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Soil Erosion Help (page 3)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Rock Falls And Slides

Rock slides and falls with little to no loose flowing material. Think of it more like huge, shifting boulders than a bunch of loose, flowing mud and pebbles. Giant boulders or sections of earth shift position because of shear stress and move as a whole down a slope.

A rockfall takes place when large amounts of rock free fall or shift downward from very steep areas of a mountain slope.

Generally, a rockfall happens along the sides of a steep mountain, with little plant life, but can also involve cliffs, caves, or arches. Small rockfalls are fairly common, but huge rockfalls are rare because of the amount of force needed to move tons of rock. Small rockfalls take place mostly because of weathering, while earthquakes use their energy to cause large and small rockfalls.

Talus is the pile or cone-shaped mound of broken, blocks of rock that gather at the foot of a cliff or steep mountain edge from rockfalls. When there are massive amounts of broken rock below a deep scar, geologists know that this is a result of a major sudden rockfall or many smaller rockfalls. Figure 15-6 shows how talus collects at the base of a cliff. A good example of this type of large, blocky talus pile can be seen at Devil’s Postpile National Monument in the western United States in California.

Weathering and Topography Rock Falls And Slides

Fig. 15-6. A talus pile builds up at the base of a cliff from rockfall rubble.

Rock and soil can experience a rockslide or landslide when the moving rock slips on an underlying layer, like sedimentary rock, and moves in a solid sheet downward. Again, there is little loose flow.

A rock slide takes place when a fairly solid chunk of rock or soil slides downward along a clearly visible surface or plane.

Slides take place because of the buildup of: (1) internal stress along fractures; (2) undercutting of clay layers and slopes by water (rivers and glaciers); and (3) earthquakes. Rock slides form deep, wide scars and a massive pile of highly broken debris (talus). Although rockslides are fairly rare, when they do happen, they can move at speeds of up to 150km/hr.

When an area of rock or soil moves a fairly short distance, it is sometimes called a slump . A slump of soil can rotate a bit along a slippage plane. Although the plants and soil on top may not move much, the entire area can turn slightly as it slips down the slope. When a section of soil breaks away and slumps downward from a cliff, the original site of the break is called a scarp . Figure 15-7 shows an area of slump below a scarp.

Weathering and Topography Rock Falls And Slides

Fig. 15-7. When rock slumps down a valley, it forms a scarp where it breaks away.

Rock Flow

When rock flow takes place, there is usually some amount of water involved. This includes soil flow, snow avalanches, and pyroclastic flows from volcanic eruptions that we saw in Chapter 11. However, any disorganized flow is considered a rock flow .

When soil is the main flowing substance, it is called an earthflow and involves less water than mudflows. Earthflows happen along low-angle, less stable parts of a cracked slope that contains some amount of clay in its makeup. These are not too dangerous because they are slow, but earthflows can destroy property.

When the ground becomes saturated with water, it’s called a mudflow . Mudflows are often seen in deserts and dry environments during heavy rains following a dry period, when the soil becomes saturated. After a mudflow, a layer of debris is smeared across the valley floor. It can happen in minutes or an hour. It’s dangerous because of its speed and the fact that it carries mud, rock, boulders, and whatever else is flowing (like buildings) down valleys or steep foot hills. Mud flows usually follow an old stream route. When completely soaked, everything flows together in a single, swift gush of water, soil, rocks, plants, and trees. This is known as a debris avalanche.

People who live in fairly dry climates like southern California make the mistake of building homes and businesses on hills or cliffs overlooking the ocean or some scenic view. This is fine as long as the weather stays dry, but weeks of rain can cause mud slides that undercut the building and cause supporting soil to slide down the hill. Besides being dangerous and costly, this hazard could be easily avoided if houses were only built on solid rock.

The slow flowing movement of soggy soil from higher ground to lower ground is called solifluction . Soil flow can take place in the thawed soil layer covering permafrost. It is limited to tundra beyond the tree line and is affected by the summer sun’s heat. When the top soil layer melts and begins moving over the solid permafrost below, then there is solifluction.

Soil creep is the most common mass wasting process; it takes place on almost all land surfaces and slopes. The main difference between other types of movement and creep is rate of movement. Creep is extremely slow. It causes the land to be deformed from its original shape, but is too slow to cause shear problems like in a landslide.

Soil creep takes place as a very slow (less than 1 cm/year) downhill movement of soil and regolith.

Soil creep can be seen most easily on steep, bare, wet slopes. Depending on how damp the soil is, creep can move along as fast as 5 cm/year. A lot of different conditions can cause creep. Some of these include: drying and wetting, heating and cooling, freezing and thawing, walking and burrowing by animals, and shaking by earthquakes.

When you can see sedimentary layers curving gently downhill along with tilted fences, poles, and walls, it can be caused by soil creep.

Practice problems of this concept can be found at: Weathering and Topography Practice Test

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