Welcome to Module 4!

Mountain Building, Obduction, & Tsunamis

Introducing Obduction

The folded Appalachian Mountains of southern Pennsylvania and adjacent Maryland. A collision between the “Old World” (Europe and Africa) and North America squeezed the rocks in a process called obduction, making folds in the rocks that are something like rumples in a carpet, and pushing the rocks up to form the Appalachians.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

"No matter how sophisticated you may be, a large granite mountain cannot be denied—it speaks in silence to the very core of your being."
—Ansel Adams, The Spirit of the Mountains

Video: Introduction to Module 4 (0:00)

The Great Smokies are geologically attached to the whole Appalachian mountain range, including the ridges near Penn State’s University Park campus. There, if you’re so inclined, you can visit the beach in the mountains—all thanks to Africa.

We saw in Module 3 that an old, cold sea floor is subducted beneath the warmer sea floor or a continent, but what happens when a high-floating continent or island arc tries to go down beneath another continent or island arc?

In this module, we’ll see that the answer is  obduction, a BIG collision. The Great Smoky Mountains, Mt. Nittany near Penn State's University Park campus, and all the rest of the Appalachians were formed by just such a collision, between North America on one side, and Africa and Europe on the other side. Folding and thrust-faulting in the collision zone thickened and shortened the crust and upper mantle. This produced high mountains—probably about as tall as the Andes today. And, as we will discuss, high mountain peaks float on deep roots. When erosion lowers a mountain range, the root floats up, bringing metamorphic rocks to the surface that have been "cooked" by heat and pressure deep within the Earth.

Before we go any further, look at the following short video introduction by Dr. Anandakrishnan.

Video: Duck Mountain (2:23)

And....A Word About Tsunamis

Plate tectonics causes earthquakes, volcanic explosions, and steep slopes that can experience landslides. If any of these happen under the ocean or in a deep lake, or if a landslide falls into an ocean or lake, a lot of water can be moved in a hurry. The resulting great waves are called tsunamis and can have catastrophic consequences. Fortunately, warning systems can be devised to reduce the loss of life, and we can use our knowledge to build in ways that increase safety for people and property. We'll look at some of these issues as we wrap up our multi-week exploration of Plate Tectonics and Mountain Building.

Learning Objectives

• Identify the push-together faults and folds formed in obduction zones when continents or island arcs collide.
• Explain the three basic tectonic styles: pull-apart, push-together, and slide-past.
• Explain how obduction zones can produce thickening of the crust that leads to the formation of metamorphic rocks.
• Explain how erosion can reveal metamorphic rocks formed in obduction zones.
• Explain how tsunamis are caused, and why they are dangerous.

What to do for Module 4?

You will have one week to complete Module 4. See the course calendar in Canvas for specific due dates.

• Take the RockOn #4 Quiz.
• Take the StudentsSpeak #5 Survey.
• Continue working on Exercise #2: Geology is All Around You.

Questions?

If you have any questions, please feel free to send an email through Canvas. Remember to include all of the teachers and all of the TAs in the "To" line. Failure to email all teachers and all TAs may result in a delayed or missed response. Directions for how to send an email can be found in the Resources module.

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Main Topics: Module 4

Overview of the main topics you will encounter in Module 4.

Plate Tectonics III: Obduction

• In subduction, the denser side sinks under the less dense side.
• But the density of continents and island arcs is too low to allow them to go down very far — ”You can’t sink a continent”, as geologists like to say.
• Collisions involving continents and island arcs cause folding, push-together (thrust) faulting, and thickening of the crust and upper mantle.
• We call this OBDUCTION.
• Obduction generally makes the biggest mountain ranges, such as the modern Himalayas, and the Appalachians, which were much higher when they formed 200 million years ago than they are now.
• This push-together process sometimes pushes older rocks on top of younger ones.

A Little History—The closing of the proto-Atlantic Ocean and opening of the Atlantic

• Long ago, the Americas were separated from Europe and Africa by the proto-Atlantic Ocean (sometimes called the Iapetus Ocean).
• The proto-Atlantic Ocean shrank before disappearing, as subduction made large explosive volcanoes in island arcs that sometimes collided with one of the continents.
• The proto-Atlantic Ocean disappeared entirely in the great obduction that pushed up the Appalachians, including the Great Smoky Mountains and Mount Nittany near Penn State’s University Park campus.
• When the push-together motion ended, the huge pile of rocks that made the Appalachians was hot and soft down deep and began to spread under its own weight, with Death-Valley-type faulting and thinning of the crust and upper mantle.
• This thinning reduced pressure on the mantle beneath, which triggered convection there as hot rocks rose and melted, and thus led to the slow and still-occurring opening of the modern Atlantic Ocean.

The Three Basic Tectonic Styles

• PUSH-TOGETHER: subduction (Olympic, Crater Lake, Mt. St. Helens) or obduction (Great Smokies)
• PULL-APART: rifting/spreading/sea-floor-production (Death Valley)
• SLIDE-PAST: faulting (San Andreas)
• Intermediates can occur, such as a little push-together while sliding past, or a little pull-apart while sliding past.
• The three types of plate boundaries, plus hot-spot activity poking up through plates, create the great majority of mountain-building, earthquakes, volcanoes, etc.

Meanwhile, Out West:

• As the Atlantic Ocean opens, Asia and the Americas are approaching each other, narrowing the Pacific.
• Subduction under the western US started with cold rock, but as the continent moved toward the Pacific spreading ridge, more buoyant ("float-ier") rock was forced down, scraped along under the US rather than sinking deep, and rumpled up the lithosphere to make the Rockies, etc., far inland.
• Where the subduction zone has reached and swallowed the spreading ridge in the Pacific, rock is no longer going down under the western US; the subduction zone was push-together plus slide-past, and the slide-past remains as the San Andreas Fault.
• Where and when the push-together of the subduction ended, the pile of the western US spread under its weight, giving Death Valley-type faulting at Death Valley and across much of Nevada and nearby.
• (This may seem complex, but the full story is likely even more complex than this, and not all of it is fully known. This is close, though.)

Old Mountains and Metamorphism

• The upper layers of the Earth float on lower layers.
• When the collision of an obduction event thickens the upper, crustal rocks, the mountains sticking up into the air float on a root sticking down into the mantle (like an iceberg, but icebergs have about 1/10 sticking up and 9/10 sticking down, whereas mountains are closer to 1/7 up and 6/7 down).
• If you cut or melt off the top of an iceberg, the bottom bobs up; if you erode off the top of mountains, the bottom bobs up.
• Bobbing up of eroding mountains brings rocks to the surface that had been buried deeply, where they were heated and squeezed.
• Heating and squeezing turn sedimentary rocks (pieces of older rocks) or igneous rocks (frozen from melted rock) into metamorphic rocks, which often are pretty and may contain valuable ores or gems.

Tsunamis

• Undersea earthquakes, volcanoes, landslides, or meteorite impacts can move lots of water.
• Such water motion makes a wave (a tsunami) that is long and low in the ocean, but the wavefront slows down as it enters shallow water, and the back catches up and piles up.
• Most tsunamis are tiny, but the biggest ones can run up on land to elevations above 1000 feet; the 2004 Indian Ocean tsunami killed over 300,000 people, and the tsunami from the Tohoku, Japan earthquake in 2011 did much of the damage in what was probably the most expensive natural disaster ever.
• We know of no way to stop tsunamis, but we can give real-time warnings (earthquakes, etc., make seismic waves that go faster than tsunamis; we can “listen” for those seismic waves with seismometers, then warn people to go inland fast).
• We can enact and enforce zoning codes to build only in safe places, and keep reefs and barrier islands healthy to break some of the energy of tsunamis.

Still More Plate Tectonics, The Great Smoky Mountains

The Smokies--and State College?

Left: Scenic view of the folded Appalachian Mountains in Great Smoky Mountain National Park, North Carolina, and Tennessee. Right: Location of the Great Smoky Mountains in both North Carolina and Tennessee.

Credit: R.B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

As usual, we start with a little background on our featured National Park, this time The Great Smoky Mountain National Park of North Carolina and Tennessee. Then, we get into the material that might be on the RockOn Quiz, starting with the first virtual tour below and then discussing obduction zones. The Great Smokies include 16 mountains over 6,000 feet (about 2,000 m) high, making this generally the highest region in North America east of the Mississippi River. The tourist town of Gatlinburg is a mile (1.6 km) lower than Mt. Le Conte, a difference almost as large as in many of the great mountain parks of the west, where the peaks are higher but so are the valleys. The Smokies were preserved in a park in 1926, with much of the funding for land purchases provided by J.D. Rockefeller. The Great Smokies today are the most-visited National Park because they combine spectacular scenery, rich biological and historical diversity, proximity to major population centers, the lure of a quick stop-off on the drive from the northeast USA to Florida, and a shortage of other nearby national parks to draw off the crowds. (Although we should not forget Shenandoah National Park, connected to the Smokies by the Blue Ridge Parkway, another beautiful park.)

In case you’re interested (and no, you do not need to memorize these!), the National Park Service keeps track of visitation, and you can easily find the numbers by searching online. As this text was being written, the Blue Ridge Parkway was the most-visited “park” managed by the National Park Service, but if we restrict attention to the actual National Parks, the numbers of visitors for the most popular parks are outlined in the table below.

Much interest in the Smokies centers on its historical aspects. For example, how did the early European settlers survive and flourish in this region? At Cades Cove, wonderful relics of a bygone lifestyle are maintained in a living museum. Many visitors are also seeking to learn about the earlier Native Americans. Biologically, the Smokies host an amazing array of tree species, flowering bushes (azaleas, rhododendrons, and mountain laurels, in particular), wildflowers (including many orchids), and more. Approximately one-third of the park is covered with "virgin" timber that escaped being cut by European settlers in the high, remote landscape, and the regions that were logged are growing back rapidly with impressive stands of diverse trees.

Abundant rainfall and snowfall “scraped” from the sky by the high peaks feed numerous cascades and waterfalls, with trout in the pools and kingfishers by their banks. Rainfall is roughly 50 inches (1.3 m) per year in the valleys and more than 85 inches (2 m) per year on the peaks, so the Smokies share some characteristics with temperate rainforests of the west such as in Olympic. Especially during “off-peak” times when fewer travelers are present, you can get lost in the Smokies, and imagine what the Appalachians must have looked like without humans; approximately 3/4 of the park is wilderness. And, because so many of the visitors are traveling by car and heading somewhere else, even a short walk down a trail can get you away from almost everyone.

To see a little of the park, and to get started on the key ideas that will be on the RockOn Quiz, join Dr. Alley and his team as they take you on a "virtual tour" of Great Smoky Mountain National Park that illustrates some of the key ideas and concepts being covered in Module 4.

Virtual Field Trip: Great Smoky Mountains

Join us as we go on a virtual tour of the. Great Smoky Mountains.

Obduction Zones: The Push-Together Boundaries

The Smokies are a small part of the great Appalachian Mountain chain, which extends along the coast of North America from Newfoundland through the Smokies, and then bends westward into Oklahoma. In the Great Smokies, the mountains display a truly remarkable feature—in some places, older rocks sit on top of younger rocks! The very high peaks are composed of hard, resistant, old metamorphic rocks (which we will explain soon), of the sort that one finds deep in a mountain range. Beneath them are younger, sedimentary rocks that were deposited in shallow seaways. Between these is a surface called a thrust fault or push-together fault. Thrust faults often show scratches that form when the rocks on one side of the fault slide past the rocks on the other side. Thrust faulting has been observed during earthquakes in some cases elsewhere in the world where push-together deformation is still active. In the Smokies, the older rocks have been shoved as much as 70 miles (110 km) to reach their present position on top of the younger rocks. The pictures below the video show two very much smaller thrust faults, with the upper rocks shoved up to the left only a few inches, but the idea is the same.

Video: Cades Cove (4:21)

You may recall that we started with pull-apart faults at Death Valley. As shown in the video above, thrust faults are of the push-together type. Squeezing from the sides caused one set of rocks to be pushed over another set. Each set is right-side up, but where they meet, the older rocks are on top of the younger ones. This is seen clearly in the Great Smokies.

Farther north, near Penn State’s University Park, Pennsylvania campus, where Drs. Alley and Anandakrishnan teach and where Dr. Alley wrote most of this material, we see a different way that rocks can respond to push-together stresses. There, in addition to some push-together thrust faults, many folds occur. Take a piece of paper, lay it on your desk, and squeeze the opposite sides towards the center. The paper will buckle into a fold. You may achieve the same effect by trying to push a carpet along the floor. Clearly, there are push-together forces involved here.

Watch this 1:38-minute video about the push-together thrust faults found in Capitol Reef National Park.

Video: Monocline Explained: Capitol Reef National Park (1:38)

Thrust faults

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Thrust faults

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

These photos show two small thrust faults, with one bed of sandstone thrust a few inches over another in each fault, and Dr. Alley’s index finger for scale, in a cliff below the Glen Canyon Dam in Arizona. The same principles apply to thrust faults, whether they are tiny or huge —push-together forces shove some rocks over other rocks along a break in the rocks. The rocks were squeezed from left and right, and the yellow arrows show the motion along the faults, which are indicated in white. The turquoise line segments were connected end-to-end as one straight, horizontal line before the rocks moved along the faults.

Virtual Field Trip: Blue Ridge Mountains

To see more on obduction zones, look at the Blue Ridge Mountains VTRIP. It has some pretty pictures, and then a little geologic background. There is another thrust fault, shown by yellow arrows, in the second-to-last picture.

We saw that pull-apart forces occur at spreading ridges. And, if rocks can be pulled apart, they can be pushed together, with push-together forces at subduction zones, or other collision zones. Today, you can look from the Appalachians and from the east coast of South America across the quiet seafloor of the Atlantic, across the spreading center of the mid-Atlantic ridge, to the coastlines of Africa and Europe. The coastlines on either side of the Atlantic are nearly parallel to each other and the mid-Atlantic ridge—slide the new and old worlds back together again, and they fit like a jigsaw puzzle. You can put all the modern continents back together jigsaw puzzle style. This fact, and especially the wonderful fit across the Atlantic, has figured prominently in suggesting the idea of drifting continents to scientists and other observers almost since the first decent maps were available of the Atlantic coasts. More importantly, putting the continent shapes back together in jigsaw-puzzle style puts the “picture”—the geology—back together, as well, for events that happened while the continents were joined. For example, we will see in Module 7 that, if glaciers flow over rocks and then melt, scratches are left that show which way the glacier was flowing. These tracks from a long-gone glacier run out into the Atlantic from Africa, and then out of the Atlantic onto South America. But if you put the continents back together jigsaw-puzzle style, the tracks fit together to show the path of a single glacier from a time when the Atlantic Ocean did not exist. Many other such matches are seen, from the times after the proto-Atlantic Ocean closed and before the modern Atlantic Ocean opened.

The oldest rocks on the Atlantic seafloor are about 150 million years old, approximately the same age as sediments that were deposited in a Death-Valley-type setting in the Newark Basin of New Jersey and elsewhere along the U.S. East Coast. The modern situation of a spreading Atlantic began about then, splitting apart a supercontinent to form the Atlantic Ocean in the same way that Baja California is being split off to open the Gulf of California.

Photo of a road cut along State Route 322 just east of Penn State’s University Park campus. As the proto-Atlantic ocean closed, subduction-zone volcanoes formed. Ash from these volcanoes can be found in the rocks exposed in the road cut.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

But the Appalachian Mountains are much older than that. A story begins to emerge of a cycle—older push-together forces led to the closing of a proto-Atlantic Ocean that produced the Appalachians. When the proto-Atlantic was closing, subduction-zone volcanoes formed and spread ash layers across the land, much as Crater Lake/Mt. Mazama and Mt. St. Helens did more recently. (Some of those ash layers can be found in many places, including the road cut along the Route 322 expressway just south of East College Avenue in the State College, PA area). Sometimes, the proto-Atlantic subduction zones formed offshore, formed volcanic island arcs, and then their volcanoes moved and collided with the North American continent.

When the pushing stopped, the giant pile of the Appalachians, with deep, hot rocks beneath, began to fall apart in Death Valley style, and red sandstones and mudstones accumulated in Death Valley-style valleys along parts of the U.S. East Coast. Some of these have interesting fossils, such as the dinosaur tracks at Dinosaur State Park in Connecticut; the real tracks are shown in the picture with an artist’s rendition of how the landscape might have appeared. The drop in pressure deep in the earth as the Appalachians fell apart probably caused a convection cell in the deep mantle to rise right there, eventually forming the mid-Atlantic Ridge and the Atlantic Ocean.

Eubrontes [or Bipedal theropod] fossilized trackway and diorama at Dinosaur State Park’s exhibit center, Connecticut’s first NNL.

Credit: National Park Service (NPS) (Public Domain)

Such a cycle, with subduction or continental collisions building mountain ranges that then spread Death-Valley style and eventually split to make ocean basins, has been played out many times over the history of the Earth. And the fun isn’t over yet; North and South America are cruising westward toward Australia and Asia, as the Atlantic widens and the Pacific narrows. Africa is still bumping into Europe and pushing up the Alps, and India has not yet grown weary of ramming Asia to raise the Himalayas. At fingernail-growth speed, the next 100 million years or so should lead to a lot of geological high drama, but the next 100 years won’t see a whole lot of change. The 1:15 minute silent animation below provides a good visualization. (The animation would be more accurate if it made the crust a little thicker in the collision at the end, but otherwise, it is quite good. The animation shows that after the ocean spread for a while, the sea floor near the continent became cold enough and thus dense enough to start a new subduction zone. That might happen on one or both sides of the Atlantic in the future.

Video: Geology: Wilson Cycle (1:15) This video is not narrated.

The key to most of this is that you can sink old, cold sea floor, but you can’t sink a continent. Island arcs and continents float on the mantle too well. So rather than going down the subduction zone with the oceanic lithosphere, an island arc or continent will ride across the subduction zone for a major collision. In such a collision, called obduction, layers of rock are bent into folds such as those near Penn State’s University Park campus, or broken into thrust faults such as those under the Blue Ridge and the Great Smokies. In the case of the Appalachians, the thrust faulting was very efficient, with older rocks sliding tens or hundreds of miles (or kilometers) over younger ones in some places.

Alfred Wegener realized more than a century ago that putting the continents together jigsaw puzzle style also put together the “picture” of the geology on the continents from certain times.  Wegener used evidence such as is sketched in this USGS map.  For example, fossils of the Triassic (between ~201 and 252 million years ago) land reptile Lystrosaurus are found in India, Antarctica, and Africa (shown by the brown band), seemingly impossible for creatures that could not have swum across today’s ocean to Antarctica.  But, with the continents put together as shown in the figure, the Lystrosaurus fossils and the others shown make perfect sense, as do many other lines of geologic evidence.

Credit: USGS (Public Domain)

To see drifting continents in the past and the future, see the video 240 million years ago to 250 million years in the future. 

The Three Structural Styles

You have now seen, at least briefly, the three structural styles that are possible:  pull-apart  (Death Valley, spreading ridges);  push-together  (Crater Lake and Mt. St. Helens subduction, University Park and the Great Smokies obduction); and  slide-past  (the San Andreas Fault in California). Pull-apart behavior involves stretching of rocks until they break, forming pull-apart or gravity faults (after being pulled apart, gravity pulls one block down past the other). Pull-apart action occurs at the spreading centers, probably where the convection cells deeper in the mantle spread apart. Push-together behavior occurs at subduction and obduction zones, and produces squeeze-together folds and faults, with the faults also known as thrust faults. Slide-past boundaries, also called transform faults, occur where two large blocks of rock move past each other but not toward or away from each other. Slide-past motion produces earthquakes without mountain ranges.

Video: San Andreas Fault (3:16)

Now, you might imagine that we have oversimplified just a little. There is no law that rocks must move directly toward each other (push-together), exactly parallel to each other (slide-past), or directly away from each other (pull-apart); sometimes you see an oblique motion with rocks approaching on a diagonal. Or the rocks may pull apart on a diagonal. And, a bend in a slide-past boundary may produce pull-apart or push-together features, depending on which way the bend goes relative to the motion, as shown in the video above. A large bend in the San Andreas Fault just north of Los Angeles gives push-together motion, with some impressive mountain ranges and dangerous earthquakes.

Mountain ranges correspond directly to the main boundary types. Fault-block mountains—the Sierra Nevada, the Wasatch Range, the flanks of the great rift valleys of Africa, and the mid-oceanic spreading ridges—form at pull-apart margins. The mountains are high because the rocks beneath them, in the mantle, have expanded vertically because they are the hot upwelling limbs of convection cells. Volcanic-arc mountain ranges form over subducting slabs, where some of the downgoing material melts and is erupted to form stratovolcanoes; smaller ranges (such as the Coast Ranges of the Pacific Northwest, including Olympic National Park) may form from the sediments scraped off the downgoing slab just above the trench. Continent-continent or continent/island-arc obduction collisions occur at push-together convergent boundaries as well, producing folded and thrust-faulted mountain ranges.

Why the Great Smokies Are Still So High

Remember that crustal rocks are the low-density “scum” that floats on the denser mantle. When obduction occurs, this crustal scum is crunched—it goes from long and thin to short and thick, in the same way, that the front end of a car is changed when it runs into a brick wall, or a carpet is changed if you shove the ends together and rumple it in the middle. Then, much like an iceberg floating in the water, a mountain range is a thick block of crust floating in the mantle, with most of the thickness of the mountain range projecting down and only a little bit sticking up.

Notice something else fascinating; when a mountain range is being eroded, the top is taken off, and rocks below bob up almost as high as before. Erosion continues to remove those almost-as-high rocks, allowing more rocks from below to rise. Pretty soon, the rocks at the surface have come from far down in the Earth, where temperatures and pressures are high. And as you might imagine, those rocks were changed by the high temperatures and pressures. The rocks around Penn State’s University Park campus have not been “pressure-cooked” much, but the rocks around Philadelphia have been - they tell the story of a great mountain range that fell apart, leaving the remnant that we know as the Appalachians. The rocks in Rocky Mountain National Park are like those in Philadelphia, in the sense that they once were deep in the Earth and now are at the surface. This is similar to how icebergs work. See the animation below about icebergs to learn more about how isostasy works.

With an iceberg, about 9/10 of the thickness is below the water and 1/10 above. As shown in the narrated diagram below, if you could instantly cut off the 1/10 that is above water, the iceberg would bob up to almost as high as before. A 100-foot-high berg would have 10 feet above the water and 90 feet below. Cut off the top 10 feet, and it is a 90-foot berg with 9 feet, or 1/10, above the water and 81 feet below. So, removing 10 feet from the top shortens the ice above the water by 1 foot and the ice below the water by 9 feet. With mountain ranges, the density contrast between crust and mantle is larger than that between ice and water—only about 6/7 of a mountain range projects down to form the root, and 1/7 projects up to form the range. 

Video: Icebergs (3:02)

Still, if rivers or glaciers erode a mountain range (something we’ll study in modules 5, 6, and 7), some of the root is freed to float upward. Only by eroding the equivalent of 7 mountain ranges can you eliminate the mountain range entirely. So, the Appalachians, despite having been deeply eroded, are still high because they still have a root.

The idea that things on the surface of the Earth float in softer, denser material below is called  isostasy, which means “equal standing”—each column of rocks on Earth has the same weight or standing. Lower-density columns then must be thicker to weigh as much as thinner, higher-density columns. The continents stand above the oceans because the silica-rich continental crust is lower in density than the silica-poor sea-floor crust. The mountain ranges stand above the plains because the thick, low-density roots of the mountains have displaced some of the high-density mantle that is found beneath the plains, or because the rocks beneath the mountains are especially hot and so low in density. Look back at the animation about icebergs to learn more about how isostasy works.

Put a big weight on a piece of crust (say, an ice sheet, or the Mississippi Delta, or a mountain range) and that piece of crust sinks, pushing up material around it in the same way that the surface of a waterbed sinks beneath your posterior when you sit down, while the surface is pushed up around you by the water that is shoved sideways. The rising and sinking of the land are slower than for a waterbed—thousands of years rather than seconds—because the hot, soft, deep mantle flows a lot slower than water does. But for a mountain range over 100 million years old, a few thousand years doesn’t mean much.

Still More Plate Tectonics: The Rocky Mountains

Left: Map showing the location of the Rockies. Right: Bierstadt Lake in Rocky Mountain National Park, Colorado.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Mountain Building and Metamorphism

Rising high above Estes Park, Colorado, and almost within shouting distance of the population centers of Boulder and Denver, Rocky Mountain National Park is a natural destination for the crowds that throng to this mountain playground. Long’s Peak, at 14,256 feet (about 4300 m), dominates the south-central part of the park; the peak was first climbed in 1868, by a party that included John Wesley Powell, the man who later commanded the first boat passage of the Grand Canyon and then led the United States Geological Survey. Numerous peaks over 13,000 feet (4000 meters) in Rocky Mountain lure climbers.

Small and rapidly shrinking active glaciers still carve the mountains, and much greater glaciers of the past left the numerous tarn lakes, moraines, and other features that decorate the park. Trail Ridge Road surmounts the high tundra of the park, giving the visitor a first-hand look at periglacial processes and ecosystems (those of cold regions; more on this later). The Colorado River rises on the west slopes of the park, and lovely little trout streams such as the St. Vrain flow down the east slope. Bighorn sheep and elk attract traffic jams in Horseshoe Park.

Virtual Field Trip: Rocky Mountains

Join us as we go on a virtual tour of Rocky Mountain National Park.

Raising the Rockies

It is a tad embarrassing to say that we don’t fully understand the geological history of the Rocky Mountains yet, including the history of Rocky Mountain National Park. The long history of mountain building, erosion, glaciation, etc., is well-known—we can tell the story. But most mountain ranges hug coasts or are formed as coasts disappear in an obduction collision when the ocean closes and obduction occurs, whereas the Front Range of the Rockies is as far as almost 1,000 miles (1600 km) from the coast, yet the Rockies are not the direct result of obduction.

The U.S. West is a complicated region (see the  Optional  Enrichment  section for a little more on this). The continent has been approaching and overriding the East Pacific Rise spreading ridge, which is much like the mid-Atlantic Ridge but is no longer in the middle of an ocean. The San Andreas Fault formed as the East Pacific Rise reached the trench. Before these met, subduction had been occurring beneath the western USA from push-together motion, but with a little slide-past motion thrown in. After the meeting, the subduction stopped, so the push-together stopped, but the slide-past remained to make the San Andreas Fault. To the north of the San Andreas Fault, subduction is still active, forming the Cascades including Mt. Rainier and Mt. St. Helens.

Long ago, the west-coast subduction zone started in the usual way, with the old, cold ocean floor going down into the deep mantle. But as the continent approached the spreading center, the down-going ocean floor became progressively younger, and thus warmer and more buoyant because not as much time had passed for it to cool after its volcanic origin. This warmer ocean floor didn’t “want” to go down, but it was still attached to the older, colder floor ahead of it that was going down. So, the ocean floor that went under the continent stayed high, rather than sinking, and rubbed along the bottom of the crustal rocks rather than plunging steeply into the mantle. The friction between this buoyant subducted ocean floor and the crust above, in turn, caused thrust-faulting and crustal thickening far inland (see the figure below). Because the western part of the country has been built up of many old rock bodies and sediment piles bulldozed from the Pacific, there are scars of many old faults and other geological features that have been reactivated by recent events, so mountains and valleys have formed along the old weaknesses in response to the new pushes.

The Rocky Mountains are surprisingly far from the coast for mountains linked to a subduction zone. The diagram shows the most likely explanation, which is that the subducted slab did not sink as rapidly as normal for a while, and friction along its upper surface rumpled the overlying rocks of North America to raise the Rockies.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

As to exactly how this came to produce 14,000-foot (4300-m) peaks in the Rockies, geologists can tell the story, but it isn’t clear that any geologist could have predicted this story without seeing the rocks first. Science moves from explaining (easier) to predicting (harder), so we still have some work to do. (And we've oversimplified a bit here; see the  Optional  Enrichment  for more.)

The video below provides additional information in support of the diagram above.

Video: The Rockies (1:57)

The Rockies, like the Smokies, were formed by push-together stresses, and the high peaks float on a thick root. Erosion of the peaks has allowed the root to bob upward, so the rocks revealed at the surface include types that formed far down in the Earth and then were brought to the surface. This includes rocks such as granite that solidified from melted rock far below, and the changed—metamorphic—rocks we will discuss below. The bobbing up of the mountains tends to drag surrounding rocks upward. If you drive toward the Rocky Mountains in Colorado from the plains to the east, you can see these dragged-up rocks adjacent to the high peaks. See the narrated diagram for more background.

Video: Red Rocks (2:36)

Cooking the Earth

Think about cooking. If you mix up a bunch of ingredients to make a cake batter, throw the mixture into a pan, and put it into a warm oven, the cake you obtain will not be very similar to the mixture you started with. Grill a steak or a meat substitute, and the original cow part or vegetable-based material will come out quite different. Marinate the steak or meat substitute before grilling, and more differences appear. It is common knowledge that a material that is stable in one environment will change if it is placed in a different environment. This is true of everything (and everyone!) on Earth.

The Earth has a great range of conditions. The inside of a mountain range is hotter, has higher pressure, and is less affected by acidic groundwaters than the surface. Materials that are stable at the Earth’s surface (such as the clays in a piece of shale) are not stable deep in a mountain range. The minerals change, grow, and produce new types even without melting. This process is called metamorphism. Metamorphism makes rocks that many people consider to be especially pretty (see the video on “Toothpaste Rocks” from the Grand Canyon), produces some wonderful gems, and contributes rock names that make good puns. (The Geoclub at Wisconsin liked puns and used a metamorphic rock, a volcanic rock, and a sedimentary rock in claiming that geologists are “gneiss, tuff, and a little wacke.”) You can read a little more about rocks and minerals in the  Enrichment  section.

Toothpaste Rocks: Grand Canyon National Park

Metamorphic rocks—those cooked and squeezed deep inside a mountain range—are often especially pretty. At the bottom of the Grand Canyon, you can see such rocks. They were formed long ago, and many miles down, and then reached the surface as erosion removed the mountains above and the deep roots of those mountains floated upward. Later, these rocks were buried again under sediments from oceans, rivers, and wind, and finally revealed to us as the Grand Canyon was carved by the Colorado River. Some people—including Dr. Alley—think that these rocks are so beautiful that they're worth the overnight hike into the canyon all by themselves!

Video: Toothpaste Rocks - Grand Canyon National Park (2:22)

Where Tectonics Meet People: Tsunamis

Tsunamis

We have been looking at the ways that rocks move around on Earth and make mountains and some of the ways that this mountain building can threaten humans. Volcanoes and earthquakes are sometimes truly dangerous and damaging. But it is worth remembering that, in the developed world, only a few percent of us die in “accidents,” and car crashes greatly dominate those deaths (so the great majority of us die of other things, such as heart disease, cancer, etc.). With good scientific warnings, good zoning codes, trained medical personnel, hospitals, and ambulances to take care of us, nature kills very few of us (something like 0.03% of deaths in the US in most years). (In the less-developed world, this is, sadly, less true.) For the developed world, things we do to ourselves (smoking, eating, drinking too much, not exercising enough) are far, far more destructive to health and life than anything the planet does to us.

But it is still wise to know about the dangers from the Earth—part of the reason so few of us die from natural disasters is that we are already doing wise things to avoid being killed by nature! Some of those wise things involve preparing for giant waves—tsunamis (and preparing for earthquakes, landslides, volcanoes, floods...). Tsunamis are not directly related to our National Parks in this module, but tsunamis are related to some of the processes that helped make the Great Smokies and the Rockies. Anything that makes earthquakes, volcanoes, or steep slopes in or near the sea might be involved in a tsunami. And tsunamis can be truly horrific. We’ll start discussing a long-ago tsunami from a hot-spot setting and then look at other tsunamis closer to us.

Anomalous deposits are found on the flanks of many of the Hawaiian Islands, including Lanai, Molokai, and Maui, to at least 1,600 feet (500 m) above sea level. These deposits are composed of broken-up, mixed-up, battered corals, other shells, and beach rocks. Corals are undersea creatures and surely don’t grow 1,600 feet above sea level. It is true that some corals grow just below sea level and later are raised above the water by mountain-building processes; however, these Hawaiian deposits occur on islands that are sinking as they slide off the “hill” made by the Hawaiian hot spot, and the deposits are geologically too young to have been raised so far by mountain-building processes. Clearly, something strange happened.

This is a map of the sea floor off Goleta, California, from the Monterey Bay Aquarium Research Institute. Landslides started from the cliffs at the top of the picture, in water about 100 m (300 feet) deep, and extended as much as 9 miles (15 km) into the water slightly more than 600 m (almost 2000 feet) deep, toward the bottom of the picture.  The landslides may have been triggered by earthquakes sometime between 6,000 and 8,000 years ago and likely shoved aside so much water so rapidly that they generated huge tsunamis. Similar landslides that generated tsunamis have occurred off Hawaii and along many other coasts.

Credit: Goleta Underwater Landslide by Pacific Coast and Marine Science Center, USGS (Public Domain)

One of the deposits, in particular, is the same age as a nearby, giant underwater landslide, as nearly as the age can be measured. The Hawaiian volcanoes have rather gradual slopes above the water, where the hot, low-silica lavas spread out to make shield volcanoes. But when lava hits the water, the hot flow cools and freezes very quickly, and can make steep piles. When a slope is too steep, it can fail in a great landslide, perhaps when melted rock is moving up in the center of the island and shoving the sides out to make them steeper. Surveys by specially equipped research vessels using side-scanning sonar have shown where several such slides have slipped. Such landslides can be miles thick, tens of miles wide, and over 100 miles long.

If a chunk of rock miles thick and tens of miles wide suddenly starts moving, maybe at hundreds of miles per hour, it will shove a LOT of water out of the way. Where will the water go? The answer is that it will make a huge wave, or tsunami, that will race across the ocean, and up onto any land it encounters. Imagine a wave so huge that it would run far inland and reach heights of 1,600 feet above sea level. Fortunately, the highest deposits in Hawaii are from a tsunami about 110,000 years ago, long before people were living there. Although many such tsunami-generating landslides have occurred, they typically are spaced thousands of years apart or more. But we can’t guarantee that there won’t be another one. See the animation below for a worst-case scenario.

Video: Tsunami visualization (2:05)

The word tsunami comes from two Japanese words, for harbor and wave, a sort of shorthand for a wave that devastates a harbor. Most tsunamis are generated by undersea earthquakes, but undersea landslides, volcanic eruptions, and even meteorite impacts in the water and can generate tsunamis.

Tsunamis move rapidly across the deep ocean, with speeds of 300 to 500 miles per hour (600 to 1000 kilometers per hour). In the deep ocean, the “bump” of water that is the wave of a big tsunami may be only a very few feet high but may extend well over 100 miles in the direction it is moving. Waves slow down as they enter shallower water, and the leading edge of a wave hits shallow water before the trailing edge. So, the leading edge slows as it nears the coast, the trailing edge that is still in deep water catches up, and the wave goes from being long and low to being squashed and high. Even so, the tsunami wave is usually not a towering wall of water, but a strong surge, something like the tide coming in but higher (hence the mistaken name “tidal wave”).

A tsunami wave is long and low in deep water, but “friction” with the ocean bed as the wave enters shallower water slows the front of the wave down while the back catches up, causing the wave to become high and short, as shown in this diagram.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

An especially nasty feature of a tsunami is that the water often goes out before it comes in. (Waves consist of troughs and crests, and while the crest arrives first in some places, the trough arrives first in other places.) The sudden retreat of water and exposure of the sea floor tempts people to walk out and look around. Then, the ocean returns faster than a person can run. The outcome is very unpleasant.

Video: NOAA Tsunami Animation (1:27) This video is not narrated.

Terrible tsunamis have occurred. The greatest loss of human life from a tsunami was probably the Indian Ocean tsunami of 2004, which was triggered by the second-largest earthquake ever recorded, and killed roughly 230,000 people. Second was the tsunami from the 1755 earthquake near Lisbon, Portugal that killed about 60,000 people, especially in Morocco, Portugal, and Spain. The massive 1883 explosion of the volcano Krakatau in Indonesia essentially destroyed the island, with tsunami waves observed as far away as England. Floods raced miles inland on Java and Sumatra, killing approximately 40,000 people. The volcanic eruption of the Greek island volcano Santorini in the 1600s BCE pushed a tsunami perhaps 300 feet (100 m) or higher across the coast of Crete and may have contributed to the eventual demise of the Minoan civilization there. Many commentators have suggested that this is the source of the myth of Atlantis. The great 1964 Alaska earthquake generated a deadly tsunami that killed 118 people, with deaths as far away as California. In 1958, an earthquake-caused landslide in Lituya Bay, Alaska, caused a tsunami that included a wave 50-100 feet high in the bay, which a father and son safely rode out in a boat. They watched in awe as the wave then ran 1,800 feet up an adjacent coast; five people were killed in the event. Many other destructive tsunamis have occurred.

There isn’t a whole lot that can be done to stop tsunamis, but the loss of life and property damage can be limited. Tsunami warning systems are functioning in many places and are being extended rapidly. When instruments (called seismometers) sense the shaking of the Earth from a large undersea earthquake, volcano, or other disturbance, the signals are analyzed rapidly to see if characteristics suggest that a tsunami is likely, and if so, communications are sent out to various agencies concerned with safety, and sirens or other warnings on beaches are activated to get people away from the coast before the tsunami arrives.

The Indian Ocean tsunami of 2004 seems to have been especially deadly in places where human activities had caused damage to the coral reefs and coastal vegetation that would have blunted the strength of the wave, so maintenance of such natural buffers along these and other coasts can help protect the people living nearby from any future tsunamis. Scientists can figure out where tsunamis are likely, how big and how frequent they are likely to be, and then zoning codes can be enforced so that people build in safe ways on safe land if they want to live in an area.

Virtual Field Trip: Tsunamis

Please join us on a virtual field trip of Tsunamis.

Optional Enrichment Article

Raising the Rockies

As noted in the text, the geologic history of the US West is quite complex, with some big questions not yet answered, and more work to be done. The most widely accepted history, with the most scientific support, is sketched in the text and given in a bit more detail just below and in the Optional Rock Video Review, The Hanging Wall.

Much evidence indicates that before about 100 million years ago, the down-going rocks in the subduction zone in the west went down steeply, but after that time the rocks' angle reduced (so, the rocks continued to move horizontally, but with less downward motion). The Pacific seafloor that was sinking under what is now Seattle is called the Farallon Plate, and rather than going down somewhat smoothly into the deeper mantle, the Farallon began to move along just beneath the lithosphere of the North American plate. The text notes that the plate was slowly getting warmer over time and thus more buoyant because the ridge and trench were getting closer together, so the plate was having less and less time to cool off before it went down the subduction zone. Another reason, and perhaps a more important reason, may be that a huge volcanic outpouring on the sea floor more than 100 million years ago made a thick layer of not-very-dense rock, which began going down the subduction zone about 100 million years ago. Where smaller bumps on the sea floor are going down subduction zones today, as off Costa Rica, features like those seen in the US West are forming, so the bigger features of the West are easy to explain this way.

Anyway, friction between the top of this buoyant Farallon Plate and the rocks above it—the rocks that we see in the Rocky Mountains and elsewhere in the US West—began to squeeze, bend, and break those rocks above. The breaks are thrust faults, with older rocks thrust upward and over younger rocks to make the mountains. You will recall that the rocks above the fault are the “hanging wall”, which gives rise to the pun in the Rock Video.

Where a fault didn’t break all the way to the surface, it often raised the rocks above (think of lying on your back in bed under covers and then raising your knees—the cover draping over your knees represents the unbroken rocks). If the rocks were raised in a more-or-less circular pattern when viewed from above, we call the feature a dome; if shaped more like a US football when viewed from above, it may be called an arch, and a few other names are sometimes used. Such uplifts gave us the Black Hills of South Dakota (with Wind Cave National Park), the Waterpocket Fold of Capitol Reef National Park, the Kaibab Uplift that the Grand Canyon cuts through, the beautiful San Rafael Swell of central Utah, and many other features of the West. These events mostly happened soon after 100 million years ago, during what we now call the Sevier Orogeny, and somewhat more recently in the Laramide Orogeny—we see more faults from the Sevier and more uplifts from the Laramide.

The squeezing and thrusting of mountains caused some layers to bend down while other layers bent up. One of the low places this caused held the lake in which the pink limestones were deposited that we now see at Bryce Canyon National Park and Cedar Breaks National Monument, and similar lakes gave us the Green River limestones that produce such fantastic fish fossils from farther north in Utah. Oil is found in the west in some of the rocks that were bent down, too.

Fossilized fish from the Eocene Green River Formation, Wyoming. Rock section is five inches across.

Credit: Educational Resources, USGS (Public Domain)

The Farallon Plate probably eventually broke, and a new, “normal” subduction zone started up, feeding Mt. St. Helens and Mt. Rainier in the west. The Farallon Slab is now sinking beneath the eastern part of the US. As it sinks, it creates space into which hot rock flows slowly, and this may be helping “rejuvenate” the Appalachians, so the Great Smokies and other parts of the Appalachians are a bit higher than they would be otherwise because of processes traceable back to the Rockies and the Farallon. Why the Farallon “decided” to sink eventually may be because, as it ran into thick rocks beneath the west, either some of the low-density parts were peeled off so the higher-density ones could sink, or the low-density ones were shoved deep enough that the pressure changed the mineral structures, making denser minerals that can sink.

Anyway, there is still much to discover about the western US. But, to catch a light-hearted version of what we know, check out the tale of a geology student trying not to be hung on the Hanging Wall.

Video: Hangin' Wall (4:29)

Still more about plate tectonics and related topics

What happens when you "cook" a rock metamorphically depends on how hot it becomes, and what chemically active fluids are present, and the pressure and deviatoric stress. Both pressure and stress have units of force per area and represent a "push" on a material. Pressure is the part of the stress that is the same in all directions--it squeezes rocks or fluids to make them smaller but doesn't tend to change their shape. Deviatoric stress, sometimes just referred to as stress, is an extra push or pull in one or more directions, and does change the shape. Deviatoric stress has been involved in aligning the mineral grains of most metamorphic rocks into layers, or folia. Chemically active fluids—water, carbon dioxide, methane, etc.—can add or subtract chemicals, lower melting points, and dissolve and reprecipitate chemicals.

Materials that are stressed deviatorically can have one of three responses. The materials may bend and, when you release the stress, snap back (elastic deformation), they may bend permanently (plastic deformation or creep), or may break. We have already seen examples of all of these. Earthquakes are caused by the snap-back of rocks near faults following bending. Faults such as the San Andreas are the result of breakage, and folds such as those around University Park, or those in the rocks exposed in the heart of the Rocky Mountains, are the result of plastic deformation.

Whether a rock bends elastically or plastically or breaks, depends on the rock itself and on several other factors: heat (or more properly, how close a material is to melting), pressure, deviatoric stress, and even the chemically active fluids, acting over time. Elastic deformation is favored by low stresses, high pressures (to prevent breakage), and low temperatures (to prevent creep). Plastic deformation is favored by low stresses, high pressures (to prevent breakage), and high temperatures (to allow creep). Fracture is favored by high stresses and low pressures (to allow breakage), together with low temperatures (to prevent creep). Pressure matters in breakage because, to break a material, the two sides must be pulled apart, which increases the size of the material. When pressure is high, this is difficult to achieve. Fluids generally soften rocks and promote creep, although the details depend on the fluid and the rock involved. (“Fracking” to recover oil and gas from rock involves first pumping high-pressure fluids into holes to fracture the rocks, so while fluids generally soften rocks to promote creep, they don’t always do so). You will often see that the rocks in a region will have undergone both permanent folding and breakage, and may be bent and ready to snap back, with different modes of deformation more important in different places. Figuring this all out is fascinating, as well as useful (miners, well-drillers, and many others want to know what they'll hit in the rocks, whether the rocks are about to break, what kind of cracks the oil or gas or ores may be hiding in, and much more).

Melting and Freezing with Chemistry

If you take water and freeze it, you obtain ice that is made of the same stuff as the water. Melt the ice, and you have the water back. Melting and freezing without separating chemicals is easy to understand but is more an exception than the rule. If you take beer and start to freeze it slowly, the crystals that form will be almost pure water ice, although each crystal typically will start to grow on a small impurity particle in the beer. Filter the crystals out, and you will have cleaned the remaining beer a little, and you will have increased its alcohol content. Hire some advertising agents, and you have a “new” product to pitch: ice beer. (If you're underage, please substitute iced root beer. Freeze that, and you'll end up with a lot of ice and a little sugar water.)

In the world of rocks, things are even more complex than with (root) beer. Suppose you take a piece of granite (containing quartz, mica, potassium feldspar, and a sodium-rich sodium-calcium feldspar) and melt it by heating it hot enough to melt basalt. Suppose you then start to cool it. The first minerals to crystallize may be a little bit of olivine, and a calcium-rich sodium-calcium feldspar, minerals that were not in the original granite at all. Cool the melt a little more, and the olivine and remaining melt react to make pyroxene, while the feldspar and melt react to make more feldspar that is richer in sodium. Keep cooling, and eventually, you will grow crystals of the original sodium-rich feldspar and the mica, followed by the potassium feldspar and the quartz, regaining the original granite.

This ideal sequence may not be observed in many situations, but portions of it are well-known in laboratory experiments and in nature. In general, the first things to crystallize are poorer in silica, sodium, potassium, and aluminum, and richer in iron, magnesium, and calcium than the melt from which they grew. As the temperature drops, the early minerals react with the melt to make new minerals that are more like the original melt in composition. However, if the early minerals are removed from the melt, perhaps by settling to the bottom, they may be preserved, and exceptionally silica-rich rocks will be formed from the minerals that grow from the remaining melt. For more on the minerals, look at our "Sidebar" next.

So Why Did North America Run Over the Spreading Ridge in the Pacific?

Good question. We’re not sure. But we do know that mid-ocean ridges are high because they are hot. Plates will slide off high spots, heading toward low places. So, North and South America are being pushed westward because they are sliding off the mid-Atlantic spreading ridge. If the spreading center in the Pacific simply sat still for a long while (which it probably did, more-or-less), then eventually the Americas would get to it, which they seem to be doing now.

A Rocking Review

Peaceful Easy Obduction

The Eagles rock music group sang about the peaceful, easy feeling they got while going into the desert, but they didn’t tell us whether they planned to do a lot of geology on the way. Whatever; in this parody, you can get a peaceful, easy feeling about a whole mountain range, the beautiful Appalachians. The proto-Atlantic Ocean did close before re-opening as the modern Atlantic Ocean, acting like a very slow accordion. While the proto-Atlantic was closing, three "collisions" happened, raising mountains near the modern east coast of the US, and erosion of the mountains produced sand that now is the sandstone of the ridges of central Pennsylvania and other places along the Appalachians. The first two collisions—small obduction events—involved North America hitting island arcs containing explosive volcanoes formed at former subduction zones. The third collision was the major obduction event as the proto-Atlantic disappeared, with Africa and Europe meeting the Americas as the great supercontinent of Pangaea was assembled.

Geologists long ago decided that the place where subduction occurs is a "subduction zone” but have been slow to adopt an "obduction zone" for the place where obduction occurs. We just couldn't make the rhyme scheme work in our parody with "the mountain range formed where an obduction event happened", so we call it an obduction zone. No one is quite sure whether the mountains in our "obduction zone" were as high as the Himalayas, or how many glaciers eroded the top of our obduction zone, but the mountains probably were quite high (more or less the height of the Andes today) and at least some of the early Appalachians may have been glaciated. The diagrams in this rock video have been simplified a bit, to make it easier for you.

So, relax, get out your Irish flute, and let's go obducting.

Video: Peaceful Easy Obduction (5:02)

Module 4 Wrap-Up

Review the Module 4 Introduction

You have reached the end of Module 4! Double-check the list of requirements on the Module 4 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.

Comments or Questions?

If you have any questions, please feel free to send an email through Canvas. Remember to include all of the teachers and all of the TAs in the "To" line. Failure to email all teachers and all TAs may result in a delayed or missed response. Directions for how to send an email can be found in the Resources module.