Soil Erosion Help (page 2)
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.
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