The Movement of Soil, Part 2
By Trista L. Pollard
In The Movement of Soil, Part 1, you learned about the processes both human and
natural that cause soil erosion. There are other forces that cause the movement of soil
and rock. Gravity is one of those major forces that pull rock fragments down slopes in
events called mass movements. Mass movements vary in their speed, intensity, and
fragment size. When mass movements occur rapidly, they tend to be the most
destructive and dramatic. Rockfalls occur when rock fragments fall from steep cliffs.
This is the fastest type of mass movement. The fragments may be as tiny as pebbles or
as huge as giant boulders. Landslides occur when large amounts of loose rock
combined with soil fall suddenly down a slope. The steepness of the slope may affect
the amount and the size of the rock material in the landslide. Larger landslides tend to
occur on steeper slopes and involve larger blocks of bedrock. If you live in California or
in areas where highways run through mountains, then you may have witnessed small
landslides. Earthquakes and volcanic eruptions may cause landslides. Less dramatic
events like heavy rainfall and spring thaws may also produce conditions that cause
landslides.
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Rocks and boulders are not the only material involved in mass movements.
Mudflows, which occur in dry mountainous areas, contain large amounts of mud that
move rapidly. Heavy rainfall that happens suddenly and volcanic eruptions can cause
mudflows. The traveling mud churns and tumbles its way down slopes. It continues
through valleys and eventually spreads out in a large fan-shape at the slope's base.
Mudflows are often mistaken for landslides, especially in communities along hillsides.
Heavy rainfall may also cause slumps. Overlying soil that has been over saturated from
rain may lose friction with underlying rock. As a result, large blocks of soil and rock may
break off and slide down the slope in one piece. The block follows along the curved
slope of the surface, slipping over solid rock towards the bottom of the slope. Slumps
usually occur on very steep slopes.
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Solifluction and creep are the slowest types of mass movement. The majority of
the rock material that moves on our planet's surface is moved by slow mass
movements. Solifluction occurs when soil that has been saturated with water slips over
hard frozen layers of subsoil. This type of movement tends to happen in areas where
the subsoil is permanently frozen, like the Arctic or in mountainous areas. Warmer
regions that have subsoils made of hard clay may experience solifluction. Creep is the
slowest of all the mass movements and moves the most soil out of all the mass
movements. The only way to detect soil creep is to observe the fences, buildings, and
other surface objects that may be in that area. As the loosened soil moves slowly
downslope, these objects will move along with the soil. Soil creep may be caused by
water loosening rock particles; plants producing pressure as they grow that wedges in
between rocks and soil; or burrowing animals that loosen rock material as they build
their homes. Constant freezing and thawing may also cause creep. Once the particles
are loosened, gravity takes hold, and the soil slowly moves downhill. Eventually, these
particles may accumulate at the base of the slope forming piles of talus. Talus cones
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contain these particles and will continue to weather into smaller particles. These smaller
particles are then carried by runoff into gullies and other waterways.
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Quick Bit: Are there human activities that may cause soil creep to occur in an area?
The landforms located on our planet's surface take a beating from the agents of
erosion. In fact, it is the erosion process that has formed some of these landforms.
Mountains, plains, and plateaus are shaped by weathering and erosion. Other
landforms like hills, valleys, and dunes are also shaped by these processes. The
composition of the rock determines the shape of the landform. The landforms on our
surface are vulnerable to two opposing forces-- the uplifting of the Earth's crust and the
weathering and erosion on the surface. In its infancy, a mountain goes through uplift
caused by the tectonic forces under the surface. During this time, the mountain rises
faster than it erodes. Uplifted mountains usually have sharp peaks and deep narrow
valleys. The mountains that have more rounded peaks and gentle slopes have stopped
being uplifted and are weathered and eroded at a faster rate. Neither of these
processes happens overnight. It takes millions of years before the effects of uplifting,
weathering, and erosion can be visible. Peneplains are formed by mountains that have
not been uplifted for millions of years. These landforms, which are found in New
England, usually have low, rolling hills. The name peneplain means "almost flat."
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Plains are landforms that are relatively flat and near sea level. They tend to erode at
a slower rate than plateaus, which have a higher elevation and are broad and flat. The
Colorado Plateau, which is located in the southwestern United States, has deep stream
valleys separating regions that are broad and flat. The Catskill region in New York,
however, contains older plateaus that are surrounded by rugged hills and valleys. The
climate and composition of the rock determine the effect of weathering and erosion on
plateaus. If you were to visit an area with a dry climate, you would notice that here
plateaus have flatter tops. These structures were produced from weather-resistant rock.
Over time, these plateaus may erode into smaller tablelike areas called mesas. The
erosion does not stop with the mesas! Weathering and erosion may cause mesas to
turn into small, narrow-topped structures called buttes. Mesas and buttes tend to have
steep walls and flat tops in drier regions. In other areas, these structures may have
more rounded features. So the next time you see one of the majestic landforms on our
planet, think about the amount of weathering and erosion it took to produce these
extraordinary structures.
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Copyright © 2013 edHelper
Name _____________________________
Date ___________________
The Movement of Soil, Part 2
1. What is a rockfall?
2. Based on the composition and
characteristics of the plateaus in the
Catskill region, what can you conclude
about the Colorado Plateau?
3. The most soil transported by a mass
movement is done through ______.
Slump
Mudflow
Solifluction
Creep
4. What are the three major landforms
shaped by weathering and erosion?
What is the opposing force that affects
these landforms?
5. True or False: More rock material is
moved by a greater amount of
catastrophic landslide than slow mass
movements.
6. What important characteristic helps
scientists recognize uplifted
mountains?
7. Piles of rock fragments that
accumulate at the base of slopes are
called ______.
Buttes
Talus
Peneplains
Mesas
8. What is the relationship between
gravity and the movement of rock
fragments?