(WES) Erosion Lesson

Erosion

Erosion happens when a bit of earth material is separated from the surface. Once detached, a force like moving water or wind transports the material to a new location where it is deposited.

Erosion is responsible for some of the earth's most remarkable natural features, like the Grand Canyon. The term "erosion" comes from "erodere," a Latin verb meaning "to gnaw." Erosion is what usually happens to the material loosened by weathering. Plant roots help to hold soil together. When plants and vegetation are not present, erosion can be more dramatic. Erosion can create sandbars, floodplains, and river deltas. Four main processes have an eroding effect:

• gravity
• wind
• water
• ice

Gravity

Gravity is a force that will move material, once it is broken down by weathering, downhill. Gravity is always working on all things on Earth. Gravity pulls materials toward the center of the Earth making rocks fall from mountain tops and sand to settle to the bottom of oceans. It causes mass movement, any one of several processes that move sediment downhill. Mass movement can be rapid or it can be slow. There are different types of mass movement. Read about each below.

Wind

Wind is an effective agent in causing erosion in dry, arid climates like deserts. The main way that wind causes erosion is by deflation.

Deflation is the process by which wind removes surface materials. Wind blows over the land and picks up the smallest particles of sediment. Clay and silt make up this sediment. Stronger winds can pick up larger particles. Some materials that are somewhat heavy might gain lift from strong winds but will appear to almost bounce or skip along. Deflation can move the topsoil in an area leaving only stones and rocky materials.

In the 1930's heightened soil erosion was caused by a combination of natural climatic fluctuations and human disturbance. Barren, windswept fields, buried farmhouses, and rolling tumbleweed attest to the devastation. Read the excerpt from the novel Out of the Dust by Karen Hesse:

 

1. 63 Nightmare

I was coming home through a howling dust storm, my lower face was scrubbed raw by dirt and wind. Grit scratched my eyes, it crunched between my teeth. Sand chafed inside my clothes, against my skin. Dust crept inside my ears, up my nose, down my throat. Shuddered, nasty with dust.

In the house, dust blew through the cracks in the walls, it covered the floorboards and heaped against the doors. It floated in the air, everywhere.

 

 

Farmers had planted wheat before the 1930's. The wheat could not survive without regular water so when the plains experienced massive drought the wheat suffered. When the rain finally came the wheat shriveled and died. Now with no vegetation or plant life, severe drought conditions and flat unblocked expanse there was nothing to stop the wind from picking up the tiny bits of sediment and soil not held to the ground by anything more than gravity. Although the numbers are not known, hundreds if not thousands of Plains residents died from 'dust pneumonia', a euphemism for clogging of the lungs with dirt.

Water

The most common form of erosion is that done by water.

Rain splash erosion is caused by the impact of water striking the surface. Rain splash erosion generally takes place in two steps. As precipitation is absorbed by the surface it fills the pore spaces, loosening soil particles and driving them apart. The impact of other rain drops hitting the surface splash the particle away from the point of impact.

Sometimes the soil in a location can't hold anymore water. Surface runoff forms when the rainfall from a storm is more than the soil can contain. This is when sheet erosion happens. It is caused by the unconfined flow of water running across the surface of the land. The effects of sheet erosion often go unnoticed because such thin layers of soil are being removed. It isn't until several years later that people begin to to notice that a great deal of soil is missing.

Rill erosion is caused by water concentrating into innumerable, closely-spaced small channels. Left unchecked, rills can cut vertically and horizontally and when joined form gullies.  Gullies are steep-sided trenches formed when many rills combine. Once started they are difficult to stop.

Soil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rainsplash or indirectly by rill and gully erosion. Rain may move soil directly: this is known as 'rainsplash erosion' (or just 'splash erosion'). Splash is only effective if the rain falls with sufficient intensity. If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles a short distance. Rainfall may also move soil indirectly, by means of runoff in rills (small channels) or gullies (larger channels, too big to be removed by tillage). In many parts of the world, rill and gully erosion is the dominant form of water erosion.

Running water is the main force of erosion. Rivers, streams, and runoff change the surface of the earth. Runoff is rainwater that flows over the earth's surface. During heavy rains there is a lot of runoff. As runoff flows over the earth's surface, it carries away soil pieces. Runoff empties into rivers and streams. The action of running water has made more changes on the surface than any other force.

When it comes to water - speed makes a difference. Streams that move very quickly will carry more pieces and larger pieces of soil and rock eroded from the bottom and sides of the stream. Streams flow very fast when they go down steep slopes. Streams flow more slowly when they go down gently slopes. Heavy rainfall makes a difference also. During heavy rains, streams may swell with water speeding up the flow. As the flow speeds up it will also increase that water's ability to carry pieces of weathered rock and materials with it.

Summary of Water Erosion

Splash Erosion

The force of falling irrigation or rainwater displaces soil particles.

Sheet Erosion

Impermeable surfaces, compacted soil, or bare soil lets water run across it, washing away disturbed surface particles.

Rill Erosion

Sheet erosion wears down soil to establish a definite path, forming rivulets in the soil referred to as rills. Rill erosion is much more visible to humans than splash or sheet erosion.

Gully Erosion

Over time, rills widen and deepen into a gully, accelerating the effects of erosion by creating more and more surface area susceptible to disturbance.

Bank Erosion

Fast water flows (often caused by influx of stormwater from impermeable surfaces) wear away stream sides at an accelerated pace, often causing bank failure.

Ice

Ice erosion is the process of large chunks of ice, known as glaciers, eroding an area over a long period of time with the help of gravity.

Ice erosion is caused by movement of ice, typically as glaciers. Glaciers can scrape and break up rock and then transport it, leaving moraines, drumlins, and glacial erratics in its wake typically at the terminus or during glacial retreat. Ice wedging is the weathering process where water trapped in tiny rock cracks freezes and expands, causing the breakup of the rock. This can lead to gravity erosion on steep slopes. The scree which form at the bottom of a steep mountainside is mostly formed from pieces of rock broken away by this means. It is a common engineering problem wherever rock cliffs are alongside roads and morning thaws can drop hazardous rock pieces onto the road.

Weathering, Erosion, and Deposition

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