Erosion

views updated

Erosion

Erosion is a group of processes that, acting together, slowly decompose, disintegrate, remove, and transport materials on the surface of Earth . Among geologists, there is no general agreement on what processes to include as a part of erosion. Some limit usage to only those processes that remove and transport materials. Other geologists also include weathering (decomposition and disintegration). This broad definition is used here.

Erosion is a sedimentary process. That is, it operates at the surface of the earth to produce, among other things, surficial materials. The material produced by erosion is called sediment (sedimentary particles, or grains). A thin layer of sediment, known as regolith, covers most of the earth's surface. Erosion of the underlying solid rock surface, known as bedrock , produces this layer of regolith. Erosion constantly wears down the earth's surface, exposing the rocks below.

Sources of erosional energy

The energy for erosion comes from five sources: gravity, the Sun , Earth's rotation , chemical reactions , and organic activity. These forces work together to break down and carry away the surficial materials of Earth.

Gravity exerts a force on all matter , Earth materials included. Gravity, acting alone, moves sediment down slopes. Gravity also causes water and ice to flow down slopes, transporting earth materials with them. Gravity and solar energy work together to create waves and some types of ocean currents. Earth's rotation , together with gravity, also creates tidal currents. All types of water movement in the ocean (waves and currents) erode and transport sediment.

Solar energy, along with gravity, produces weather in the form of rain, snow, wind , temperature changes, etc. These weather elements act on surface materials, working to decompose and disintegrate them. In addition, chemical reactions act to decompose earth materials. They break down and dissolve any compounds that are not stable at surface temperature and pressure . Organic activity, by both plants and animals, can also displace or disintegrate sediment. An example of this is the growth of a tree root moving or fracturing a rock.

Erosional settings

Erosion can occur almost anywhere on land or in the ocean. However, erosion does occur more rapidly in certain settings. Erosion happens faster in areas with steep slopes, such as mountains , and especially in areas where steep slopes combine with flowing water (mountain streams) or flowing ice (alpine glaciers ). Erosion is also rapid where there is an absence of vegetation, which would stabilize the surficial materials. Some settings where the absence of vegetation helps accelerate erosion are deserts, mountain tops, or agricultural lands. Whatever the setting, water is a more effective agent of erosion than wind or ice—even in the desert .

Weathering

The first step in erosion is weathering. Weathering "attacks" solid rock, produces loose sediment, and makes the sediment available for transport. Weathering consists of a number of related processes that are of two basic types: mechanical or chemical.

Mechanical weathering

Mechanical weathering processes serve to physically break large rocks or sedimentary particles into smaller ones. That is, mechanical weathering disintegrates earth materials. An example of mechanical weathering is when water, which has seeped down into cracks in a rock, freezes. The pressure created by freezing and expanding of the water breaks the rock apart. By breaking up rock and producing sediment, mechanical weathering increases the surface area of the rock and so speeds up its rate of chemical weathering.

Chemical weathering

Chemical weathering processes attack the minerals in rocks. Chemical weathering either decomposes minerals to produce other, more stable compounds or simply dissolves them away. Chemical weathering usually requires the presence of water. You may have noticed during a visit to a cemetery that the inscription on old marble headstones is rather blurred. This is because rainwater, which is a weak acid, is slowly dissolving away the marble. This dissolution of rock by rainwater is an example of chemical weathering.

Chemical weathering results in the formation of dilute chemical solutions (minerals dissolved in water) as well as weathered rock fragments. Chemical weathering, along with biological activity, contributes to the formation of soils. Besides surface area, both the temperature and the amount of moisture present in an environment control the rate of chemical weathering. Chemical weathering usually happens fastest in warm, moist places like a tropical jungle, and slowest in dry, cold places like the Arctic.

Agents and mechanisms of transport

Transport of sediment occurs by one or more of four agents: gravity, wind, flowing water, or flowing ice. A simple principle controls transport; movement of sediment occurs only as long as the force exerted on the sediment grain by the agent exceeds the force that holds the grain in place (friction due to gravity). For example, the wind can only move a grain of sand if the force generated by the wind exceeds the frictional force on the bottom of the grain. If the wind's force is only slightly greater than the frictional force, the grain will scoot along on the ground. If the wind's force is much greater than the frictional force, the grain will roll or perhaps bounce along on the ground. The force produced by flowing air (wind), water, or ice is a product of its velocity .

When gravity alone moves rocks or sediment, this is a special type of transport known as mass wasting (or mass movement). This name refers to the fact that most mass wasting involves a large amount of sediment moving all at once rather than as individual grains. Along a highway, if a large mass of soil and rock from a hillside suddenly gives way and rapidly moves downhill, this would be a type of mass wasting known as a landslide. Mudflows and rockfalls are two other common types of mass wasting.

Products and impacts of erosion

As already mentioned, sediment grains and chemical solutions are common products of erosion. Wind, water, ice, or gravity transport these products from their site of origin and lay them down elsewhere in a process known as deposition. Deposition, which occurs in large depressions known as basins, is considered to be a separate process from erosion.

Soil is also an erosional product. Soil is formed primarily by chemical weathering of loose sediments or bedrock along with varying degrees of biological activity, and the addition of biological material. Some soil materials may also have undergone a certain amount of transport before they were incorporated into the soil.

Another important product of erosion is the landscape that is left behind. Erosional landscapes are present throughout the world and provide some of our most majestic scenery. Some examples are mountain ranges such as the Rocky Mountains, river valleys like the Grand Canyon, and the rocky sea cliffs of Northern California. Anywhere that you can see exposed bedrock, or where there is only a thin layer of regolith or soil covering bedrock, erosion has been at work creating a landscape. In some places one erosional agent may be responsible for most of the work; in other locations a combination of agents may have produced the landscape. The Grand Canyon is a good example of what the combination of flowing river water and mass wasting can do.

In addition to producing sediment, chemical solutions, soil, and landscapes, erosion also has some rather negative impacts. Two of the most important of these concern the effect of erosion on soil productivity and slope stability.

Soils are vital to both plants and animals. Without soils plants cannot grow. Without plants, animals cannot survive. Unfortunately, erosion can have a very negative impact on soil productivity because it decreases soil fertility. Just as erosion can lead to the deposition of thick layers of nutrient rich material, thereby increasing soil fertility, erosion can also remove existing soil layers. Soil forms very slowly—a 1-in (2.5-cm) thick soil layer typically takes 50-500 years to form. Yet an inch of soil or more can be eroded by a single rainstorm or windstorm, if conditions are right. Farming and grazing, which expose the soil to increased rates of erosion, have a significant impact on soil fertility worldwide, especially in areas where soil conservation measures are not applied. High rates of soil erosion can lead to crop loss or failure and in some areas of the world, mass starvation. On United States farmland, even with widespread use of soil conservation measures, the average rate of soil erosion is three to five times the rate of soil formation. Over time , such rates cut crop yields and can result in unproductive farmland.

Erosion is also a very important control on slope stability. Slopes, whether they are small hillsides or large mountain slopes, tend to fail (mass waste) due to a combination of factors. However, erosion is a significant contributor to nearly all slope failures. For example, in California after a drought has killed much of the vegetation on a hillside, the rains that signal the end of the drought lead to increased erosion. Eventually, due to the increased erosion, the slope may fail by some type of mass wasting, such as a mudflow or landslide.

Controls on erosion

The average rate of erosion at the surface of Earth is about 1 in (2.5 cm) per thousand years. However, the rate of erosion varies tremendously from place to place. Soil erosion in some areas exceeds one inch per year—one hundred times its rate of formation. This range in rates is dependent on several different controlling factors. These factors include the type and amount of plant cover and animal activity, the climate, the nature of surface materials, the slope angle , and human land use . However, many of these factors routinely help increase erosion in some ways, while decreasing it in others. In addition, a complex interplay between the different factors may exist. For example, a particular combination of surficial materials and plant cover may accelerate erosion in one climate, while decreasing it in another. The individual controls can be difficult to recognize and their effects difficult to discern as well.

Vegetation

Generally, plants tend to secure and stabilize sediment, but they may also be instrumental in helping to weather bedrock (for example, by prying open cracks during root growth). Animals may increase erosion by loosening soil, but they can also help stabilize it. An earthworm's sticky slime, for example, increases soil particle cohesion and helps the particles resist erosion.

Climate

As was mentioned above, warm, moist climates increase the rate of weathering and so speed up erosion as well. However, the plant cover in this setting usually helps decrease soil loss. Deserts tend to be very susceptible to erosion due to the limited amounts of vegetation. Fortunately, the low rainfall characteristic of deserts helps to limit erosional effects.

Surface material

Bedrock is more resistant to erosion than are sediments and soil. However, bedrock does display a range of susceptibility to erosion due to the different types of rock that may be present. Here again, the type of climate can have a major impact on erosion rates. In the desert, nearly all types of bedrock are very resistant to erosion, whereas in the tropics, nearly all types of rock weather rapidly.

Slope angle

The angle of a slope is one of the few consistent controls on erosion. The steeper the slope, when all other factors being equal, the more susceptible the slope will be to erosion.

Land use

Agriculture increases the likelihood of erosion by exposing soil to wind and rainfall. However, agriculture is not the only human land use that increases the likelihood of erosion. Logging, construction, landscaping, as well as many other activities make land more susceptible to erosion. Generally, any land use or activity that disturbs the natural vegetation or involves a change in slope, surface materials, etc., will increase the likelihood of erosion. There are some obvious exceptions, though. For example, pavement can temporarily halt erosion in almost all cases. However, nothing resists the erosive power of nature forever—all man-made structures will eventually weather and then fail.

Erosion and rejuvenation

Studies of erosion and the landscapes it leaves behind have been going on for over a century. This area of geologic inquiry, known as geomorphology, has long recognized that a balance exists between the erosion of land and its rejuvenation. If this were not the case, after a few tens to hundreds of millions of years, the earth's mountains would wear down to flat, relatively featureless plains and the basins would fill up with the sediment shed by the mountains. Instead, after billions of years of erosion, we still have mountains such as Mt. Everest in the Himalayas, which stands over 5.5 mi (8.8 km) above sea level , and ocean trenches such as the Marianas Trench, which reaches depths of more than 6.5 mi (10.4 km) below sea level.

The continued existence of rugged landscapes on the face of the earth is a result of a process of rejuvenation known as plate tectonics . Forces within the interior of the earth periodically re-elevate, or uplift , the earth's surface in various regions, while causing the lowering, or subsidence , of other regions. Plate tectonics therefore serves to maintain existing landscapes or build new ones. Currently, the Himalayas are an area of active uplift, but someday uplift will cease and erosion will slowly, but completely, wear them down. Someday the Marianas Trench may be filled in with sediment deposits. At the same time, new dramatic landscapes will be forming elsewhere on the Earth.

Erosion research

Research continues to focus on the factors that control erosion rates and ways to lessen the impact of land use on soil productivity. New methods of soil conservation are continually being developed and tested to decrease the impact of soil erosion on crop production.

Conventional tillage techniques leave fields bare and exposed to the weather for extended periods of time, which leaves them vulnerable to erosion. Conservation tillage techniques are planting systems that employ reduced or minimum tillage and leave 30% or more of the field surface protected by crop residue after planting is done. Leaving crop residue protects the soil from the erosive effects of wind and rain. Direct drilling leaves the entire field undisturbed. Specialized machines poke holes through the crop residue, and seeds or plant starts are dropped directly into the holes. No-till planting causes more disturbance to the crop residue on the field. Using this technique, the farmer prepares a seedbed 2 in (5 cm) wide or less, leaving most of the surface of the field undisturbed and still covered with crop residues. Strip rotary tillage creates a wider seed bed, 4-8 in (10-20 cm) wide, but still leaves crop residue between the seed beds. Conservation tillage techniques are particularly effective at reducing erosion from farm lands; in some cases reducing erosion by as much as 90%.

Other erosion research is focused on the factors that control mass wasting, especially where it is hazardous to humans. Stabilization of slopes in high risk areas is an increasingly important topic of study, as more people populate these areas every day.

Resources

books

Dixon, D. The Practical Geologist. New York: Simon & Schuster, 1992.

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Leopold, L.B. A View of the River. Cambridge: Harvard University Press, 1994.

Woodhead, James A. Geology. Boston: Salem Press, 1999.

periodicals

Reganold, J. P., R. K. Papendick, and J.F. Parr. "Sustainable Agriculture." Scientific American (June 1990): 112-120.

Clay Harris

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bedrock: —The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.
Chemical weathering: —The decomposition and decay of Earth materials caused by chemical attack.
Deposition: —The accumulation of sediments after transport by wind, water, ice, or gravity.
Geomorphology: —The study of Earth's land forms and the processes that produce them.
Mechanical weathering: —The break up or disintegration of earth materials caused by the creation and widening of fractures. Also known as physical weathering.
Regolith: —A thin layer of sediment that covers most of the earth's surface. Erosion of the underlying solid rock surface produces this layer.
Slope stability: —The ability of the materials on a slope to resist mass wasting.
Soil productivity: —The ability of a soil to promote plant growth.
Surficial material: —Any type of loose Earth material, for example sediment or soil, found at the surface of Earth.