EROSIONAL FORCES
7.1
Gravity
I. Erosion and Deposition
A. Erosion – process that wears away surface materials and
moves them from one place to another.
B. Major causes – gravity, glaciers, wind, and water
C. Deposition – dropping of sediment in a new location.
a. The final stage of erosion.
II. Erosion and Deposition by Gravity
A. Gravity – force of attraction that exists between all objects
B. Gravity causes loose materials to move down a slope.
C. Mass Movement – erosion that occurs due to gravitational
pull on a down slope.
D. Slump
a. Slump- mass movement that happens when loose materials or rock
layers slip down a slope.
b. Happens when layers underneath can not support the rock and
sediment above.
E. Creep
a. Creep – sediments that slowly inch their way down a hill.
b. Frequent in areas of freezing and thawing
F. Rockslides
a. Large blocks of rock break loose form a steep slope ad start tumbling
b. Occur in mountainous areas or where there are steep cliffs
c. Most often happen after heavy rains, or during earthquakes
G. Mudflows
a. Occur in dry areas where there are thick layers of dry sediments
b. Often after a heavy rain
III. Erosion-Prone Land
A. Building on Steep Slopes
a. Prone to slumps because of weak sediment layers underneath.
b. Removal of vegetation or increasing slope speeds up erosion.
B. Making Steep Slopes Safe
a. Plant vegetation
b. Build walls or terraces
c. Can only slow erosion, never eliminate
Pg. 189 1-4
EROSIONAL FORCES
7.2
Glaciers
I. Continental and Valley Glaciers
A. Weight of snow becomes great enough to press the bottom
layers into ice.
B. Pressure on ice on the bottom causes partial melting and
becomes putty-like.
C. Glacier – moving mass of ice and snow.
D. Continental Glaciers
a. Glaciers covered up to 28% of Earth during the last ice age (2 to 3 million
years ago)
b. Average air temperature was 5 degrees C lower than today.
c. 20,000 years ago ice sheets began to melt.
d. Today glaciers only cover 10% of Earth.
E. Valley Glaciers
a. Occur in current climate in high mountains where temperature is low
enough to stop snow from melting.
b. These glaciers grow and creep along.
II. Glacial Erosion
A. Glaciers pull loose material and deposits it in a new location.
B. 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.
C. As the glacier moves, plucked rock fragments rub against
soil and bedrock speeding up erosion.
D. The rocks leave grooves or striations.
E. Evidence of valley Glaciers
a.
b.
c.
d.
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
III. Glacial Deposition
A. Till
a. Till – different size sediment dropped from a melting glacier.
b. During the last ice-age glaciers dropped till in the northern United
States to fill valleys and make land appear flat.
c. Moraine – ridge formed when a glacier stops moving and a large
amount of till is dropped
B. Outwash
a.
b.
c.
d.
e.
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
EROSION FORCES
7.3
Wind
I. Wind Erosion
A. Deflation and Abrasion
a. Deflation – blowing loose sediment, removing small particles such as
clay, silt, and sand.
b. Abrasion – windblown sediments strike rock, and wear away the
surface.
c. Abrasion and deflation happened mainly in deserts, beaches, and
plowed fields.
d. Abrasion works like sandblasting, with the wind acting as the machine.
B. Sandstorms
a. Sandstorm – low cloud, just above the ground
b. Occur in deserts and sometimes on beaches and dry riverbeds
C. Dust Storms
a. Dust storms – higher off the ground and can be carried far distances
b. Storms blow top soil from open fields, overgrazed areas, and place
where vegetation has disappeared.
II. Reducing Wind Erosion
A. Windbreaks
a. Rows of trees used to reduce the energy of motion in wind to stop soil
erosion
B. Roots
a. Plants with fibrous root systems, help to stop erosion
III. Deposition by Wind
A. Loess
a. Loess – glacial outwash of fine-grained sediments, deposited by wind
b. Creates much of the fertile farmland of the Midwestern US
B. Dunes
a. Formed when sediments build up behind an obstacle
b. Build where there is sand and winds or sea breezes that blow daily.
c. Sand builds up becoming an obstacle itself
d. Sand will continue to build and form a dun until the sand runs out or the
obstacle is removed.
C. Dune Movement
a. Side facing wind has gentler slope, side away is steeper
b. Without grasses, dunes will migrate form the direction of the wind
D. Dune Shape
a. Shape can tell the direction of the wind
b. In a crescent shaped dune, the points are directed downwind.