GEOSC 10
Geology of the National Parks

Main Topics: Module 5

Main Topics: Module 5

 

Overview of the main topics you will encounter in Module 5

First, a quote from President Teddy Roosevelt, who looked for ways to slow the soil erosion that was making it hard for farmers to feed us:

There are certain other forms of waste which could be entirely stopped—the waste of soil by washing, for instance, which is among the most dangerous of all wastes now in progress in the United States, is easily preventable, so that this present enormous loss of fertility is entirely unnecessary. The preservation or replacement of the forests is one of the most important means of preventing this loss.

—President Theodore Roosevelt, Seventh Annual Message to Congress, Dec. 3, 1907

In this Module, we start by visiting Redwood and Sequoia National Parks, and Death Valley, to learn a little about the weather. You may want a jacket to stay warm and dry in the foggy drizzle of Redwood National Park on the California Coast, yet, above the clouds, Redwood gets about the same amount of sunshine as toasty Death Valley just over the mountains where California meets Nevada. Much of Death Valley's heat can be traced to the energy that was stored when water evaporated from the ocean and then released to warm the air as clouds formed to rain on the redwoods.

After learning about the weather, we visit Badlands National Park. The Badlands are carved in old soils and other soft deposits, not hard rocks. Yet, most of the material in the Badlands started out as hard rocks. Weather, helped by living things, causes the “weathering” of hard rocks, breaking them apart and changing them chemically. Some chemicals from those rocks dissolve in water that flows toward the sea, while other materials stay in place and make soil.

To finish this Module, we visit the Grand Tetons and the Gros Ventre Slide. Naturally, soils and loose rocks move downhill about as rapidly as new ones are produced by weathering. We call this downhill motion “mass wasting” when it happens without the help of a river or glacier or wind. Most mass wasting is slow, but not all, and the Gros Ventre Slide provides dramatic evidence of the dangers of landslides that can be really fast. The Gros Ventre slide is now being washed away by a river and carried toward the sea, a topic for Module 6.

Humans have greatly accelerated mass wasting in many places. This is dangerous if a landslide threatens to bury you, but also if soil erosion reduces our ability to grow food. The quotation above from President Teddy Roosevelt, more than a century ago, highlighted the importance of saving soils.

Weather, Weathering, and Landslides

  • The Sun hits the equator just about straight-on, but gives the poles a glancing blow, so the equator gets more of the Sun's energy.
  • The Sun heats the Earth, which drives convection in the atmosphere.
  • This convection in the air, when combined with Earth's rotation, makes interesting winds including breezes blowing from the Pacific onto the US West Coast.
  • These winds rise up the Coast Ranges and the Sierra Nevada, cooling and making rain that waters the redwoods and sequoias.
  • These winds then sink down into Death Valley, heating and drying it.

Why are the Redwood Wet, and Death Valley Dry?

  • Warmer air can hold more moisture.
  • Rising air expands, which cools the air; sinking air is compressed, which warms the air.
  • Evaporation requires heat and condensation releases heat (you cool as your sweat evaporates, taking heat from you).
  • If water vapor is not condensing to form clouds, the air cools about 5°F for each 1000-foot rise.
  • If water vapor is condensing to form clouds (which then can produce snow or rain), the air cools about 3°F for each 1000-foot rise.
  • Air from the Pacific is "wet", carrying about as much vapor as it can.
  • This air blows up the Coast Ranges and the Sierra, cooling, which reduces its ability to carry water, forming clouds that rain on the trees.
  • Dry air comes down the other side of the mountains into Death Valley.
  • The air cools 3°F for each 1000-foot rise going up, warms 5°F for each 1000-foot fall coming down, and must go up about 15,000 feet to get over mountains, so the air comes down about 30°F warmer than when it went up.
  • This is the main reason Death Valley is hotter than Redwood; the lack of clouds over Death Valley also allows more warming from the Sun.
  • The energy that warms the air at 30°F going from Redwood to Death Valley was stored in the air when water vapor evaporated from the ocean, and released to warm the air when condensation made the rain for Redwood.

Rocks Are Not Forever

  • "Weathering" includes the physical changes that make small rock pieces from big ones, and the chemical changes that make new minerals.
  • Physical weathering is caused by crystal growth in cracks (especially ice) and other processes.
  • Granite is a common rock composed of quartz (silicon+oxygen, sometimes called silica), feldspar (which is silica+aluminum+(calcium or sodium or potassium)), and a dark mineral (which is silica+iron+magnesium).
  • Chemical weathering leaves the quartz as quartz sand, changes the feldspar to clay (silica+aluminum+potassium) while the calcium or sodium wash away, and rusts the iron of the dark mineral while the magnesium and silica wash away.
  • The "chunks"—rust+sand+clay—plus worm poop and similar materials make soil.
  • The calcium and silica go to make shells in the ocean.
  • The magnesium reacts with hot sea-floor rocks to make new minerals there.
  • The sodium makes the ocean salty.
  • The soil eventually is washed into the ocean.
  • Subduction takes sea water, sediment, shells, soils, and the sea floor with its new minerals down to melt; the melt rises and solidifies as granite (or andesite, if it erupts), in a nearly balanced cycle.

Mass Movement

  • Mass movement is the downhill transport of soil and rock that occurs in many places without major help from rivers or glaciers or wind.
  • Mass movement ranges from huge, destructive landslides to barely measurable soil creep.
  • Rivers usually pick up and carry away the material delivered by mass movement.
  • In most places and times, there is a natural balance between soil production by weathering and soil removal by mass movement or other processes.
  • Humans are upsetting this balance in many places, typically making soil removal much faster than soil formation, so that soil is getting thinner.
  • We often can figure out where mass movement is potentially destructive, and stabilize slopes or stay out of the way.