Forces of Erosion LESSON


    Erosion is the movement of rocks, sediment and soil from one location to another by the forces of nature: wind, gravity, glaciers and water.

    We will study water erosion next week. This lesson will be about gravity, glacier and wind erosion.

    Erosion is the moving of sediments. Where the sediments get dropped off, that is, where
    they are deposited is call deposition.


    Erosion by Gravity
    When a large mass of land moves either quickly or slowly that is a mass movement caused by gravity. Rocks and mud and large sections of earth can move by gravity pulling it down hill or down further into the ground. This is called Mass Movement (or Mass Waste). Here are some examples.

    Slump, a large section of ground on a hill or mountain slumping down to a lower elevation.



    Creep, a large section of ground slowly creeping downhill and the ground bends the fences and trees in the direction of the creeping land.



    Mudflow a large section of wet ground (muddy) that is broke loose by it's increased weight (with all the water in the mud) and suddenly and dangerously flows down hill destroying anything in its path like houses.

    A mudflow happens when the soil is soaked with water like during or after a rain.
    The water is heavy and increases the weight of the top layers of soil. The water
    also makes the soil loose from the bedrock below it and so gravity just takes
    over and the muddy earth slips down the mountain.



    Rockslides and Rock falls are situations where rock after being weathered for a long time will finally fall off the face of a cliff or start rolling down a steep hill side due to mild earth tremor. Rock slides and falls can be very dangerous to whatever they may hit on the way down the mountain or hill side.




    Erosion by Glaciers
    Glaciers are so enormous and massive that we can hardly imagine how heavy they are, yet though they are beyond imagination to sense their weight they do move like a slow river of ice. When a big mass like a glacier moves nothing can stop it. A moving glacier is really a moving, flowing river of ice.

    There are two kinds of glaciers: sheet glaciers and valley glaciers. Sheet glaciers are also called continental glaciers because they cover continents.




    Today there are sheet glaciers, one over Greenland and the over Antarctica (South Pole).

    Many thousands of years ago a sheet glacier covered Canada and where the northern states of the United States are today. When a glacier stops moving forward and starts to melt away the glacier is retreating.

    As the glacier moves it is so massive that it literally carves deep grooves into the bedrock below it. Glaciers pluck out large rocks and scrapes up the soil and sediment and rocks it passes over like a huge bulldozer. This movement of the sediment with the glacier is
    glacial erosion    .



    When the glacier has advanced as far as it can and starts to melt back all the rocks and sediments it has been pushing/carrying is deposited right there at the furthest point it traveled to. The glaciers are so strong that they pluck out large boulders easily and carry them along. As glaciers scour over bedrock they cut out grooves and striations in the rock. Grooves are deeper cuts into rock than are striations. But both grooves and striations show the direction that the glacier moved in. See the picture below.

    Glaciers leave behind Till through the melting water: Outwash
    All the sediment glaciers leave behind when they melt away is called till. Till, the remaining sediment a glacier leaves is spread out over a great area. Till later becomes plains and fertile soil, like we see in the Midwestern states. All the water that melts from the glacier spreads out and carries some of the rock and sediment with it. That is called outwash. Washout is the water from the melting glacier pouring out upon the land and carrying with it the till, dirt, sand silt and rocks. The heavier rocks will be deposited first and then the sand in silt will travel a further distance from the melting glacier. Today the sand and gravel that glaciers deposited thousands of years ago is use to build roads and for construction. Below is a picture of Till left behind by a glacier.

    In valley glaciers the till is called moraine instead and is not as much in quantity as a sheet glacier because it is a valley glacier is smaller than a sheet glacier. The moraine (sediments) that a valley glacier melts is the sediments that were at the tail end of the glacier. The moraine is left behind as a large accumulation of rocks and can look like a hill of sediment.

    Valley glaciers come off the top of very high mountains. They follow the curvature of the mountain similarly to a stream of water, but they carve out a large valley shape as they go. The shapes of valleys that are created by a glacier are U-shaped. The shape of valleys carved out by water rivers are V-shaped. Below you see a U-shaped valley carved out by a valley glacier.



    The speed of a glaciers movement
    Glaciers move at a speed of from one foot a day to up to 50-60 feet a day. The speed of glaciers vary from day to day. The bottom of a glacier may move slower than the top. The speed depends on the slope it is traveling, the roughness of the terrain it is traveling over, the height of the glacier. If a new snow falls and the glacier gains 2 feet of height and is now heavier it may move faster for a while and then slow down again.


    Wind Erosion
    The wind will cause both weathering and erosion. When the wind blows there is sand particles in it, those sand particles will hit existing rocks and will act like an abrasive force brushing off tiny particles from the rock. This is similar to a sand blaster that can aggressively remove paint from a car. The very unusual rock forms you see in deserts are created by the wind carving out these beautiful shapes.



    But the same wind that can weather rocks will also carry away sediment to another location. Sand and very fine sand called silt and clay will fly through the air on a windy day and be deposited elsewhere. This is called wind erosion.

    Deposition refers to the fact that eventually the force of erosion that is carrying sediments will eventually deposit the sediment somewhere. That is, gravity, glaciers and wind all deposit the sediments they are carrying. That is called deposition.

    Sandstorms are an extreme example of wind erosion. Dust storms are the same thing as a sand storm but a dust storm has smaller, lighter, particles of dust and silt in it so the dust will fly higher into the sky.

    Soil is comprised of smaller grains particles of silt and clay. Dry soil has smaller grains than say the sand on a beach.  Dust storms pick up this dry silt and clay particles. A sandstorm picks up the bigger sand particles.



    Sandstorms and dust storms can be dangerous as it can be very hard to breathe in if caught in such a storm, and people will get sand in their lungs and possibly die. In the 1930’s there were severe dust storms all over the Midwest that forced farmers and residents to flee to the east or west coast. Many people has severe lung problems from the dust that devastated their homes.



    Deflation
    Wind will blow away the sand and silt right off the land and leave the land barren of soil and unable to farm.  Land that does not have vegetation on it can have it’s sand blown away. When the wind blows the sand away from the underlying bigger or coarser rocks it is called deflation. Below is a picture of deflation. The sand has been blown away.

    Protect the soil with vegetation
    We know know that soil will not be blown away if the land has vegetation on it. Grasses especially will hold the soil down when the wind blows.

    Wind velocity and duration, the size of sediment particles, the size of the area subjected to the wind, and the amount of vegetation present all affect how much soil is eroded by wind.

    Here is a picture of land purposes planted with vegetation of protect the soil from the wind.





    Sand Dunes
    When the sand blowing in the wind hits an object the sand will pile up around it. Eventually the sand builds up so high a sand dune is created. Sand dunes can travel as the sand is still being pushed and blown by the wind in a direction.