High School Earth Science/Erosion and Deposition by Gravity

So far in this chapter, you have learned about erosion and deposition by moving water in rivers and the ocean, erosion and deposition by glacial ice and erosion and deposition by wind. With this long list, you may think that we have covered all the types of erosion and deposition that can possibly occur. The force you may have forgotten is gravity! Perhaps because it is a constant force or perhaps because it is invisible, students often forget that gravity also acts to shape the Earth's surface. The examples we will consider here include sudden, dramatic events like landslides, as well as slow steady movements that happen over long periods of time. Whatever the example, we know that the force of gravity will always be there and it is changing the Earth's surface right now.

Lesson Objectives

Describe the ways that material can move downhill by gravity.
Discuss the factors that increase the possibility of landslides.
Describe the different types of gravity driven movement of rock and soil.
Describe ways to prevent and be aware of potential landslides or mudflows.

Gravity Moves Material Downhill

There are several ways that gravity can move material from a higher place to a lower one. Sometimes this happens along a cliff or a very steep slope. Material that has been loosened by some type of weathering simply falls away from the cliff because there is nothing to keep it in place. If you were to keep nudging your notebook towards the edge of your desk, eventually enough of the notebook would be off the desk to cause it to fall. Landslides happen when large amounts of rock suddenly fall down a cliff or mountainside.

Other times gravity simply makes things slide along rather slowly. You may have seen this as a classmate moves further and further down in their seat. It's not a very fast or dramatic movement, like your notebook falling to the floor, but slowly your friend is no longer sitting up straight in the seat. The same thing can happen to rock or even whole parts of a hillside. This might happen over a period of days or even weeks. In the end, the whole area of soil or rock has slid to a lower spot.

The last way that gravity moves material along is when it becomes very wet. Saturated soil flows downhill, often removing trees, homes and bridges that are in the way. To help you understand how water increases the chances of movement, think about playing in sand at the beach. If you were making a sand castle with dry sand, you could not build walls very well. If you add a little bit of water, it helps your walls to stand. A little bit of water helps to hold grains of sand or soil together. However, if you added lots of water, what would happen? Too much water causes the sand to flow quickly away. There are a couple of ways that soil or rock can get very wet and flow. Sometimes this happens if it has been raining for a very long time or if it rains very hard. In the spring, snow and ice begins to melt and much of this water moves into the ground. Springtime is a particularly dangerous time for landslides because there are heavier and more frequent rainfalls at this time of year and it is also the season when snow and ice melt. Extra water in the soil adds more weight to the slope and also makes the grains of soil lose contact with each other, allowing them to flow.

Contributing Factors

There are several factors that increase the chance that a landslide will occur. Some of these we can prevent and some we cannot. Whenever we dig into the base of a slope, this contributes to the likelihood of a landslide. There are many reasons why we might need to do this. We may want to build a house on flat ground, so we level out an area by cutting into a hillside. Roads and railroad tracks also need to be flat and level, so excavation of a slope could be necessary in areas where we travel frequently. This is particularly dangerous when the underlying rock layers also slope towards the area that is cut away or when layers of clay are present. If rock layers slope towards the region that is removed, then the support for those layers is gone and the overlying rocks can slip away, causing a landslide (Figure 10.46).

Figure 10.46: The slope of underlying materials must be considered when making road cuts.

Soils rich in clay are water holding types of soils. If you have ever worked with clay in art class, you know that when clay is wet, it is very slippery. If there is a lot of clay in the soil, the clays hold onto the water when it rains. This slippery clay layer provides an easy surface for materials to slide over.

Figure 10.47: It is important to reinforce a slope that has been cut away in order to prevent landslides.

When construction workers need to cut into slopes for building a home or road, it is important to stabilize the slope to help prevent a landslide (Figure 10.47). Some ways that you may have seen on steep slopes along a highway include building supports into the slope or planting vegetation to keep the soil in place. Trees have deeper roots than grasses, but each type of plant produces benefits for particular areas. It is also a good idea to provide drainage for groundwater so that the slope does not become saturated.

One well known cause of landslides is from the ground shaking. Sometimes the ground shakes from an earthquake, a volcanic eruption or even just a truck going by. We can't control earthquakes or volcanoes and some of the most devastating landslides have been started by these other natural hazards. Skiers and hikers need to be aware of the ways they disturb the snow they travel over or through to avoid setting off an avalanche. Most people buried by an avalanche do not survive, either because they freeze, are crushed by the weight of the snow or are unable to breathe. If you ski or snow board in deep powder, you should carry a small shovel in your backpack and attach a long, red lightweight cord to your waist or carry a GPS radio transmitter to help rescuers locate you in the event of an avalanche.

Types of Movement Caused by Gravity

Figure 10.48: Pieces of rock regularly fall to the base of cliffs and form slopes known as talus slopes.

Mechanical weathering loosens pieces of rock as water seeps into cracks in the rock and freezes. As these rocks fall, they form a big pile of angular rocks at the base of a cliff called a talus slope (Figure 10.48). If you travel along a road or highway through regions such as these, you may see signs warning of the danger along the road side. Sometimes as one rock falls, it hits another rock lower down, which hits another and so on and so forth. This is one way that a landslide or an avalanche can begin.

Landslides and Avalanches

Figure 10.49: Landslides are called rock slides by geologists.

Landslides and avalanches are the most dramatic, sudden and dangerous examples of earth materials moved by gravity. Usually the term landslide is used to mean solid rock that falls suddenly, whereas an avalanche is formed from snow. Most landslides happen along convergent plate boundaries, in regions of the world that are tectonically active. These regions are often mountainous and are places of frequent earthquakes and volcanic eruptions. When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Figure 10.49). Scientists believe that air gets trapped under the falling rocks and acts as a cushion that keeps the rock from slowing down. Landslides and avalanches can move as fast as 200 to 300 km/hour (Figure 10.50).

Figure 10.50: An avalanche of snow moves suddenly and quickly downslope, burying everything in its path.

Landslides are exceptionally destructive. They can bury everything in their path, including entire villages. Some landslides have created lakes when the rocky material dams a river or stream. Often homes are destroyed as hillsides collapse. If a landslide flows into a lake or bay, they can trigger a tsunami. In July of 1958, a landslide of 30.6 cubic meters of rock fell from 914m up on a steep slope at the end of Lituya Bay in Alaska (Figure 10.51). As that large volume of rock suddenly pushed away all the water, a 524m tsunami was formed. The tsunami produced by the landslide knocked down all the trees and vegetation surrounding the bay. In the area directly opposite the landslide, trees at elevations higher than the Empire State Building were scoured off the valley walls. Fortunately, this event happened in an area where very few people were living. Most of the people who witnessed this event were in boats and most of them were able to survive because their boats rode on top of the wave, rather than being smashed by it.

Figure 10.51: This photograph of Lituya Bay in Alaska shows (in light gray) the areas damaged by the tsunami produced by a landslide sent 30.6 million cubic meters of rock into the bay.

Landslides occur often in dry or semi-arid climates in areas with steep slopes or mountains. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is more prone to landslides than many other regions. In areas where landslides are a frequent hazard, communities have put together warning systems, to help people be better prepared. Around San Francisco Bay, the National Weather Service and the United States Geological Survey have a set of rain gauges that monitor the condition of the soil. If soil becomes saturated, the weather service will issue a warning. Earthquakes, which can happen along western California's abundant faults, can also trigger landslides.

Mudflows and Lahars

Mudflows and lahars are also dramatic and dangerous natural hazards produced by the force of gravity (Figure 10.52). Mudflows tend to follow existing stream channels or ravines. Mudflows often occur on hillsides with soils rich in clay and with little sand or gravel. Where there is little rain, there is not much vegetation to hold the soil. That means mud will flow when a large storm produces a lot of rain in a short time. The saturated soils, without plant roots to keep them in place, flow downhill, following river channels, washing out bridges, trees and homes that are in their path.

Figure 10.52: The white areas on the otherwise green mountainsides mark scars from numerous mudflows. Mud deposited by the flow can be seen along the river channels.

Some mudflows are as small as a few meters in length, width and depth. Others can travel for thousands of meters, moving materials tens of meters deep and hundreds of meters wide. On steep slopes, a mudflow might travel very quickly, ending abruptly when it reaches flatter ground. Thicker, more viscous mudflows move over a period of days or even years. The movement could be as slow as several millimeters/day or perhaps several meters/day.

A lahar is a particular type of mudflow that flows down the steep sides of a stratovolcano (Figure 10.53). These explosive volcanoes produce tremendous quantities of ash and dust as they erupt. Snow and ice from the top of the volcano melt, producing floods of meltwater.

Figure 10.53: A lahar is a mudflow that forms from volcanic ash and debris.

This now hot water mixes with volcanic ash to produce exceptionally hazardous flows that move as fast as 60 km/hour. In Columbia, the eruption of Nevado del Ruiz in 1985 produced a lahar that killed more than 23,000 people as it swept over villages and flattened everything in its way. In 1991, a typhoon arrived just after Mt. Pinatubo in the Philippines erupted. The rains soaked the volcanic ash and dust that blanketed the entire region and produced lahars that killed 1,500 people and displaced thousands more from their homes.

Slump and Creep

Fortunately not all types of erosion by gravity cause so many problems. Some less dramatic types of movement, correctly called slump and creep, move earth materials slowly down a hillside. When materials slump down a hillside, they tend to move as a large block along a curved surface. This type of earth movement often happens when a slope is undercut, leaving little or no support for the overlying materials. It can also happen when too much weight is added to an unstable slope. It is very unfortunate when that extra weight comes from building someone's home on a slippery slope. When earth materials slump down a hillside, a crescent shaped scar marks the place they moved from (Figure 10.54). A wise homeowner will look for these crescent shaped scars along surrounding hillsides when considering buying a new home. If they are present, it is a good possibility that earth materials have slipped before.

Figure 10.54: Material that slumps down a hillside often moves as a whole unit, leaving behind a crescent shaped scar.

Figure 10.55: Trees with curved trunks are often signs that the hillside is slowly creeping downhill.

The term creep is used to describe the very gradual movement of soil downhill, because it just barely creeps along. Creep is such a slow way that earth materials move that no human would likely notice. One way to tell that earth materials are slowly moving downhill is to look at the growth of trees. Have you ever seen a tree whose trunk bends almost horizontally to the ground and then grows upwards? If there are many trees growing this way, it is likely that the ground is slowly moving down hill (Figure 10.55). Tilted telephone or power company poles are also signs that this type of motion is occurring.

Lesson Summary

Gravity moves earth materials from higher elevations to lower elevations.
Landslides, avalanches and mudflows are very rapid and dangerous examples of erosion by gravity.
Slump and creep happen slowly but surely, moving material downslope.
Planting trees and vegetation, building retaining walls, and providing good drainage are ways to help prevent this type of erosion.

Review Questions

Name three ways that gravity moves materials. Describe each.
What natural events and human actions can trigger a landslide or avalanche?
What makes landslides and avalanches move at such great speeds?
Compare and contrast a mudflow and a lahar.
Name two ways that soil can move slowly down a slope.
What can people do to help prevent landslides or mudflows?

Vocabulary

avalanche: Mass of snow that suddenly moves down a mountain under the influence of gravity.
creep: Exceptionally slow movement of soil downhill.
lahar: Volcanic mudflow formed when heavy rains or snow and ice melt and combine with volcanic ash and dust.
landslide: Rapid movement downslope of rock and debris under the influence of gravity.
mudflow: Saturated soil that can flow very rapidly or slowly down a slope depending on the viscosity of the flow.
saturated: Soil that has become completely soaked with water.
slump: Downslope slipping of a mass of soil or rock, generally along a curved surface.
talus slope: A pile of angular rock fragments formed at the base of a cliff or mountain.

Points to Consider

Why might someone build a home on top of land where a landslide has happened before?
Could a landslide happen anywhere in the world? What would make it likely or unlikely in your area?
What new technologies might help people to know when a landslide will occur?

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