Soil Erosion Help (page 3)
Erosion converts soil into sediment. Chemical weathering produces clays on which vegetation can grow. A mixture of dead vegetation and clay creates soil which contains necessary minerals that plants need for growth.
Soil exists as a layer of broken, unconsolidated rock fragments created over hard, bedrock surfaces by weathering action.
Most geologists talk about soil as being part of three layers called soil horizons or soil zones . These three soil horizons are commonly recognized as “A,” “B,” and “C,” but it is important to remember that not all three horizons are found in all soils. Figure 15-4 illustrates the way soil horizons are stacked on top of each other.
Soil horizons are described from the top soil layer down to the lowest soil and bedrock level and are as follows:
- “A” horizon includes the surface horizon, a zone of leaching and oxidation, where penetrating rain water dissolves minerals and carries the ions to deeper horizons. It also holds the greatest amount of organic matter.
- “B” horizon describes the middle horizon, a zone of accumulation, where ions carried down by infiltrating rain water are reconnected to create new minerals. Blocky in texture it is made up of weathered rock mixed with clay, iron, and/or aluminum.
- “C” horizon includes the bottom horizon, a zone of unconsolidated, weathered original rock.
Just as there are three different soil horizons, there are also several factors that determine which type of soil will form. These include structure, rainfall (lots or little), solubility, temperature (hot or cold), slope (gentle or steep), vegetation (types and amount), and weathering time (short or long.) Singly or in combination, soils form as a result of many different factors. A key factor in naming major soil types is rainfall amount. Everyone from toddlers making mud pies to petroleum geologists looking for oil can tell whether a soil is wet or dry, hard or soft.
Geologists have named three basic soil types based primarily on water content. These are the pedocal , pedalfer , and laterite .
The pedocal is found in dry or semi-arid climates with little organic matter, little to no leaching of minerals and is high in lime. Most nutrient ions are still present. In places where water evaporates and calcite precipitates in the “B” horizon, a hard layer called the caliche or hardpan , is formed. Pedocal soil also collects in areas of low temperature and rainfall and supports mostly prairie plant growth.
Pedalfer soil is found in wetter environments and contains greater amounts of organic matter and leaching. It is enriched with aluminum and iron after many other soluble nutrients are leached out. This type of soil is found in areas of high temperatures and humid climates with a lot of forest cover.
Laterite , the soggiest type of soil, is found in tropical and sub-tropical climates and is high in organic matter. Because of high equatorial rainfall in very wet climates, there is widespread leaching of silica and all soluble nutrients. Iron and aluminum hydroxides are left behind and cause well-drained laterite soils to be red in color. Besides iron and aluminum ores, laterites can also form manganese or nickel ores.
Regolith And Slopes
Regolith is a collection of many different soil and rock types. It’s the loose rock matter, like volcanic ash and wind-driven deposits that are scattered around on bedrock.
All rock surfaces, except for the super steep and the fairly new (geological time) are covered by a layer of weathered material. Generally, the growth of plants in a specific area contributes to soil development by holding it in one place. This allows the soil that builds up on a rock to take place in a layered way.
At the deepest level is bedrock, with different types of overlying regolith and finally a scattering of soil on top. This is shown simply in the series below.
bedrock regolith residual regolith transported regolith soil
Of course, depending on the region, this soil progression might not all be visible. But just knowing how it all stacks up can give you an idea of what is missing in any one rock formation. Table 15-2 describes what each layer in the soil development column contains.
top layer of regolith (1–3 m) mixed with organic matter
sediment transported and laid down by erosion (water current, waves, wind, ice) and mass wasting
weathered rock from settled lower bedrock
layer of weathered rock
solid unchanged rock
In general, mass wasting doesn’t happen with a lot of flow-type movement. It moves material a fairly short distance, compared to the longer distances that sediments are carried by rivers.
Mass wasting describes the slow or sudden movement of rock downslope as a result of gravity.
Mass wasting has several factors that affect it. These include gravity, types of soil and rock, physical properties, types of motion, amount of water involved, and the speed of movement.
Gravity is the main influence on mass wasting. It is always pulling things down. When rocks are piled on a steep mountain slope, there is a high amount of friction that holds them to the slope. However, gravity is pulling the rock downward. The downslope pull of gravity that causes mass wasting is known as shearing stress . The steeper the slope, the greater the shearing stress. Figure 15-5 illustrates the difference of slope angle on shearing stress.
The counteracting force that works against shearing stress is friction or with a large body of rock, it is called shear strength . When the amount of shear stress is higher than the shear strength, something has got to give. A quick movement like an earthquake acts as a trigger. It provides just enough energy to overcome the last bit of friction and allow gravity to pull everything down.
There are several different mass wasting rock movements.
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.
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.
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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