Constructive and Destructive Forces

Erosion

Dr. Pamela Gore
Georgia Perimeter College

1. Explain erosion.
2. List the agents of erosion on the earth's surface.
3. Explain how soil particle size and weight affect deposition.
4. List the forces that cause shoreline erosion.
5. List ways in which erosion can be controlled.
6. Tell how erosion can be reduced in high-risk areas.
7. Explain the problems that develop when people develop land prone to erosion.
8. Tell some ways in which natural resources can be conserved.
9. Explain how ocean and air currents benefit plant and animal life.

Erosion

The agents of erosion are:

1. Running water
2. Wind
3. Ice (glaciers)
4. Waves
5. Gravity (mass wasting)

Please note that weathering and erosion are two different things. Many people mistakenly say erosion when they really mean weathering.

Weathering = breakdown.
Erosion = transport.

Erosion by Running Water - Rivers and Streams

1. Abrasion of the stream bed by gravel and sand carried by the water. Potholes may form.
2. Dissolution of soluble rock types (such as limestones and marbles, or rocks with calcite cements)
3. Scour or lifting of loose particles due to the turbulence of the water.

River deposits

In mountainous areas, at the break in slope between the mountain front and the flat valley, running water decreases rapidly in veolcity as it reaches the flat valley, and sediment is deposited as an alluvial fan.

Today, much eroded soil will be trapped behind dams across rivers, filling reservoirs. Loss of sediment transport to the coast also depletes beaches of sand, and can lead to accelerated beach erosion.

Erosion by Wind

1. Deflation (lifting of loose particles by the wind)
2. Abrasion (natural sandblasting caused by wind-borne particles of sediment striking the Earth's surface)

The Dust Bowl of the 1930's was due to wind erosion of soil following extended drought and over-tilling of the soil.

Wind Deposits

Layers of fine sand and silt deposited by the wind form loess deposits. Windblown silt from the Pleistocene glaciations formed thick loess deposits in the and central parts of the Mississippi River Valley. They form very fertile soils.

How can we control erosion from running water and wind?

• Contour plowing
• Terraces on a hillside
• Planting groundcovers; roots hold the soil. (Kudzu)
• Windbreaks - a barrier or baffle that causes the wind to slow down. When windspead is decreased, the load carried by the wind is dropped. Fences and plants are often used as windbreaks.

Fences on the beach at Tybee Island, Georgia
are used to slow wind and cause sand deposition.
Note the dune-building that has occurred between the fences.

Erosion by Ice

Types of glaciers:

1. Alpine glaciers or valley glaciers - small glaciers in mountainous areas such as the Alps and the Rocky Mountains
2. Ice sheets - huge sheets of ice covering large areas of land such as the ice sheets on Antarctica and Greenland. During the Pleistocene Ice Ages, ice sheets covered much of North America and Europe.

Glaciers erode the surface of the Earth in two ways:

1. Abrasion (rock materials carried by the glacial ice scrape and grind against the floor and walls of the valley, or the bedrock beneath the ice sheet, making scratches or grooves called glacial striations on the rock)
2. Plucking (the process by which loose particles become frozen into the glacial ice as the glacier moves over them, and then they are carried along by the glacier)

Glacial Deposits

Types of glacial deposits:

1. Till - a mixture of grain sizes ranging from boulders to fine clay particles deposited by a glacier. The material is poorly sorted and not layered.
2. Moraine - a ridge of till deposited by a retreating (melting) glacier
3. Drumlin - a streamlined oval-shaped mound of till; the tip points in the direction that the glacier was moving.
4. Meltwater deposits - streams of water from melting glaciers carry sediment which is sorted and deposited in layers

Erosion by Waves - Shoreline Erosion

Water movements along the shoreline:

1. Waves - caused by wind blowing over the water
2. Tides - caused by gravitational pull of moon on the water
3. Currents - unidirectional flow of water

Sediment is transported along the beach by the waves.

Longshore drift (or longshore transport); also called beach drift.
Waves rush onto the beach at a slight angle, but they rush straight back out to sea because of gravity.
Because of this, sediment in the surf zone is transported along the beach in a zig-zag pattern.
It is referred to as a longshore current.

Ocean currents benefit plant and animal life by bringing in nutrients and oxygen-rich water, and by carrying away wastes.

Waves erode the shoreline in several ways:

1. The pounding force of breaking waves can break fragments off of rock formations
2. Abrasion (sand and rocks carried by waves abrade other rocks on the shoreline)
3. Waves can force water into cracks in rocks along the shoreline, causing water pressure to build up in the cracks. Over time the cracks become larger and the pressure breaks the rocks.
4. Dissolution (some rocks dissolve in salt water)
5. Scour (turbulence of the water picks up small particles)

Erosional features formed by waves:

• Wave-cut cliffs
• Wave-cut platform
• Sea arches
• Sea stacks

Sea arch - a feature of coastal erosion

Shoreline deposition

Depositional features along the shoreline formed by waves:

• Beaches and barrier islands
• Spits and sand bars
• Bay-mouth bars
• Tombolos

Controlling Shoreline Erosion

Seawalls may be built along the beach to prevent the waves from eroding the shoreline, and to protect homes or other structures built along an eroding beach.

Sea wall attempting to prevent coastal erosion, south end of St. Simons Island, GA

Groins and jetties are built perpendicular to the coastline in an attempt to trap some of the sand being carried by the longshore current. Sand is deposited and the beach is built outwards on the upcurrent side, but on the downcurrent side of the groin or jetty, erosion occurs.

Jetties along Lake Ontario near Detroit, Michigan.
Can you tell the direction of longshore drift?

Unfortunately, man-made structures along the coastline often have the unwanted side effect of enhancing coastal erosion.

For images and more information on coastal erosion go to this web address: http://www.gpc.edu/~pgore/geology/geo101/coastal.htm

Erosion by Gravity or Mass Wasting

Types of mass wasting:

1. Rapid movement
   1. Rock fall - The free fall of detached pieces of material of any size; may fall directly downward or bounce and roll. May occur as result of freeze-thaw, or the loosening action of plant roots.

   2. Slump - Slumps involve a mass of soil or other material sliding along a curved, rotational surface. (Shaped like a spoon.) Causes the formation of a small, crescent-shaped cliff or "scarp" at the upslope end. (Sometimes more than one scarp.) At the bottom (or toe) of the slump, earthflow, or flow of soil occurs.

      Slumps are sometimes seen along interstate highways where the graded soil on the sides of the road is a little too steep.

      
      Slump along I-675 south of Atlanta, GA

   3. Rockslide or debris slide - Also called "landslides". Occurs when blocks of rock, or masses of unconsolidated material slide down a slope. These are among the most destructive of mass movements. May be triggered by rain or melting snow, or earthquakes.

   4. Debris flow or mudflow - Commonly occur in volcanic areas, where they are called lahars. Mudflows generally follow established drainage patterns (valleys).

   5. Earthflow - Form in humid areas on hillsides following heavy rain or melting snow, in fine-grained materials (clay and silt). Also occurs at the toe of slumps. Rate of movement varies (less than 1 mm per day to several meters per day), but may be long-lived (days to years). Includes the liquifaction associated with earthquakes.

2. Slow movement
   1. Creep - A SLOW downhill movement of soil and regolith. Creep results in tree trunks that are curved at the base, tilted utility poles, fence posts, and tombstones, and causes retaining walls to be broken or overturned.

   2. Solifluction - Occurs in areas underlain by permafrost. Occurs in the "active" surface layer that thaws in summer.

Return to Earth & Space Science page

Return to Georgia Geoscience Online

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

February 3 - 4, 2001