Earth image Erosion

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  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.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E5h. Describe the effects of human activity on the erosion of the Earth's surface.
  • S6E5c. Describe processes that change rocks and the surface of the Earth.
  • S6E5e. Explain the effects of physical processes (plate tectonics, erosion, deposition, volcanic eruptions, gravity) on geological features including oceans (composition, currents, and tides).
  • S6E5f. Describe methods for conserving natural resources such as water, soil, and air.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Waves, wind, water, and ice shape and reshape the earth's land surface by eroding rock and soil in some areas and depositing them in other areas, sometimes in seasonal layers.
  • Some changes in the earth's surface are abrupt (such as earthquakes and volcanic eruptions) while other changes happen very slowly (such as uplift and wearing down of mountains). The earth's surface is shaped in part by the motion of water and wind over very long times, which act to level mountain ranges.
  • Sedimentary rock buried deep enough may be reformed by pressure and heat, perhaps melting and recrystallizing into different kinds of rock. These re-formed rock layers may be forced up again to become land surface and even mountains. Subsequently, this new rock too will erode. Rock bears evidence of the minerals, temperatures, and forces that created it.
  • Human activities, such as reducing the amount of forest cover, increasing the amount and variety of chemicals released into the atmosphere, and intensive farming, have changed the earth's land, oceans, and atmosphere. Some of these changes have decreased the capacity of the environment to support some life forms.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Land forms are the result of a combination of constructive and destructive forces. Constructive forces include crustal deformation, volcanic eruption, and deposition of sediment, while destructive forces include weathering and erosion.
  • Human activities also can induce hazards through resource acquisition, urban growth, land-use decisions, and waste disposal. Such activities can accelerate many natural changes.

Erosion

Erosion is the process by which soil and weathered rock particles (sediment - gravel, sand, silt, and clay) are transported, or moved from one place to another.

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

Running water erodes soil and sediment in several ways:
  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

Soil that is eroded and transported by streams will eventually be deposited as sandbars in streams, as pointbars on the inside curve of a meandering stream, on floodplains and levees, or at the mouth of the river in a delta.

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.


Providence Canyon - An example of the effects of human activity on erosion


 
Providence Canyon

Providence Canyon State Park, near Lumpkin, GA (south of Columbus) contains a spectacular series of canyons or deep erosional gullies as much as 200 feet (66 m) deep. Sometimes called " Georgia's Little Grand Canyon", Providence Canyon is one of the Seven Natural Wonders of Georgia. The canyon is actually a geologically young feature. It was not here when the first settlers reached the region in the early 1800's. The story goes that it was formed by a woman throwing her dishwater out into the yard, year after year. That is not quite accurate. The story is a little more complex.

The canyon is a testimony to poor farming practices, which led to runaway soil erosion, and illustrates the need for sound soil conservation practices. When the land was first cleared for agriculture in the early 1800's, farmers plowed straight up and down the hills. (Contour plowing did not come into fashion until much later). The furrows were a good conduit for rainwater runoff. By 1850, gullies ranging from 3 to 5 feet deep (1 to 2 meters) had begun to appear in the fields. Once gullies appeared, erosion rates accelerated, and the land became useless for farming. The gullies deepened and widened into canyons, which continue to expand.

The rate of downcutting by erosion was calculated to be about 8 inches per year between 1820 and 1930 (based on the total volume of sediment removed by erosion). In addition to downcutting, headward erosion (or erosion at the head of a canyon) caused the canyons to lengthen. Rates of headward erosion were measured from aerial photos. Between 1955 and 1968, the average headward erosion of the canyons was calculated to be about 6 feet (2 m) per year. The softness of the sediments in this area, and poor farming practices led to the severe erosion that formed Providence Canyon.


Erosion by Wind

Wind erodes the Earth's surface in two ways:

  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)

  
  
  
Sand Storm, 26 April 2005. Al Asad, Iraq. A wall of sand blowing at 60 mph.

Particle size and weight affect the transportation and deposition of soil and sediment. Fine, light weight particles can be high up into the air, whereas larger, heavier particles may only roll or bounce along the ground.

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

Sand transported by the wind may be deposited as sand dunes.

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?


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

A glacier is mass of ice and snow moving under the influence of gravity. Snow accumulating over many years without melting eventually compacts into glacial 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

When the edge of the glacier begins to melt, rock materials carried by the glacier are deposited.

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

The powerful force of waves constantly erodes and shapes the shoreline.

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:


Sea arch - a feature of coastal erosion


Shoreline deposition

Waves can deposit sediment, in addition to eroding it. Sediment that is eroded in one area will be deposited in another area.

Depositional features along the shoreline formed by waves:


Controlling Shoreline Erosion

Man-made structures are sometimes built along the shoreline to try to reduce or prevent 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.

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.
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.


Erosion along the Georgia coast

In 1964, Hurricane Dora struck coastal Georgia, and eroded large portions of the beach along St. Simons Island, and washed several homes out to sea. President Lyndon B. Johnson ordered the construction of a seawall or revetment composed of rip rap or huge boulders, known as the Johnson Rocks, along the southern end of the island, to help slow the coastal erosion. Critics note that armoring shorelines with rocks, however, promotes scouring of beach sand, as the waves are reflected off the rocks. It also makes nesting difficult for sea turtles, and makes it difficult for people to access the beach.


St. Simons Island Lighthouse at the southern end of the island. Note the seawall of boulders between the beachand the lighthouse (Johnson Rocks), and the wooden stairway built for public access to the beach.


A view of the seawall, Johnson Rocks, between the beach and a house at the southern end of St. Simons Island.

Erosion occurs in many areas along the Georgia coast, but it is an important problem on a populated island like St. Simons. Man-made structures along the beach hamper the natural landward migration of beaches as sea level rises.

How do you stop coastal erosion in a place like St. Simons Island? One possibility is beach renourishment, or the construction of a concrete groin to hold the sand in place. Unfortunately, installation of a concrete groin to hold the sand in place would accelerate erosion rates on the downcurrent side of the groin. In addition, there are some negative aspects to beach renourishment that may not be immediately obvious. Renourishment would bring a different type of sand to the beaches. Currently, the sand on Georgia's beaches is well sorted and has a low percentage of shell fragments. Sand used to renourish the beach would be dredged from offshore. Offshore sand has a high concentration of broken shell fragments, which would make it difficult to walk barefoot on the beach. In addition, dumping on the beach would bury and kill the organisms that inhabit the beach. Beach renourishment would cause a significant decrease in marine life and an interruption of the food chain. In fact, fisheries, tend to suffer following dredging and beach renourishment, possibly as a result of suspended sediment in the water.

Any sand added to the beach will ultimately be eroded away again, because the energy of the waves, currents, and tides does not change. Renourished beaches typically last several years, or perhaps as long as five or six years before the sand washes away and renourishment again becomes necessary. And renourishment comes with a price tag of several million dollars.


Erosion by Gravity or Mass Wasting

Mass wasting is the downslope movement of rock, regolith, and soil, under the influence of gravity. Also called mass movement or mass wastage.

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.
For more information and images of mass wasting, go to this web address: http://facstaff.gpc.edu/~pgore/geology/geo101/masswasting.html.


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

Page created March 9, 2005
Images added May 4, 2005
Image links updated February 21, 2009