000 years. And as that changes, the amount of sunlight hitting the Earth changes.
And let's look at that picture. So on the left, diagram A is showing the orbit of the Earth spinning around the sun. And it's more squashed in blue and more rounded in the black, dashed line. And it goes between those two shapes every 100,000 years, slowly. It takes 100,000 years to do it, but it cycles back and forth, and back and forth
And we can measure this. Astronomers and physicists are really good at this, and they can measure the shape of the Earth's orbit, and they can tell us what it's doing. Similarly, on panel B there, on the right, we have the Earth's orbit tilted over by 23 and 1/2 degrees. And the amount of that tilt changes a little bit every 40,000 years. It goes from 23 and 1/2, down to 21, back to 23. And it takes about 40,000 years to do that.
And then finally, you have that precession, which is the direction of that axis moves around. Today, the Earth's access points at the North Star. You know that. You go out at night, and you can look up, and you can always tell which way is north because all you got to do is look for the Pole Star, right? You all remember how to find the Pole Star? Find the Big Dipper— that's that Big Dipper shaped set of stars— and then follow the end of the Dipper, and you'll go straight to the Pole Star.
And you always know that the Pole Star is where the axis is pointing at. Well, 10,000 years ago, if you had been standing around and you would try to see which way is north, you wouldn't have seen the Pole Star because the Earth's axis was going somewhere else.
So, those things happen over time. And as those three things happen, the amount of sunlight hitting the Earth changes. The sunlight in the Earth changes, the temperature changes, the glaciers grow, and the glaciers shrink.
Milutin Milankovitch predicted this in the 1920s. It's known as the Milankovitch hypothesis. He was a Serbian mathematician. Long before any of these isotopes data were available, he said this ought to happen. The amount of sunlight hitting the Earth is changing. He was an astronomer. He wasn't a geologist. But he calculated that the amount of sunlight hitting the Earth was changing, and he said, I'll bet you that that would have had effects on climate. And he was absolutely correct.
The amount of sunlight in the far northern hemisphere seems to control the ice ages. This is a huge topic of research right now. This is what everybody is really interested in in the climate community. Why? Because we as humans are changing the amount of CO2 in the atmosphere. We're changing the composition of the atmosphere. We're starting to warm the globe because of our activities.
And so it's no longer these solar orbital cycles that are controlling temperature. We as humans are starting to do that. And so we need to understand the natural cycles much, much better, so that we can pull out how much it is that humans are affecting it. And so, this is a huge area, topic of research. And we'll find out more about climate change as we go forward.
Changes in sunlight are relatively small, but have very big effects because of feedbacks. And you'll find out about feedbacks down the road a little bit.
So, I hope you've enjoyed your tour through some of these beautiful places on the planet. You've seen how glaciers are built. You've seen some of the evidence for glaciers changing their size and shape over time. And now, here is a good hypothesis for why those changes take place. It's still an active area of research, and maybe we'll change some of the details, but it seems to be a pretty tight one.
Thank you very much. We'll see you next time when we talk about coastlines, and sea shores, and how the ocean can help to tear down mountains.
Want another look?
Check out the Unit 7 Presentation used in the online lecture.
A Rocking Review
The most recent ice age may have ended, but there is still a lot of ice remaining in Antarctica and Greenland. Here’s a little more about the Antarctic ice, who lives around it, how it behaves, and why we might care. We’ll talk about the warming effect of rising CO2 in Unit 12; for now, just note that we are raising CO2 in the air, and it does have a warming influence, based on fundamental physics discovered in part by the Air Force for military applications.
DR. RICHARD ALLEY: Oh, snowflake out on a chilly night. Over the ocean of blue and white. Southern Cross as a guiding light and heading for South Pole-Oh, Pole-Oh, Pole-Oh. Southern Cross as a guiding light and heading for South Pole-Oh.
Over the albatross soaring through. Penguins plying the krill-flecked blue, seals and skuas and great whales, too, all playing around South Pole-Oh, Pole-Oh, Pole-Oh. Seals and skuas and great whales, too, all playing round South Pole-Oh.
One flake's a miracle, two a display. Three and you might wreck your car today, but make a two-mile pile and they're on their way flowing from South Pole-Oh, Pole-Oh, Pole-Oh. Two-mile pile and they're on their way flowing from South Pole-Oh.
The physics are simple, all piles spread, water, batter, or cats on a bed, or a continent-wide two-mile pile snow fed and heading from South Pole-Oh, Pole-Oh, Pole-Oh. A continent-wide two-mile pile snow fed and heading from South Pole-Oh.
Slow in the middle, thick and cold, down through the mountains carved and old, picking up speed in the ice streams bold, don't fall in a crevasse-oh, crevasse-oh, crevasse-oh, picking up speed in the ice streams bold, don't fall in a crevasse-oh.
Ice streams at the sea don't make bergs right away, they flow cross the ocean for many a day. As great ice shelves in a rock-bound bay with friction from sides-oh, sides-oh, the sides-oh. A great ice shelf in a rock-bound bay with friction from the sides-oh.
If we raise CO2, warm the air and sea, melt the shelves away and let the piles spread free, it'll raise the oceans towards you and me while it shrinks that two-mile pile-oh, pile-oh, pile-oh, raise the oceans towards you and me while it shrinks that two-mile pile-oh.
Arctic fox on a chilly night by another ocean of blue and white, if we melt their ice, would that be right? Both poles are on the same trail-oh, trail-oh, trail-oh. If we melt their ice, would that be right? Both poles are on the same trail-oh.
Oh, a snowflake out on a chilly night.
Optional Enrichment Article
Types of Glaciers
Many types of glaciers exist, with fairly loose or imprecise definitions. An ice sheet is a continent-scale mass of ice that spreads in all directions. An ice cap or ice dome is a smaller version of an ice sheet, sitting on a mountain top or high plateau, and also spreading in all directions (or at least in several directions). For pretty glaciers flowing down from the mountains, different people may use different terms: mountain glacier (it flows down a mountain), valley glacier (it flows down a valley on the side of a mountain), and plain old glacier. An outlet glacier drains an ice sheet or ice cap between rock walls, and an ice stream is a fast-moving “jet” of ice within an ice sheet or ice cap flowing between slower-moving regions of ice. But if an ice sheet is drained by a fast flow with ice on one side and rock on the other, is that fast flow in an ice stream or an outlet glacier? Classifications such as this help us talk about things, but are not precise.
Flowing Solids and Hot Ice
Dr. Alley has spent months of his life living on the great ice sheets of Greenland and Antarctica. (And Dr. Anandakrishnan, who has worked so hard on this course, has spent a lot more time on the ice sheets than Dr. Alley has!) Eating and sleeping and working at -30º, it is hard to think of ice as being a hot material, but that is exactly what it is.
Recall that heat is the vibration of atoms or molecules in a material, and that in most solids including ice, the atoms or molecules are arranged in regular, repeating patterns. Melting of ice occurs when the typical molecule vibrates fast and hard enough to break free from the bonds that tie it to its neighbors and escape from that regular arrangement. When a material is almost hot enough to melt, the atoms are vibrating almost hard enough to break free from their neighbors and move around, so it is relatively easy with a little extra push to move a few molecules at a time past their neighbors. The gravitational stresses caused by the surface slope of a glacier supply that little extra push, and the ice deforms (primarily by dislocation glide, for those of you with materials-science backgrounds). When a material is not nearly hot enough to melt, the molecules are not even close to vibrating hard enough to break free from their neighbors, a whole lot of extra push is required to move molecules, and moving even a few molecules at a time is very difficult. The material then deforms elastically, or it breaks, but it does not creep and deform permanently in the way that a glacier does.
Most people measure temperature on a scale that gives “nice” numbers (something between 0 and 100) for typical daytime temperatures, so that talking about the temperature is easy for us. But, there are other temperature scales that make more sense in physics. If you slow the vibrations of molecules by cooling them, you can imagine that there must be some temperature at which vibration stops because all the heat has been removed. We call that temperature “absolute zero” or just zero in an absolute temperature scale. (Yes, in a quantum world, the Heisenberg uncertainty principle means that the last tiny bit of vibration can’t really be removed, but absolute zero comes darn close, so live with it.) If we set the zero on our temperature scale to this “absolute zero,” and then use degrees that have the same size as in the commonly used Celsius or Centigrade scale, we get the Kelvin scale. Ice melts at 273ºK and water boils at 373ºK; there are 100 degrees between melting and boiling in Kelvin, just as in Celsius. (The Rankine scale uses Fahrenheit-sized degrees and absolute zero as zero, with ice melting at 460ºR and water boiling at 640ºR, but almost nobody uses Rankine any more, so you are welcome to forget you ever heard about it. Or, you can practice sniffing disdainfully.)
As a general rule, little or no permanent deformation (creep) occurs when the temperature (in Kelvin or Rankine!) is less than about half the melting temperature, and creep occurs rather easily when temperatures exceed about three-quarters of the melting temperature. The coldest mean-annual temperature on Earth today is about eight-tenths of the melting temperature of ice (that is 217ºK, which is also -56ºC or -69ºF, in case you still like old-fashioned thermometers). Most ice is as close to melting as is red-hot or even white-hot iron being worked by a blacksmith. This is why glaciers usually flow rather than breaking—although breaking is still possible where deformation is very fast and where the pressure is very low, producing crevasses. So, the next time you are tempted to “pull down your pants and slide on the ice,” remember that ice is a “hot” material, even if you may not look very hot when you’re through. (We recommend that you don't "pull down your pants and slide on the ice," for many reasons related to public decency, avoidance of frostbite, and not sliding over a cliff or falling into a lake.)
Glacier Erosion
As noted in the chapter, glaciers that are frozen to their beds don’t erode much, but if the basal ice is at the melting point, glaciers can erode very rapidly. Such thawed-bed glaciers have three ways to erode: plucking, abrasion, and subglacial streams.
Ice is an unusual material—higher pressure lowers its melting point rather than raising it (because ice becomes smaller when it melts; the tinker-toy-structure of ice has much open space, and squeezing ice tends to force it to become denser water, whereas most materials contract as they freeze so that higher pressure favors the solid). If a glacier is sliding across a bump in a bed, ice will tend to melt on the upglacier side of the bump where the pressure is higher. The meltwater will flow around the bump to the downglacier side, where the lower pressure will allow the water to refreeze. The heat given up by the refreezing will be conducted back through the bump, to allow more melting. But, you may remember that melting and freezing can open cracks in a rock. So, a glacier sliding over its bed can work rocks loose, and then freeze those rocks onto its base, in a process known as plucking. (When water spreads over the bed of a glacier in the spring as melting on the surface starts to feed water downward, the friction with rock that holds the ice back becomes concentrated on smaller regions of the bed not lubricated by the water, and this stress concentration breaks rocks, causing most plucking.)
Once glacier ice contains rocks at the bottom, it is like sandpaper—it drags those rocks over other rocks, scratching and polishing and knocking loose smaller rocks. This process is called abrasion. If you examine rocks on the walls of Yosemite, many still retain a polished appearance with parallel scratches or striations, showing where abrasion was active. Bumps are polished on one side—the upglacier side—but may be rough and jagged on the downglacier side where rocks were plucked off of them.
Melting of glaciers can produce a lot of water. The toe of a fast-melting glacier may supply more water to streams than does a similar-sized region in the rainiest place on Earth. The glacier acts to collect snowfall from a big area and take the snow to melt in a much smaller area, and trees and grass do not grow on glaciers to use the melt but they do grow on ground to use rainfall. Glacier melt usually flows down holes in the glacier, called moulins, that often form at the bottoms of crevasses. (Some brave or foolhardy people like to go caving in moulins after they drain during the winter.) The moulins eventually reach the glacier bed, where they feed large, steep, fast-moving streams. These erode in the same ways as streams outside of glaciers. Glaciers with much melt water usually cause erosion to be faster than in nonglaciated regions. Fluctuations in water pressure, as moulins fill with water during daytime melting and drain as melting slows at night, contribute to cracking rocks for plucking.
When Is the Next Ice Age?
In the text, we noted that the history of ice ages generally has involved 90,000 years of cooling, followed by 10,000 years of warming, then repeat. The rate of cooling initially is slow, and some people prefer to refer to 10,000 years of warmth followed by cooling. The northern hemisphere has been in the not-much-change/slight-cooling phase for almost 10,000 years already, and you might expect that we are ready to drop into the next ice age. Some people have suggested that humans have already headed off that ice age, or that global warming is a good thing because it will head off the ice age.
The 100,000-year pacing of a 90,000-year/10,000-year world is linked to interaction of the different orbital cycles, but the 100,000-year cycle in the out-of-roundness of the orbit is important. The orbit goes from nearly round to more squashed and back in about 100,000 years. But, there exists a slower modulation that takes about 400,000 years. The orbit goes nearly round, a little squashed, nearly round, more squashed, nearly round, even more squashed, nearly round, not as squashed, nearly round, barely squashed, repeat, with the nearly-rounds spaced 100,000 years apart. We are in the barely-squashed part now, and the last time that the orbit was in the barely-squashed mode, the warm time of the ice-age cycle lasted 30,000 years rather than 10,000 years. Climate models have confirmed that this should be our natural future; another 20,000 years of warmth (or maybe 40,000 years) before the next ice age starts. However, human burning of fossil fuels may extend the warmth beyond the next 20,000-40,000 years.
Also, note that the 19,000-year cycle noted in the text is an oversimplification. There is instead a “quasi” periodicity ranging from 19,000 to 23,000 years, as we mentioned briefly, and this was calculated by Milankovitch and is observed in the data collected to test Milankovitch's calculations, beautifully confirming his predictions.
Central Pennsylvania and Glaciation
During at least one old glaciation (probably over 1 million years ago), ice dammed the West Branch of the Susquehanna River and formed a lake in the Lock Haven area of Pennsylvania. If that lake filled to the next lowest bedrock outlet (into the Juniata River along the Bald Eagle Valley at Dix), then the water would have lapped at the steps of Old Main on Penn State’s University Park campus. There is no evidence of such a large lake, and before the lake filled all the way, it probably drained through failure of the ice dam, but we’re not sure. With ice so close, however, the State College area was cold during the ice ages.
Isotopic Ratios of Dead-Bug Shells
In the main text, you learned how the changes in ice volume control the isotopic composition of water in the ocean, and how we can reconstruct the ice-age cycle from the history of shell isotopic compositions in a sediment core because the shells record the water isotopic composition. As usual, things are a bit more complicated than that. Shell isotopic composition also is affected by temperature. Bigger ice gives heavier isotopic ratios in shells, and colder temperatures also give heavier isotopic ratios in shells. (At high temperature, both heavy and light atoms have plenty of energy to get up and go; at low temperatures, the heavy ones tend to get stuck in shells while the light ones can jump out.) Because both colder and bigger ice favor isotopically heavier shells, it is hard to tell how much of the signal in a shell is from temperature or from ice volume.
One way around this is to go to a place that is really cold today; the water was above freezing during the ice age (there were shells living in it…), so there the signal must be primarily one of ice volume. Other approaches include finding additional paleo-thermometers, such as estimating the temperature from the species living in a place and leaving their shells, or using changes in other “contaminant” ratios in shells that depend on temperature. Yet another way is that there is water in spaces in mud, and the water in some sediments is from the ice age, so just measure the isotopic composition of that water.
The result of this is that isotopic ratios did change because there was much more ice during the ice age than today, and because most places were much colder during the ice age than today.
Wrap Up
Review the Unit 7 Introduction
You have reached the end of Unit 7! Double-check the list of requirements on the Unit 7 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.
Unit 7 Overview
Review of the main topics and ideas you encountered in Unit 7.
Ice Is Nice: Yosemite, Glacier, Rocky Mountain, Bear Meadows, and NE Greenland
- Glacier = pile of ice and snow that flows
- Forms if snow exceeds melt enough to make a pile
- Takes water (as ice) and sediment from accumulation zone (snow exceeds melt) to ablation zone (melt exceeds snow) or to calve icebergs
- Flows in downhill direction of the upper surface (where ice meets air), even if that means the bottom flows uphill
- Think of pancake batter flowing on a waffle iron
Slip Sliding Away
- Glacier moves by deformation within ice, and if bed warmed to freezing point, by sliding over substrate or deforming sediment there
- Most deformation deep, but top fastest because rides along on deeper layers
- Ice deforms because almost hot enough to melt
- Glaciers erode by plucking rocks loose, sand-papering bed, and by subglacial streams
- Thawed-bed glaciers, especially those with surface meltwater reaching the bed, change landscape more rapidly than streams, etc.
Ages of Ice
- Recent (about 20,000-year-old), unique glacier tracks across broad areas now far from ice suggest past ice age(s)
- Ice-age hypothesis predicts land rising where ice was, sinking around, and that is indeed observed
- Ice-age hypothesis predicts sea level was lower when ice big, and indeed observe dead shallow-water corals of that age in growth position deep, flooded river valleys, etc.
Ice-Age Records
- Isotopically lighter water evaporates more easily
- Bigger ice-->isotopically heavier ocean and shells
- Shell-isotopic history from ocean-mud cores shows biggest ice every 100,000 years, smaller wiggles about 41,000 and 19,000 years apart
- Predicted by Milankovitch before observed--these are wiggle-spacings in Earth’s orbit
- Ice grows globally when little northern sunshine
- Orbitally changing sun controls northern ice, which affects CO2, which controls southern ice
Bear Meadows
- Ice sheets today cover about 10% of land area; at height of ice age covered about 30% of modern land; central PA just beyond edge of Canadian ice
- Rocky Mountain, coastal NE Greenland National Parks have permafrost--soil at some depth frozen year-round
- Permafrost freeze-thaw and enhanced creep (summer melt can’t drain down, so soil soggy and creep easy) make distinctive features
- Those features exist but are not forming in central PA
- So, we were really cold in the ice age
Glacier Tracks
- Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock
- Smooths upglacier, plucks downglacier sides of bumps
- Glaciers make valleys with “U”-shaped cross-sections, often with side-valley floors hanging above main-valley floor; streams make “V” shape without hanging valleys
- Glaciers gnaw bowls called cirques into mountains
- Glaciers deposit all-different-size-pieces till and washed-by-meltwater outwash, often in outlining ridges called moraines
Reminder - Exercise #3 is due and Exercise #4 opens this week. See Course Calendar for specific dates and times.
Supplemental Materials
Following are some supplementary materials for Unit 7. While you are not required to review these, you may find them interesting and possibly even helpful in preparing for the quiz!
- Website: Glacier National Park
- Website: Yosemite National Park
Comments or Questions?
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