Erosional Forces

1. Erosional Forces 7.1 Gravity

2. Erosion and Deposition • Erosion – process that wears away surface materials and moves them from one place to another. • Major causes – gravity, glaciers, wind, and water • Deposition – dropping of sediment in a new location. • The final stage of erosion. • Erosion and Deposition by Gravity • Gravity – force of attraction that exists between all objects • Gravity causes loose materials to move down a slope. • Mass Movement – erosion that occurs due to gravitational pull on a down slope.

3. Slump • Slump- mass movement that happens when loose materials or rock layers slip down a slope. • Happens when layers underneath can not support the rock and sediment above. • Creep • Creep – sediments that slowly inch their way down a hill. • Frequent in areas of freezing and thawing • Rockslides • Large blocks of rock break loose form a steep slope ad start tumbling • Occur in mountainous areas or where there are steep cliffs • Most often happen after heavy rains, or during earthquakes

4. Mudflows • Occur in dry areas where there are thick layers of dry sediments • Often after a heavy rain • Erosion-Prone Land • Building on Steep Slopes • Prone to slumps because of weak sediment layers underneath. • Removal of vegetation or increasing slope speeds up erosion. • Making Steep Slopes Safe • Plant vegetation • Build walls or terraces • Can only slow erosion, never eliminate Pg. 189 1-4

5. Erosional Forces 7.2 Glaciers

6. Continental and Valley Glaciers • Weight of snow becomes great enough to press the bottom layers into ice. • Pressure on ice on the bottom causes partial melting and becomes putty-like. • Glacier – moving mass of ice and snow. • Continental Glaciers • Glaciers covered up to 28% of Earth during the last ice age (2 to 3 million years ago) • Average air temperature was 5 degrees C lower than today. • 20,000 years ago ice sheets began to melt. • Today glaciers only cover 10% of Earth.

7. Valley Glaciers • Occur in current climate in high mountains where temperature is low enough to stop snow from melting. • These glaciers grow and creep along. • Glacial Erosion • Glaciers pull loose material and deposits it in a new location. • Plucking - Glacial ice melts and water flows into crack in rocks. As it refreezes rocks break apart and results in boulders, gravel, and sand being added to the moving glacier. • As the glacier moves, plucked rock fragments rub against soil and bedrock speeding up erosion. • The rocks leave grooves or striations.

8. Evidence of valley Glaciers • In mountains look for striations and plucking. • Valley glaciers make bowl-shaped basins called cirques. • Two or more glaciers will form arete, a ridge, or a horn. • Glaciers erode valley in U-shapes • Glacial Deposition • Till • Till – different size sediment dropped from a melting glacier. • During the last ice-age glaciers dropped till in the northern United States to fill valleys and make land appear flat. • Moraine – ridge formed when a glacier stops moving and a large amount of till is dropped

9. Outwash • More snow melting causes glacier to melt and retreat. • Outwash – material left by the meltwater • Meltwater leaves sediments and deposits in layers like a river • Can make fan-shaped deposits. • When meltingwater under a glacier forms a river in the ice, eskers, a winding ridge of sand and gravel is formed. Pg 196 1-3

10. Erosion Forces 7.3 Wind

11. Wind Erosion • Deflation and Abrasion • Deflation – blowing loose sediment, removing small particles such as clay, silt, and sand. • Abrasion – windblown sediments strike rock, and wear away the surface. • Abrasion and deflation happened mainly in deserts, beaches, and plowed fields. • Abrasion works like sandblasting, with the wind acting as the machine. • Sandstorms • Sandstorm – low cloud, just above the ground • Occur in deserts and sometimes on beaches and dry riverbeds • Dust Storms • Dust storms – higher off the ground and can be carried far distances • Storms blow top soil from open fields, overgrazed areas, and place where vegetation has disappeared.

12. Reducing Wind Erosion • Windbreaks • Rows of trees used to reduce the energy of motion in wind to stop soil erosion • Roots • Plants with fibrous root systems, help to stop erosion • Deposition by Wind • Loess • Loess – glacial outwash of fine-grained sediments, deposited by wind • Creates much of the fertile farmland of the Midwestern US • Dunes • Formed when sediments build up behind an obstacle • Build where there is sand and winds or sea breezes that blow daily.

13. Sand builds up becoming an obstacle itself • Sand will continue to build and form a dun until the sand runs out or the obstacle is removed. • Dune Movement • Side facing wind has gentler slope, side away is steeper • Without grasses, dunes will migrate form the direction of the wind • Dune Shape • Shape can tell the direction of the wind • In a crescent shaped dune, the points are directed downwind.