Chapter 8 Erosional Forces

Erosion and Deposition Erosion – The wearing away and moving of surface materials by gravity, water, wind or glaciers Deposition is when agents of erosion drop their sediments as they lose energy

Gravity Gravity is the force of attraction that pulls all objects to the center of earth Gravity slowly moves sediment along, trapped in ice.

Gravity A mass movement is any type of erosion that happens as gravity moves materials down slope. Some are slow, others happen quickly.

Gravity Slump – material slips down a curved surface as one large mass Sediments slowly shift downhill in the process of creep Rockfalls and rock slides occur when rocks break off or slip suddenly down a hill A mudflow is a thick mixture of water and sediments flowing downhill

Gravity Consequences of erosion – buildings on slopes eventually have problems due to erosion by gravity Sometimes builders and residents make slopes more unstable by making them steeper. Another source of instability is the removal of vegetation. Steep slops can be made safer with vegetation, drainage pipes, and walls of concrete or railroad ties.

Gravity Define the term erosion and name the forces that cause it. Explain how deposition changes the surface of the Earth. What characteristics do all types of mass movement have in common? Describe ways to help slow erosion on steep slopes. Explain how to control the effect man has had on erosion due to building houses or roads. How do slump and creep differ?

Glaciers: Glaciers - A large mass of ice and snow moving on land under its own weight. When snow does not melt, it piles up. The increasing weight becomes great enough to compress the lower layers into ice. When enough pressure builds up, the lower layers of ice become plastic like and the mass begins to move away from its source.

Glaciers As glaciers move, they pick up boulders, gravel, and sand in an erosion process called plucking Plucked rocks at the base of the glacier scour the soil and bedrock Dragged rock fragments leave scars on the bedrock called grooves Striations are shallower scars on the bedrock Grooves and striations indicate the direction a glacier moved

Glaciers As glaciers retreat, they leave behind till, a mixture of different sized sediments Till areas include wide swaths of farmland from Iowa to Montana, and Ohio to Illinois A moraine is a ridge, or deposit left at the end of a glacier Outwash – material deposited in layers by the meltwater of a glacier, with largest pieces closer to the glacier An esker is a type of outwash deposit formed as meltwater rivers within the ice deposit and gravel within their channels

Glaciers Continental glaciers – huge masses of ice and snow now covering only about 10% of the Earth in areas near the poles; in the past, as much as 28% of the Earth was covered by glaciers Periods of widespread glaciation over the last 2 million to 3 million years are known as ice ages The average air temperature on Earth was about 5o C lower during ice ages than today The last major ice age was about 18,000 years ago.

Glaciers Valley glaciers – exist in mountains Cirques are bowl-shaped basins in the sides of mountains, created by valley glaciers A long ridge or arete forms when two valley glaciers erode a mountain side-by-side A horn forms when valley glaciers erode a mountain from several directions Glacially eroded valleys have a U shape, as opposed to a V shape left by a stream

Glaciers Glaciers have changed, and continue to change, the shape of the Earth’s surface; sand and gravel deposits left by glaciers are important resources for the construction of roads and buildings

Glaciers How do glaciers move? Describe two ways in which a glacier can cause erosion. Till and washout are glacial deposits. Explain how they differ. How do moraines form? What are moraines made of? What do you think might cause the milky blue coloring of rivers and streams that receive glacial waters?

Wind When air moves, it picks up loose light material and transports it to other places. Air differs from other erosional forces because it cannot pick up heavy sediments. Wind carries and deposits sediments over large areas.

Wind Wind Erosion – can scatter dust or volcanic as over thousands of kilometers Deflation – wind removes small particles of loose sediment, such as silt and sand, leaving behind heavier materials. Wind behaves like a sandblaster blowing sand grains against rocks wearing them down and pitting them in the process of abrasion. Deflation and abrasion happen most often in areas where there is little vegetation to hold sediments in place.

Wind When strong winds blow in the deserts, beaches, or dry riverbeds, an airborne sand cloud or sandstorm occurs. Dust storms occur when the wind blows dry topsoil from open fields, overgrazed areas, or places with little or no vegetation.

Wind Reducing wind erosion – plant vegetation Windbreaks – rows of trees can slow down wind reducing erosion; they also trap snow to increase moisture. Roots – fibrous root system plants such as grasses help anchor soil particles

Wind Deposition by wind – airborne particles eventually return to the Earth. Fine-grained sediments known as loess helped form fertile soils in the Midwestern United States. A mound of sediments drifted by the wind is called a dune. Dunes move as the wind continues to blow against them The more gently the sloping side of a dune faces the wind.

Wind Mounds… continued Dunes have different shapes, such as crescents, lines, or stars, based on sediments, wind speed and direction, and vegetation

Wind Erosion and deposition are constantly changing the shape of the land.

Wind Compare and contrast abrasion and deflation. Describe how the energy of motion affects the surface of the Earth. Explain the difference between dust storms and a sandstorms. Describe how the energy of motion affects the deposition of sand and dust by these storms. Explain what loess is made of and how a loess deposit forms. Why do farmers plant trees along edges of their fields? If you lived close to a desert, explain how sand might be building up along the fence or the side of your apartment complex? What are two ways vegetation can help prevent wind erosion?