Welcome to Tearing Down Mountains: Weathering, Mass Movement, & Landslides!

Module 5 Introduction (2:17 minutes)

Fans of old-fogey rock music may recall that Paul Simon was "slip-sliding away." Paul was singing about human relations, not about debris flows. But, our hillsides really are “slip-sliding away,” too. Weather attacks rocks to make loose blocks, which may fall off cliffs rapidly or hang around to make soil before sliding downhill. So, crank up the tunes, watch out for rolling boulders, and let’s slip on into Module 5.

Learning Objectives

• Explain why the wind blows.
• Discuss why wind going up mountains produces rain and then becomes warmer going down the other side.
• Explain how weathering changes rocks physically and chemically at the Earth’s surface.
• Discuss mass movement and the downhill motion of loose rocks and soil.
• Explain how weathering and mass movement of old rocks are part of a cycle that leads to new rocks that experience weathering and mass movement.

What to do for Module 5?

You will have one week to complete Module 5. See the course calendar in Canvas for specific due dates.

• Take the RockOn #5 Quiz
• Take the StudentsSpeak #6 Survey
• Submit Exercise #2
• Begin working on Exercise #3

Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.

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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.

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.

Take a tour of the Redwoods

Weather and Climate in the Redwoods

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

First, as usual, here is some background material on national parks you might want to visit. This background also raises some questions that we find fascinating, about why there are such huge differences in climate between nearby national parks. After we raise these questions, we start to answer them with Why the Wind Blows, below.

Redwood National Park has the feel of a soaring, gothic cathedral—only more so. One of the great Sequoia sempervirens trees may live for over two thousand years, but when it falls, new trees will grow from the fallen trunk. (This growth of new trees from a fallen giant gave us the scientific name, which means “ever-living sequoia” in Latin.) The redwoods are the tallest trees on Earth, commonly more than 200 feet (60 meters) and with the very tallest soaring above 380 feet (more than 115 meters). If such a tree grew on the goal line of a US football field and then fell over, it would knock down the goalposts on the other end, and the top branches would extend into the stands. Ferns growing beneath the redwoods may be shoulder-high on a person, yet appear lost and inconsequential. Reports from early loggers included trees even taller than any known today.

The redwoods lie in the southern part of the great, coastal, temperate rainforest that extends north from near San Francisco along the Pacific coast to southeastern Alaska and includes Olympic National Park, which we visited earlier. The redwoods grow in soils that came from rocks much like those of the Olympic.

Redwoods are mostly restricted to a narrow band along the coast with 50-100 inches (1.25-2.5 meters) of rain per year, and with frequent to continuous fogs that help the trees avoid drying out. The trees, in turn, help maintain the fog. Cutting the trees may decrease the fog, making it difficult or impossible for the redwoods to re-grow.

The wood of redwood trees is highly resistant to fire and rot, and so is greatly sought after. Logging of old-growth redwoods thus is a contentious issue. An estimated 96% of the old-growth forest has been cut already. Those trees typically were 500-700 years old, with some about 2000 years old, so a new "old-growth" forest cannot return soon. Some people still want to cut the remaining 4% or so of the original old-growth redwood forest.

Fossil evidence shows that redwoods once were much more widespread. U.S. parks that preserve fossilized redwood logs include the Petrified Forest, Yellowstone, and the Florissant Fossil Beds.

A bit farther south and higher on the slopes of the Sierra, but still in a zone that gets plenty of rain and snow, are the great sequoias of Yosemite, Kings Canyon, and Sequoia National Parks. The Sequoiadendron gigantea are close relatives of the redwoods. Although not as tall (“only” up to 311 feet, or about 95 meters), the great sequoias are more massive. The General Sherman tree, at 275 feet tall and 102.6 feet around, is generally considered to be the largest single-trunk tree on Earth (and much larger than the largest whales). Sequoias can live for 3500 years.

The great sequoias are extremely fire-resistant and require fire to clear out competing trees and trigger the sprouting of sequoia seeds. Fire suppression instituted after the parks were established led to a period with few or no new sequoias sprouting. Dead wood, leaves, sticks, and other debris that can burn accumulated during this time of fire suppression and can make wildfires (which are being made worse by human-caused climate change) hot enough to endanger the sequoias. Carefully planned burns started by experts, and procedures that allow some natural fires to burn, are being used to help return the forest to a more natural state.

Ultimately, the wind blows for the same reasons that the mantle convects and that boiling water in a pot full of spaghetti rises over the heating element of a stove. The air, the spaghetti water, and the mantle are capable of flowing, and all are heated from below and cooled from above. The amount of heating and the rate of flow are VERY different in these different cases, which helps make the world interesting. But you might see a thunderstorm and imagine a hot-spot formation in the Earth, or a pasta dinner, and there is at least a little similarity between a cold front, a subduction zone, and spaghetti noodles sinking along the wall of the pot.

Some of the sunshine that reaches the top of Earth’s atmosphere is reflected from clouds or snow or other things without heating us, but most comes through the air and heats the surface of the Earth, which then heats the air above. The equator receives more sunshine than the poles because of simple geometry. Imagine for a moment that Dr. Alley’s head is the Earth, with his nose on the equator and the North Pole in his bald spot on top. (See the picture.) If he stands in front of a sun-lamp “sun,” he’ll never get a sunburn on his North-Pole bald spot, but he will on his equatorial nose.

Dr. Alley’s equatorial nose and ear can be sunburned by the sun lamp (you can see the back of the lamp on the right side of the picture), but his north-polar bald spot receives only a glancing blow from the “sun’s” rays and so needs no sunscreen. Similarly, the Earth’s equator is strongly heated by the sun while the poles are not, almost entirely because of the curvature of the planet; the Earth’s equator is closer to the sun than the poles by only about 0.004%, not enough to matter. This is shown in a more scientific-looking way in the diagram below.

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

The Earth works the same way—at the top of the atmosphere, the amount of sunlight passing through a square meter is the same at the equator as at the pole (see the diagram below). But because of the Earth’s curvature, the light passing through a top-of-the-atmosphere square meter at the equator illuminates a square meter at the surface, whereas the light passing through a top-of-the-atmosphere square meter near the poles is spread over many square meters on the surface, so a square meter on the Earth’s surface gets more sunshine near the equator than near the poles. (Additionally, in both cases, the rotation of the Earth causes the sunlight to be spread around the whole planet.)

The additional energy received at the equator compared to the poles means that the surface at the equator becomes hotter than at the poles. If we had no atmosphere or oceans, the equator would become too hot for life as we know it, and the poles too cold. However, the extra sunlight that the equator receives heats air and water there, driving winds and ocean currents that carry some of the excess heat toward the poles, making the whole world habitable to humans.

Video: The Drivers of Earth's Weather (1:34)

Following is a static image that was described in the video above.

At the top of the atmosphere, the amount of sunlight passing through a square meter is the same at the equator as at the pole. But because of the Earth's curvature, the light passing through a top-of-the-atmosphere square meter near the poles is spread over many square meters on the surface, causing the poles to be heated less than the equator.

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

In slightly more detail, as the sun heats the land at the equator, the land heats the air above, and the air expands, rises, and then moves poleward in convection currents. The hot air loses some of this energy to warm the land and water it passes along the way, but eventually, all the energy is radiated back to space. The sunlight that comes in is called shortwave radiation because its waves are short. (This stuff really isn’t that complex a lot of the time!). The radiation going out has longer waves and is called longwave radiation. Your eyes can see the shortwave light, but you would need infrared goggles to see the longwave. Later, we will discuss how the different way that shortwave and longwave radiation interact with certain gases in the air is important in understanding the greenhouse effect. For now, note that the global energy budget is very close to being balanced—the total amount of energy brought in by shortwave radiation and absorbed in the Earth system is very nearly equal to the total amount of energy taken out by longwave to space. (This is not perfectly balanced now, because we humans are changing the composition of the atmosphere by adding greenhouse gases, so the Earth is warming because we are sending out a little bit less energy than we receive. But, once we quit changing the composition of the atmosphere, the Earth will get back to balance.) A factory balances the total amount of stuff coming in and going out, but little auto parts come in and big cars go out; the earth balances the total energy coming in and going out, but shortwave comes in and longwave goes out. And, the uneven heating because of the Earth’s nearly spherical shape drives the wind, and the wind plus uneven heating of the oceans drives ocean currents.

Because the Earth rotates, the winds end up turning rather than going straight from the equator to the pole, and this makes the weather much more interesting than it would be on a non-rotating planet. If you want to explore this a little more, see the optional Enrichment about the Coriolis effect.

Take a tour of Death Valley

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

Some people are surprised to learn that the rain on the redwoods is partly responsible for the heat of Death Valley, as we explore next in the text and in the video just below.

Warm air can hold more water than cool air. All air in nature has at least some water vapor in it, so if you cool air enough, water will begin to condense out of the air to make clouds that can rain or snow.

In nature, the air cools in two major ways—by losing energy to warm up its surroundings (by radiation or conduction), or by expanding as it is lifted. If warm tropical air flows toward one of the poles over colder land or water, the air cools while the land or water warms, and fog or clouds may form in the air. And, air that is lifted upward expands, and this expansion cools the air. You can experience this by feeling air becoming cold if it escapes rapidly from a high-pressure bicycle tire. Air that moves downward is compressed and warms, something you can feel if you keep your hand on the bicycle pump while you fill the tire again. In nature, air may be lifted when one air mass moves over another along a weather front, or when air moves up a mountain range. In either case, higher elevations have lower temperatures. (If it bothers you that cooler temperatures exist higher, but that cold air sinks, see the optional Enrichment—as explained there, cold air will sink if, after it warms while sinking, it is still colder than the air it replaces.)

Evaporation of water ultimately cools the surroundings of the water, and condensation warms the surroundings. To see this, remember that almost everything we see—including water—is made of fast-moving particles called atoms, or groups of atoms called molecules. If you have some water, such as the ocean or a drop of sweat on your skin, the average speed of the molecules will be higher if the water is warmer, but there always will be a range of speeds with some of the water molecules moving faster than others. The faster-moving, hotter molecules evaporate by breaking the attraction to their neighbors, leaving the slower, cooler ones behind, so evaporation always cools the remaining water. More heat then is conducted or radiated into this remaining water from its surroundings because heat flows from warmer to cooler places—your skin feels cool as it loses heat to the drop of sweat as it evaporates, and the ocean surface cools as water evaporates into the air.

Condensation reverses evaporation. Water condensing on a glass of iced tea warms the tea and the surrounding air. Condensation of water to form drops in clouds warms them, and they warm the surrounding air, releasing the energy that was stored when the water evaporated. (This energy that is stored during evaporation and released during condensation is called latent heat.)

Now, consider the wind blowing into California from the Pacific Ocean and rising up the Coast Ranges above the redwoods, and even farther up the Sierra Nevada above the giant sequoias, as shown in the video below As the air rises, the pressure on it (the weight of the air above) becomes smaller, and the air expands and cools. Once the air has cooled enough, it becomes saturated with water, and further cooling causes condensation. But the condensation releases some heat that partially counteracts the cooling from expansion. Air in which condensation is not occurring cools by about 1ºC for every 100 meters it is lifted, but when clouds and rain are forming the cooling is only about 0.6ºC for every 100 meters it is lifted (5ºF per 1000 feet dry, and 3ºF per 1000 feet wet). The difference represents the heat released by the condensation. This heat originally came from the sun, was stored in the air when the sun’s energy evaporated water, and was released to warm the air when the water condensed.

The strong breezes from the Pacific usually are full of water vapor, and begin to form clouds and fog as soon as they start to rise. This gives the common fogs of San Francisco and the Redwood Coast.

Fog along the Redwood Coast.

Credit: National Park Service (Public Domain)

As you can see in the video below, those winds blowing over the Redwoods and the Sierra to Death Valley must rise about 15,000 feet to get over the Sierra, and cooling by 3ºF for every 1000 feet upward means the air cools about 45ºF going up. (Air that makes you feel comfortable at 70ºF at the coast will be 25ºF at the top, so take a warm jacket even in summer if you’re planning to climb in the high Sierra!) When this wind continues down the other side, it has lost almost all its water vapor and warms about 75ºF on the way back to sea level at the dry rate of 5ºF per 1000 feet. Thus, a comfortable 70ºF breeze on the Pacific Coast will be 100ºF when it nears the bottom of Death Valley. (Or, the air comes in from the Pacific at 21ºC, cools by 25ºC while rising 4200 m and cooling at 0.6ºC per hundred meters to -4ºC on top, then warms by 42ºC at 1ºC per hundred meters, reaching 38ºC at sea level on the edge of Death Valley.) Add a little solar heating through the cloudless desert air, and it is no wonder that Death Valley is hot! (And yes, the wind usually goes around the Sierra rather than over, so you haven’t learned everything about meteorology in part of one Module in a geology class, but it’s a start.)

Watch the following video that describes this process in more detail.

Video: Heating and Cooling of the Redwoods (2:23)

Take a tour of Badlands National Park

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

The Badlands of South Dakota are much more than just the land to the south of Wall Drug Store. (If “Wall Drug Store” doesn’t make sense, your favorite search engine can give you a lot of information about South Dakota's strange but popular commercial tourist trap. Among other things, they have a great collection of western art, and a jackalope you can take pictures on, as shown in the VTRIP). Today, the Badlands may be most valuable for ecological reasons, because they preserve a wonderful piece of the shortgrass prairie that once covered much of the western Great Plains. The sea of grass and flowers that nourished the bison and the Native Americans of the plains has been almost entirely plowed under in most of the US West. But, in the upper prairie of the Badlands, the grass still waves in the breeze like an ocean, the pronghorn antelopes still bound through the grass, and you can, perhaps, imagine what the prairie once was.

Badlands (left) and Yosemite (right)

Credit: Badlands, National Park Service (Public Domain), Yosemite, National Park Service/Michael Hernandez (Public Domain)

As you can see in the VTrip (and image) above, the Badlands are carved into “rocks” that occur in nearly horizontal layers. But these don’t make giant cliffs such as the 3000-foot-high granite walls we will visit at Yosemite soon. Instead, the Badlands break down easily, so hikers on the trails must be careful to avoid slipping and sliding on loose material that becomes really, really slippery in occasional rains. (Hikers in Yosemite need to be careful, too!) The material at the Badlands was washed into where we now see it by small streams or blew in on the wind, and includes some loose ash from far-away volcanic eruptions that fell on the surface. Many of the layers at the Badlands are old soils that formed where we see them today and then were buried by newer deposits. Amazing fossils have been buried in these sediments, including bones of ancient alligators, saber-toothed cats, camels, rhinos, and more. (The National Park is essential in preserving these so that everyone can enjoy them!)

Earlier, we learned about obduction and subduction, and about volcanic eruptions and intrusions, processes that make hard rocks of the sort that you can see in the cliffs at Yosemite. Clearly, many things must have happened to turn such hard rocks into the loose pieces that washed into the Badlands, and then to turn those loose pieces into soils. And, just as understanding the mountain-building processes can help keep us safe, understanding the mountain-breaking processes is important for our well-being. We look at some of these mountain-breaking processes next, starting with changes called “Weathering”, because many of them are linked to the weather.

We met metamorphism back in Module 4. If you take some Earth material (mud, for example) from one environment where it is “happy” (near the surface of the Earth), and move it into a very different environment, the mud changes. Moving the mud deep into the Earth, where temperature and pressure are high, causes new minerals to grow, and the soft mud with its tiny clay particles can become a hard metamorphic rock with big, beautiful crystals of fascinating minerals.

The materials in the mud are stable (or at least nearly so) under conditions found at the surface but not stable under conditions found deep in the Earth. And, perhaps not surprisingly, minerals produced deep in the Earth usually are not stable under surface conditions. Compared to deep in the Earth, the surface is wetter, has more oxygen, has a wider range of acid/alkaline conditions (with acid especially common at the surface), and has many more living things trying to break down the minerals to extract chemicals that are useful to them (“fertilizer”).

As a general rule, the more you change the conditions around a mineral, the faster the mineral changes into something new. (This “rule” has many exceptions, but it is often useful.) At or near the Earth’s surface, the changes that occur to a mineral at a place are called weathering. Moving the products of weathering is called transport. And weathering plus transport are lumped together as erosion.

Weathering, in turn, is divided into mechanical weathering and chemical weathering. Mechanical weathering refers to nature breaking big pieces to make little pieces; chemical weathering refers to nature making new types of materials that were not there previously.

Turning big pieces into little ones requires cracking the big ones. Cracks in rocks are caused or enlarged by processes including:

• Mountain-building stresses or earthquakes;
• Expansion as erosion removes the weight of overlying rock;
• Expansion and contraction during heating and cooling (especially very near the surface during forest fires; a really hot fire followed by a rainstorm can change temperature a lot in a hurry);
• Growth of things in cracks (tree roots, various minerals, and especially ice).

Slideshow: Examples of mechanical weathering

Probably the most important mineral that grows in cracks is ice, but others do too. For example, the mineral thenardite, Na₂SO₄ (no, you don’t have to memorize the mineral or the formula!) can add a lot of water to its structure (10 molecules of water for each Na₂SO₄, to make mirabilite, Na₂SO₄·10H₂O, and you still don’t need to memorize the mineral or the formula), expanding in the process. Some pieces of the “dry” mineral, thenardite, may fall into a crack in a dust storm during the dry season, and then change to the much bigger mirabilite during the rainy season as the air gets humid, wedging open the crack. Too much rain may dissolve the mirabilite and move it deeper into the crack where it can lose water during the next dry season and then get wet and expand again, and again… This process is breaking many of the ancient monuments of Egypt as increased irrigation and other activities give seasonal increases in humidity in some places. (The story is even a little more complex than this, but, as shown below, the growth of minerals in cracks really does break rocks!)

Video: Rock Weathering (1:44)

Chemical processes break down rocks, and some of the material dissolves in water and washes away as part of a great, slow cycle.  Here’s a short introduction from Dr. Alley at the Bear Meadows National Natural Landmark, not far from Penn State’s University Park campus. Bear Meadows is one of the few natural wetlands in central Pennsylvania, and has a lot of blueberries and bears as well as interesting geology.

Video: Weathering at Bear Meadows (:49 sec)

Chemical changes are often more interesting and more complex than physical ones. There is a great range of possible changes, and you must know a lot of chemistry to really appreciate all of them. In general, weak acids are the most important. (Strong acids would be most important, except nature doesn’t make large quantities of them!) Rainwater picks up carbon dioxide from the air and becomes a weak acid called carbonic acid. In soils, water may pick up more carbon dioxide plus organic acids from decaying organic material, becoming a slightly stronger but still-weak acid.

When acid attacks a rock, the results depend on what minerals are present, how warm, wet, and acidic the conditions are, and a few other things you don’t need to worry about. We can sketch some general patterns. Suppose we start with granite, a silica-rich rock that forms in many continental and island-arc settings. Granite is fairly common and contains a lot of the commonest elements in the Earth’s crust, so learning about granite gives you insights into weathering of other things. Don’t obsess about learning the details of the minerals we discuss; start by looking for the big picture.

In the image below, you see Penn State graduate Matt Spencer in front of white granite that was intruded into dark metamorphic rock, along Trail Ridge Road in Rocky Mountain National Park. The granite has weathered faster than the metamorphic rock in this environment, so the granite remains only where it is protected by the overhanging metamorphic rock. (These vaguely mushroom-shaped features are called "hoodoos", by the way.)

Video: White Granite Intruded into Metamorphic Rock (5:06 min)

As shown in the close-up picture of a granite boulder below, granite usually is composed of four minerals: quartz (which is almost pure silica, with silica in turn composed of the elements silicon and oxygen), potassium feldspar and sodium-calcium feldspar (mostly silica, with a little aluminum replacing some of the silicon, and potassium, sodium or calcium added for balance), and a dark silica-bearing mineral containing iron and magnesium (often a dark mica called biotite). The eight elements named in this paragraph make up almost 99% of the atoms in the rocks of the crust of the Earth. (Helping living things survive and running our economy requires many other elements that are quite rare in rocks, one reason that geologists are hired to find valuable, rare things and help mine them.)

Close-up view of a part of a granite boulder, also in Rocky Mountain National Park, with Dr. Alley's index finger for scale. The large crystal just above and right of the finger is a feldspar (there are two different kinds of feldspar here, but they look similar). The small gray spots are quartz, and the darker spots are biotite mica. The different mineral grains are identified by Dr. Alley in the video just above, beginning at about 2:00 minutes.

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

When granite interacts with carbonic acid, several things happen. Typically, for most of the minerals in most environments:

• Iron (Fe) rusts. It picks up water and oxygen and remains in the soil as little pieces of rust.
• The aluminum (Al), potassium (K), and silica (SiO₂) from the feldspars and from the dark mineral rearrange into new minerals, called clays, that also include some water.
• The calcium (Ca), sodium (Na), and magnesium (Mg) dissolve in water and wash away.
• Most of the quartz (silica as a mineral) sits there almost unchanged as quartz sand (a little of it may dissolve and wash away, but most stays).

One can write a sort of equation:

Granite → rust + clay + (dissolved-and-washed-away Ca + Na + Mg) + quartz sand

The rust, sand, and clay left behind, plus a little organic material often including worm poop, become the indispensable layer we know as soil. (And, if you have ever tried to drive a car on soft soil during a rainstorm and had your tires sink in and get stuck, you may call the soil “mud”, possibly with some bad words added.)

The calcium and silica that dissolve and wash into the ocean are used by sea creatures to make shells, the dissolved magnesium washed into the ocean often ends up reacting with hot rocks at spreading ridges to make new minerals in the seafloor or goes into some of the shells, and the dissolved sodium accumulates in the ocean to make it salty. (Eventually, the ocean loses some salt, often by the salty water getting trapped in spaces in sea-floor sediments and going down subduction zones to feed volcanoes; evaporation of water in restricted basins also may cause deposition of some salt.)

You should recognize that this is a very general description of what happens; were it this easy, there would not be hundreds of soil scientists working to understand this important layer in which most of our food grows. In general, the hotter and wetter the climate, the more stuff is removed—rust and quartz sand can be dissolved in some tropical soils, leaving aluminum compounds that we mine for use in making aluminum. In dryland soils, calcium and magnesium may be left behind forming special desert soils, or sodium may be left behind forming salty soils in which little or nothing will grow.

You also should recognize that the “chunks” in soil – rust, clay, sand, and organic materials – can be carried away by streams or wind, or glaciers, but as chunks rather than invisible dissolved materials. We discuss this loss of chunks in the next sections. If chunks are carried away more rapidly than new ones are formed, the soil will thin, and we will find it difficult to grow food to feed ourselves (this is what Teddy Roosevelt worried about in the quote at the start of this Module). The chunks eventually are carried to the oceans and deposited as sediment on the seafloor, together with a lot of shells.

Recycling

Granite may form beneath a volcano in a subduction zone. We have just seen that the granite then will begin to break down, making dissolved things and chunks. Eventually, the chunks are carried to the sea, by rivers and glaciers and wind (we will study this transport soon), while the dissolved things also go to the sea where they are turned into shells or other things. Sediment consisting of these chunks and shells, with some of the salty water in the spaces, is then taken down subduction zones to feed volcanoes that make granite. Some of the shells even contain a little carbon, and some dead things containing carbon are buried in the sediments, and some of this carbon is taken down subduction zones and supplies carbon dioxide to the volcanoes with water, helping make carbonic acid that weathers the granite.

If this looks like a cycle, it is! The Earth really does cycle, and recycle, everything! But, going around this loop once takes at least millions of years, and may take a lot longer than that, issues we'll discuss later.

Peaks of the Grand Tetons, Grand Teton National Park, Wyoming.

Credit: Left, Oxbow Bend, Nate Foong, free to use per Unsplash license. Right, R.B. Alley © Penn State CC BY-NC-SA 4.0

The Grand Tetons tower above the valley known as Jackson Hole, Wyoming, providing the epitome of western scenery for many people. A still-active pull-apart fault lies along the front of the range and slopes steeply downward beneath Jackson Hole. From the highest peaks to the fields of the Hole, where elk and moose and bear are common, is well over a mile vertically (roughly 2 km), but the total vertical offset on the fault is almost 6 miles (10 km) (we don’t see this total offset because a lot of rocks have been eroded from the top of the range and deposited in the valley). The uplifted block is primarily old metamorphic rocks that erode only slowly. The faulting is probably related to the Basin and Range extension that also gave us Death Valley, although the complexity of the region makes any interpretation difficult. Dr. Alley recalls huddling next to an overhanging rock, far up on the steep front of the Tetons, watching hailstones rattle off the trail from a black deck of clouds barely over his head. It is a truly awesome place. 

Video: The Gros Ventre Slide (3:10 min)

A few miles (few km) east of the park you can visit another interesting feature: the Gros Ventre Slide Geological Site. There, as shown in pictures and the VTrip below, a mountain-sized ridge is made of rock layers that slope steeply, almost parallel to the north slope of the ridge, down to the Gros Ventre River. Those layers include strong, resistant sandstone resting on weak, slippery shale. The river had eroded down through the sandstone and into the shale, leaving the toe of the sandstone unsupported. In June of 1925, after a particularly wet spring, the entire mountainside let loose, sliding along the soft shale down, across the river, and more than 300 feet up the other side; a rancher and his horse who were on the other side barely escaped safely. The slide mass made a dam, and the river then made a lake many miles long and as much as 200 feet (60 m) deep. The entire slide probably required only seconds to occur and moved cubic miles (many cubic kilometers) of rock. 

Such a dam of loose debris is not very strong; water flowing through its porous spaces or over it can remove rocks and weaken it greatly until it collapses catastrophically. Back in Module 2, in the West Yellowstone VTrip, we saw that an earthquake just northwest of Yellowstone in 1959 caused a similar landslide, which dammed a river to form a new lake, and that the Army Corps of Engineers had rushed in to move massive amounts of debris and prevent a collapse of the dam. The Corps knew how likely and how dangerous such a failure would be, in part because the Corps had not been tasked to act at Gros Ventre in 1925.  In 1927, the dam formed by the Gros Ventre slide failed, washing out a small town downriver and killing six people. The loss of life would have been much larger if more people had lived there.  A few of the people living there were saved when a ranger saw the start of the flood, drove downstream faster than the flood and warned the people to flee.  Unfortunately, not everyone listened.   

Take a tour of Grand Tetons National Park

The Gros Ventre slide. Notice the trees in the foreground, this was a big landslide!

Credit: Gros Ventre Slide by Ealdgyth is licensed under CC BY 3.0

The Gros Ventre slide is an especially dramatic example of an important process that usually is more boring: mass movement. This is the name given to the downhill motion of rock, soil, debris, or other material when the flow is not primarily in wind or in a glacier, or in water (if the material is washed along by a river, we call it a river)

Water is usually involved in mass movement, however, and most mass movements occur when soil or rock is especially wet. Water helps cause mass movement for four reasons: 1) water makes the soil heavier; 2) water lubricates the motion of rocks past each other; 3) water partially floats rocks (a rock pushes down harder in the air than in water) so that the rocks in the water are not as tightly interlocked and can move more easily past each other; and 4) filling the spaces in soil with water removes the effect of water tension.

Number four, above, may deserve a bit more explanation. Think about going to the beach and building sandcastles. Dry sand makes a little pile with sides rising at maybe 30 degrees (steep, but not too steep; see the diagram below). Totally saturated (wet) sand flows easily, forming a pile with a much more gradual slope. But people making sandcastles want damp sand, which can hold up a vertical face. You can even make and throw damp sand balls (be careful where you throw them).

Now watch a demonstration of the process followed by a video explanation.

Video: Mass Movement and Sandcastles Demonstration(0:55 seconds)

Video: Mass Movement and Sandcastles Explanation (2:09 minutes)

The details of the surface physics involved are a bit complicated, but basically, a drop of water will sit at the junction of two sand grains. If you pull the sand grains apart, both grains will end up wet, so you had to “break” the water from one continuous film into two. There is a similarity to a dripping faucet. A water drop doesn’t fall off immediately but first becomes large and heavy. Water molecules stick to each other, and to the faucet, so strongly that they can hold up a large drop of water before it falls. (In situations such as this, the attraction of water molecules for each other is usually called surface tension.) Damp sand thus is strong—a landslide would require some sand grains to move rapidly past other sand grains, breaking the water bonds between the grains. In fully wet sand, however, the grains move more freely in the water without ever breaking it, so motion is easy. Hence, wind can blow dry sand into dunes, damp sand tends to stay where it is, but wet sand flows easily.

Classifications of Mass Movements

There are elaborate classifications of mass movements, depending on how fast, how wet, how coarse, how steep, and how "other" they are. Most of the names make sense: falls are rocks that fell off cliffs, topples are rocks that toppled over from cliffs, landslides, debris flows, and debris avalanches are fast-moving events, and slumps are something like a person slumping down in a chair (failures of blocks of soil along concave-up curved surfaces).

One fascinating and scary type of mass movement occurs in “quick” clays. You can read about these in the Enrichment. Quite literally, in certain places at certain special times, the foundations of a town built on sediments made of certain types of clay may liquefy and flow down the river, killing people. (Most people don’t need to worry about these, though!)

Enrichment

The quick clays that cause large, dangerous landslides generally start off as clay layers deposited rapidly in a shallow ocean, that then is raised above sea level. This often occurs near a melting ice sheet at the end of an ice age. The melting ice dumps a lot of sediment including a lot of clay, and then, as the weight of the ice is removed, the land rebounds above sea level. Clay particles tend to be platy and may look a little like playing cards. When these particles are deposited rapidly in the ocean, the particles may make a house-of-cards structure, with lots of big spaces. The saltwater supplies large ions that sit in the spaces and help hold the “cards” in position, something like little bits of glue helping hold up a house of cards.

After the clay is raised above sea level, rain supplies fresh water that slowly washes out the salt, like removing the glue that was holding up the house of cards. Eventually, a small disturbance may start a collapse, and this tends to make the clay “run away”, failing catastrophically from a solid to a liquid almost instantaneously, and generating a flow.

Flows from such clays are known especially from parts of Canada and Scandinavia. A quick clay failure at Saint Jean Vianney, Quebec in May 1971 destroyed 40 houses and killed 31 people in Canada, and a similar one at Nicolet, Quebec in 1955 killed 3 people. The Norwegian Geotechnical Institute released an amazing report and video about the Quick Clay Slide at Rissa in 1978; this is generally available online, if you search for it, and is truly fascinating. A man with a new (in 1978) camera filmed part of it but then had to run for his life as the slide expanded toward him. (When this was being written, you could find the video on YouTube and elsewhere.)

Sometimes, a quick clay slide will be small and will generate a flow that crosses a road. Bulldozing the clay out of the way does little good; more just flows across. But throwing a bag of salt into the flow near the road and driving a tracked vehicle through to mix the salt and clay may cause the flow to solidify so that it can be bulldozed away.

St. Jean-Vianney's landslide. View of the southwest wall of crater, May 6, 1971.

Credit: NRCan Photo Collection photo 201665

St. Jean-Vianney's landslide. View of the west wall of crater, May 6, 1971.

Credit: NRCan Photo Collection photo 201681

The most important mass-movement type in terms of transferring material downhill is soil creep, the slow (typically inches, or centimeters, per year or less) downslope motion of soil. Creep may be just a very slow landslide. It may occur from freeze-thaw processes—a column of ice that grows under a small pebble on a cold night pushes that pebble out from the hillslope, and the pebble falls straight down when the ice melts, effectively moving a tiny distance down the hill (see the video below). When trees fall over and uproot soil, or when groundhogs and even worms dig up rock grains and allow them to move downhill, creep is occurring. If you look at a typical hill slope, streams on the lower slopes are present to move water and rock downhill, but the upper slopes lack streams. There, soil creep moves the material downhill.

Naturally, hillslopes typically reach a balance, in which weathering breaks down rocks about as rapidly as mass movement and streams take the broken rocks away. The balance may occur with bare rock sticking out (making cliffs, for example), or with a lot of soil covering the rock. If soil creep dominates the mass movement, the hillslope may be close to balance at all times. If landslides dominate, then the soil will build up for a while before suddenly sliding off, and you have to watch for a long time to see the balance. Over a very long time, the hill will usually get flatter, causing the mass movement to slow. However, the soil will very gradually thicken to slow the weathering as the hillslope is reduced, and near-balance will be maintained.

Humans are greatly upsetting this balance worldwide. Our activities—bulldozing, cutting trees whose roots held the soil, plowing, and more—are moving more material than nature moved before we were involved. Landslides are becoming more common, and causing more damage as we build in more dangerous areas. Soil erosion has increased from our farm fields, making it harder for us to feed ourselves. We could slow or reverse many of these damaging trends if we decided to work at it.

Video: Soil Creep/Frost Heave (1:59)

Here is a simplistic diagram. See if you can describe what is happening to a friend and then take a look at some truly amazing landslides from around the globe.

A Slideshow of Landslides Around the Globe

Why the Wind Spins

All large flows on Earth appear to turn along their path, because of the Coriolis effect, which arises because the Earth rotates. You can find serious discussions of the Coriolis effect in any good textbook on meteorology or oceanography. The material in this optional Enrichment article provides a more intuitive version. This isn’t a complete explanation… but we don’t know of a complete explanation that avoids serious math and physics.

The rotation of the Earth is surprisingly fast. If you buckled a belt around the Earth at the equator, the belt would need to be 25,000 miles (about 40,000 km) long. The Earth spins once every 24 hours. This means that the Earth’s rotation is moving a point on the equator at just over 1000 miles per hour (1600 km/hr). In comparison, a point near the pole is essentially stationary (the pole itself stays in place as it rotates). Dr. Alley has walked around the South Pole in three steps, but he couldn’t walk around the equator in a day. If this doesn’t make sense, get a tennis ball or the head of a friendly classmate, draw an equator, and try it out!

Wind with Earth's Rotation

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

If you stand on the equator, the wind is not blowing 1000 miles per hour. This is fortunate indeed because the “mere” 150-mile-per-hour winds occasionally reached in hurricanes can cause immense disasters. Instead, the air near the surface of the Earth is, on average, moving with the surface, dragged along by the mountains and trees and such. You may have seen something similar if you have driven down a highway at 65 miles per hour with a bug stuck to your windshield. The wind at 65 miles per hour surely is strong enough to blow a bug, but the wind very close to the windshield is very much slower than 65 miles per hour—really strong winds don’t blow close to surfaces, because of the friction of the surface, and instead, the air close to the surface does almost the same thing that the surface is doing.

Now, suppose that you watch a parcel of air that rises from the sun-warmed surface at the equator and begins moving towards the North Pole in a convection cell. Once the air has moved many miles north, the Earth under it is no longer rotating at 1000 miles per hour, but is somewhat slower, perhaps 900 miles per hour, and the farther north the air goes, the slower the surface is moving, dropping toward zero at the pole. But aloft, there aren’t trees and mountains to slow the air down from the 1000 miles per hour it had at the equator. Thus, this continues to move at 1000 miles per hour, and “gets ahead” of the Earth beneath. The Earth rotates to the east—you see the sunrise in the east as the rotation of the Earth brings the sun into view—so, the equatorial surface air is moving east at 1000 miles per hour. As this air rises and moves northward over slower-moving land, the wind will appear to turn to the right or east as it blows, getting ahead of the ground. Wind heading south from the equator will also move east ahead of the surface, making a left turn.

Similarly, wind moving from the pole to the equator in the returning limb of a convection cell will lag behind the rotating surface of the Earth, again seeming to turn to the right in the northern hemisphere (and to the left in the southern hemisphere). Thus, the wind cannot go directly to where it “wants” to go; instead, it turns and tends to go in circles. The circular airflow around low-pressure systems and hurricanes occurs because the Earth rotates.

As noted earlier, more-precise definitions are possible of this “Coriolis effect,” which explains why all large flows turn on a rotating Earth. The intuitive explanation given above will fail you if you think of a wind moving due-east or due-west because those winds also turn. Starting from the conservation of angular momentum might be better. Notice, however, that the explanation given above will get the right answer for you, in how much the wind will turn, and which way.

Notice also that Coriolis turning affects large, fast flows, not large slow flows or small ones at any speed. The geological convection deep in the Earth is large, but it is too slow to feel Coriolis much. The difference in rotation speed between opposite sides of a kitchen sink or a bathroom toilet is so tiny that Coriolis turning has no significant effect on the direction that the water swirls as it goes down. Instead, those water swirls are controlled by the design of the sink or toilet, and by any motion in the water at the time the drain was opened; get the water swirling in a sink and then pull the plug, and the water usually will keep swirling in the way you started it as it goes down. Dr. Alley has seen both clockwise and counterclockwise flows in Pennsylvania and Greenland, and in New Zealand and Antarctica.

More Enrichment: Why Cold Air Sinks but Valleys Are Warmer than Mountain Peaks

When air moves up, it expands, which requires that work is done in pushing away other air to make room for the expansion. The work requires energy, which comes from the heat energy in the air, so the rising air cools. Similarly, when air moves down, it contracts as the surrounding, higher-pressure air squeezes the sinking air parcel, and this squeezing is work that is done on the sinking parcel and warms the parcel. If this is happening near the surface of the Earth, and the air is dry, the change in temperature is about 1^oC per 100 m of vertical motion. This applies everywhere, at all times. So, it was complete nonsense in the 2004 movie, The Day After Tomorrow, when huge storms brought air down from above so rapidly that the air didn’t have time to warm up.

As noted in the main text, air can cool by expanding as it rises, but also by losing energy by radiation or by conduction into colder land or water beneath. Imagine that a “chunk” or parcel of air, sitting somewhere on the side of a hill, cools a little by losing energy, perhaps by radiating energy to space as the sun goes down in the evening. Will that air parcel sink now, flowing along the hill into a valley? The answer is that it will sink if, after it has sunk and warmed, it is colder than the air that started out in the valley and had to be displaced as the sinking air arrived.

Consider an example. You measure the temperature of your parcel, add 1^oC for the warming from sinking 100 m, and if your air is still colder than the air it must displace 100 m below, then your parcel will sink. (If you do this really carefully, friction comes in as well—if your parcel plus 1^oC would be only a tiny bit colder than the air it must displace, motion is unlikely; you need a notable difference to overcome the friction and really move.)

Overall, a balanced, stationary atmosphere will cool upward by about 1^oC per 100 m under dry conditions, and somewhat less under wet conditions, as described in the main text. Vertical motions will be triggered when cooling or warming creates air that is anomalously cold or warm relative to this stationary profile. So, cold air on a mountaintop won’t necessarily sink, unless that mountaintop air is colder than you would expect from this profile. But on an October evening in the Appalachians, when fog develops and holds heat in the valleys while the mountaintop radiates heat to space, the mountaintop air will become anomalously cold and sink to the valleys.

A Rocking Review: Somewhere Over the Puddle

If you want another look at the weather system, and the difference between the Redwoods and Death Valley, the Wizard of Odd takes you Somewhere Over the Puddle in this review revue. (The Sierra tops out over 14,000 feet but in most places is lower, so don't let it bother you that the air in the GeoClip went a little higher than the air in this song--both are right, depending on just where the air goes over.)

Video: Somewhere Over the Puddle (2:54 minutes)

Review Module Requirements

You have reached the end of Module 5! Double-check the list of requirements on the Welcome to Module 5 page and the Course Calendar to be sure you have completed all the activities required for this module.

Comments or Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.