Chapter 8: Erosional Forces
Section 1 Erosion by Gravity
A. Erosion—wearing away and moving of surface materials by gravity,
water, wind, or glaciers
B. Through the process of deposition, sediments are dropped by erosion agents
as they lose energy.
C. Mass movement occurs as gravity moves materials down a slope as one
large mass.
1. Slump—material slips down a curved surface as one large mass.
2. Sediments slowly shift downhill in the process of creep .
3. Rockfalls and rock slides occur when rocks break off or slip suddenly
down a hill.
4. A mudflow is a thick mixture of water and sediments flowing downhill.
D. Consequences of erosion—buildings on slopes eventually have problems due
to erosion by gravity.
1. Sometimes builders and residents make slopes more unstable by
making them steeper.
2. Another source of instability is the removal of vegetation.
E. Steep slops can be made safer with vegetation, drainage pipes, and walls of
concrete or railroad ties.
Section 2 Glaciers
A. Glacier—large mass of ice and snow slowly moving on land; an agent of
Erosion
B. As glaciers move, they pick up boulders, gravel, and sand in an erosion
process called plucking.
1. Plucked rocks at the base of the glacier scour the soil and bedrock.
2. Dragged rock fragments leave scars on bedrock called grooves.
3. Striations are shallower scars on bedrock.
4. Grooves and striations indicate the direction a glacier moved.
C. As glaciers retreat, they leave behind till, a mixture of different sized
sediments.
1. Till areas include wide swaths of farmland from Iowa to Montana, and
Ohio to Illinois.
2. A moraine is a ridge, or pile, of deposit left at the end of a glacier.
3. Outwash—material deposited in layers by the meltwater of a glacier,
with largest pieces closer to the glacier
4. An esker is a type of outwash deposit formed as meltwater rivers within
the ice deposit sand and gravel within their channels.
D. Continental glaciers—huge masses of ice and snow now covering only about
10 percent of Earth in areas near the poles; in the past, as much as 28
percent of Earth was covered by glaciers
1. Periods of widespread glaciation over the last 2 million to 3 million years
are known as ice ages.
2. The average air temperature on Earth was about 5°C lower during ice
ages than today.
3. The last major ice age was about 18,000 years ago.
E. Valley glaciers—exist in mountains
1. Cirques are bowl-shaped basins in the sides of mountains, created by
valley glaciers.
2. A long ridge or árete forms when two valley glaciers erode a mountain
side-by-side.
3. A horn forms when valley glaciers erode a mountain from several
directions.
4. Glacially eroded valleys have a U shape, as opposed to the V shape left
by stream erosion.
F. Glaciers have changed, and continue to change, the shape of Earth’s surface;
sand and gravel deposits left by glaciers are important resources for the
construction of roads and buildings.
Section 3 Wind
A. Wind erosion—can scatter dust or volcanic ash over thousands of kilometers
1. Deflation—Wind removes small particles of loose sediment, leaving
behind heavier materials.
2. Wind behaves like a sandblaster blowing sand grains against rocks
wearing them down and pitting them in the process of abrasion.
3. Deflation and abrasion happen most often in areas where there is little
vegetation to hold sediments in place.
4. When strong winds blow in the deserts, beaches, or dry riverbeds, an
airborne sand cloud or sandstorm occurs.
5. Dust storms occur when winds blow dry topsoil from open fields,
overgrazed areas, or places with little or no vegetation.
B. Reducing wind erosion—plant vegetation
1. Windbreaks—Rows of trees can slow down wind reducing erosion; they
also trap snow to increase moisture.
2. Roots—Fibrous root system plants such as grasses help anchor soil
particles.
C. Deposition by wind—airborne particles eventually return to Earth.
1. Fine-grained sediments known as loess helped form fertile soils in the
Midwestern United States.
2. A mound of sediments drifted by the wind is called a dune.
a. Dunes move as the wind continues to blow against them.
b. The more gently sloping side of a dune faces the wind.
c. Dunes have different shapes, such as crescents, lines, or stars,
based on sediments, wind speed and direction, and vegetation.
D. Erosion and deposition are constantly changing the shape of the land.
Chapter 9: Water Erosion
Section 1 Surface Water
A. Runoff—rainwater that flows across Earth’s surface; amount of
runoff is affected by several factors:
1. Amount of rain
2. Length of time it falls
3. Steepness, or slope, of the land
4. Amount of vegetation
B. Erosion—the wearing away of soil and rock by water
1. Rill erosion—during a heavy rain, a small stream forms and carries
away soil, creating a groove called a channel.
2. Gully erosion—Heavy rains remove large amounts of soil and sediment
from a rill channel, forming a gully.
3. Sheet erosion—Sheets of water flowing across the land pick up and
carry away sediments
4. Stream erosion—Water in a stream picks up sediments from the bottom
and sides of its channel, making the channel deeper and wider.
C. River system—the network of groundwater and streams that come together to
form a river
1. Drainage basin—the area of land from which a stream or river collects
runoff
2. Mississippi River drainage basin—largest in the United States
D. Stages of stream development:
1. Young stream—flows swiftly through a steep valley
a. May have whitewater rapids and waterfalls
b. Erodes the stream bottom faster than the sides
2. Mature stream—flows more smoothly through its valley
a. Erodes more on its sides
b. Curves form called meanders
c. Carves a broad, flat valley floor called a floodplain
3. Old stream—flows slowly through a floodplain it has carved
E. Flooding—When too much water enters a river system, it overflows its banks.
1. Dams—built to control the flow of water downstream
2. Levees—mounds of earth built along the sides of a river to prevent
flooding
F. Deposition—when water slows down, it drops, or deposits, the sediments it is
carrying.
1. Delta—formed by sediments that are deposited as water empties into
an ocean or lake
2. Alluvial fan—fan-shaped area formed by sediments deposited as water
empties from a mountain valley onto a flat open plain
Section 2 Groundwater
A. Groundwater—Water that soaks into the ground and collects in the pores of
the rock underlying the soil
1. 14% of all freshwater exists as groundwater.
2. Soil and rock are permeable if water can pass through the pore
spaces. Example: sandstone
3. Soil and rock are impermeable if water cannot pass through them.
Example: granite
4. Aquifer—a layer of permeable rock that lets water move freely
a. Zone of saturation: the area where all pores in the rock are filled
with water
b. Water table—upper surface of zone of saturation
B. Water table
1. Many people get their water from groundwater through wells.
a. Groundwater flows into a well, and a pump brings it to the
surface.
b. Good wells extend deep into the zone of saturation, past the
water table.
c. Wells can dry up during dry seasons or if demand is too high.
2. Artesian wells—water rises to the surface under pressure
3. Springs—the water table is so close to the surface that water flows out.
4. Geyser—a hot spring that erupts periodically, shooting water and
steam into the air
C. The work of groundwater
1. Groundwater mixes with carbon dioxide to form carbonic acid.
2. Acidic groundwater moves through cracks in limestone, dissolving the
rock
3. Gradually, the cracks in the rock enlarge until a cave is formed.
a. Stalactites—calcium carbonate deposit that hangs from a cave’s
ceiling
b. Stalagmites—calcium carbonate deposit that forms on a cave’s
floor
4. If underground rock is dissolved near the surface, a sinkhole forms.
Section 3 Ocean Shoreline
A. Shoreline forces—three major forces constantly change the shape of the
shoreline.
1. Waves pound against shores, breaking rocks into smaller pieces.
a. Move large amounts of sediment
b. Collide with the shore at angles, creating longshore currents,
which act like rivers of sand in the ocean
2. Currents move sediment along the shoreline.
3. Tides carry sediment out to sea and bring in new sediment.
B. Rocky shorelines—steep shorelines
1. Rocks and cliffs are the most common features
2. Softer rocks erode away before harder rocks, leaving islands of harder
rocks
C. Sandy beaches—gently sloping shorelines
1. Beaches—deposits of sediment that are parallel to the shore.
a. Made of rock and shell fragments
b. Fragile because longshore currents constantly carry sand down
the shore to form barrier islands, spits, sand-bars, and baymouth bars
2. Barrier islands—fragile sand deposits that parallel the shore but are
separated from mainland.