Tearing Down Mountains: Weathering, Mass Movement, & Landslides

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, 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.

Weathering Processes

A garnet in gneiss

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

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.

Mechanical Weathering

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).

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 Weathering

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.)

Penn Stater Matt Spencer in front of white granite that was intruded into dark metamorphic rock, along Trail Ridge Road in Rocky Mountain National Park.

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

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). Small gray spots are quartz, and the darker spots are biotite mica.

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.

The Grand Tetons tower above the valley known as Jackson Hole, Wyoming, provides 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. 

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 the 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 (: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!)

Soil Creep

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 above). 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.

Diagram of soil creep.

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

A Slideshow of Landslides Around the Globe

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)