LABORATORY INVESTIGATION 4-3 EROSION
The text and graphics on this page are to be used with the Lab 4-3 handout.
Erosion
The process known as weather breaks up rocks so that they can be carried away by the process known as erosion. Water, wind, ice, and waves are the agents of erosion that wear away at the surface of the earth.
Wind
Erosion by wind is known as aeolian (or eolian) erosion (named after Aeolus, the Greek god of winds) and occurs almost always in deserts. Aeolian erosion of sand in the desert is partially responsible for the formation of sand dunes.
Water
Water is the most important erosional agent and erodes most commonly as running water in streams. However, water in all its forms is erosional. Raindrops (especially in dry environments) create splash erosion that moves tiny particles of soil. Water collecting on the surface of the soil collects as it moves towards tiny rivulets and streams and creates sheet erosion.
In streams, water is a very powerful erosional agent. The faster water moves in streams the larger objects it can pick up and transport. This is known as critical erosion velocity. Fine sand can be moved by streams flowing as slowly as ¾ of a mile per hour.
Streams erode their banks in three different ways: 1) the hydraulic action of the water itself moves the sediments, 2) water acts to corrode sediments by removing ions and dissolving them, and 3) particles in the water strike bedrock and erode it.
The water of streams can erode in three different places: 1) lateral erosion erodes the sediment on the sides of the stream channel, 2) down cutting erodes the stream bed deeper, and 3) headward erosion erodes the channel upslope.
Waves
Waves in oceans and other large bodies of water produce coastal erosion. The power of oceanic waves is awesome, large storm waves can produce 2000 pounds of pressure per square foot. The pure energy of waves along with the chemical content of the water is what erodes the rock of the coastline. Erosion of sand is much easier for the waves and sometimes, there’s an annual cycle where sand is removed from a beach during one season, only to be returned by waves in another.
Ice
The erosive power of moving ice is actually a bit greater than the power of water but since water is much more common, it is responsible for a greater amount of erosion on the earth’s surface.
Glaciers can perform to erosive functions - they pluck and abrade. Plucking takes place by water entering cracks under the glacier, freezing, and breaking off pieces of rock that are then transported by the glacier. Abrasion cuts into the rock under the glacier, scooping rock up like a bulldozer and smoothing and polishing the rock surface.
Wind
Erosion affects the landscape with energy from the wind. Wind is a very active agent of erosion in open areas such as beaches, grassless areas, and deserts. Loose or unattached particles are open to exposure. These particles are easily picked up and carried by the wind over great distances. Wind can cause erosion through deflation and abrasion
Deflation
Wind can move particles over large areas and high into the atmosphere. It erodes through a process called deflation. Deflation is the removal and lifting of loose materials. Usually, wind can remove only fine-grained particles. Deflation is usually difficult to detect because the entire surface area is being decreased or eroded at the same time. Shallow depressions (blowouts) that occur because of deflation are much more noticeable. Deflation usually occurs in areas where the soil is dry.
Abrasion
Abrasion is the wearing away of rocks by solid particles that are transported by water and wind. Sand that has been picked up by the wind acts as a sand blaster; scraping, cutting and polishing rocks. Over time, various and unusual shapes can be carved or formed from the abrading sand.
Wind-blown sand is picked up and can be moved over a wide area. The edges of the sand can rub over or against rocks and cause them to develop sharp edges. These particles slowly wear away exposed rocks causing them to have jagged and sharp edges. This change in shapes and edges occur over a long period of time.
Water
Water can be one of the most destructive agents of erosion. Yet, water has many useful functions, such as farming, travel, commerce, and recreation. Water can erode or decrease areas of high elevation and fill in areas of low elevation. The amount of material that flowing water can carry depends on the volume of water and the speed that it is moving. These, in turn, depend on the topography over which the water is flowing. Flowing water is one of the primary sculptors of the landscape. Water can cause erosion in three ways: rivers, waves and runoff.
Rivers
Rivers are important sources of fresh water for our environment and are important for transportation and recreation as well. We rely upon rivers for many of our daily activities. Rivers are also very important agents of erosion. Flowing water can transport all types of material. Rivers change with time or age.
Aging of Rivers
As rivers develop, many characteristics become evident. Their development can be divided into three stages:
YOUTHFUL rivers are characterized by narrow, V-shaped valleys with steep slopes, and rapidly flowing water. The river channel is relatively straight. They are usually located in mountainous areas.
MATURE rivers are characterized by wide floodplains and meandering streams. Streams “meander” (their paths wander from side to side) when gradients are low and objects deflect the stream’s course. Meandering streams flow faster on the outside of their bends and thus erode their banks on that side. They flow slower on the inside of banks and deposit material there. Floodplains are produced by a meandering stream and are often covered with water during times of high volume runoff.
OLD AGE rivers are characterized by flat valley channels that have an overload of sediment. The area has been eroded nearly as low as it can go, the velocity of the water is greatly reduced and sediments continue to build up. When one meander cuts into another, oxbow lakes are formed.
Constant movement of the earth’s crust can have an effect on our river systems. Sometimes this movement causes a river to rejuvenate. Rejuvenation occurs when the stream’s gradient(slope) is increased and its power to erode increases, making it act more “youthful”. The rejuvenated stream cuts a new V-shaped valley and the remains of the old floodplain are present in the form of step-like surfaces called terraces. V-shaped valleys of the youthful stream can be seen within meanders. Evidence of a river undergoing cycles can be seen in the Mississippi River.
During the development of a river system, the landscape is continuously being reshaped and resculptured. Plateaus can be cut by rivers to form canyons. Valleys can be carved in mountains as rivers flow through them. Even in very dry regions, where streams flow only after major rainfalls, runoff is responsible for the topography of the land.
Waves
The energy produced by waves has an enormous impact on shores and coastal areas. Waves pound rock structures and, over a period of time, small cracks that are found in the rocks become larger and larger.
Waves are the most obvious movement of the ocean. They can be set in motion by winds, earthquakes and the gravitational pull of the moon, but most waves are produced by wind.
At first look, it appears that the water is moving in about the same way the waves move. Actually, as the diagram shows, the water moves in small circles as the wave passes. Each water particle in a wave moves around in a circle. As a waves passes, it carries energy along with it, but the water returns to about the same place it started.
In shallow water, waves are effective agents of erosion. When the water is sufficiently shallow, the base of the waves touches the bottom and the friction slows the waves’ motion. As the wave in front slows, the next wave presses up closer to it. As the waves move closer together, they are pushed up to become taller. The taller the wave becomes, the more unstable it is.
As the waves continue to move into shallower water, they continue to grow. The bottom of the waves slows down as it is forced to move along the bottom. Eventually the top completely overruns the bottom and collapses, creating a breaker. The crest falls and the water tumbles over itself.
Runoff
Runoff is water that flows over the surface of the land, usually after a rainfall. Runoff in an area is affected by several factors. They include:
rainfall, plants, and the slope of the land.
Rainfall
In areas of heavy rainfall, runoff is increased. Therefore, erosion is increased. Heavy rain causes loose soil and other particles to be washed or carried away. Runoff increases during heavy rainfall because there is not enough time for the water to soak into the ground, or the ground becomes saturated.
Plants
Runoff is affected by plant growth. Plants act as a buffer or deterrent to erosion. The roots help absorb water and anchor the soil to prevent it from being carried away. Leaf litter and other plant debris protect the soil. In areas where plants are scarce, runoff is greater and therefore erosion is greater.
Slope
Runoff is affected by the slope of the land. Steep-sloped areas are at greater risk of erosion. The more rapidly water moves, the more soil it can pick up and transport. Plants or other objects which slow the water’s flow can help prevent erosion.
Glaciers
Glaciers can transform mountain river valleys from a V-shape to a U-shape.
Moraines
Mounds of till are called moraines. If it is at the front of a glacier it is called terminal moraine. It is called ground moraine if the till is dropped wherever it was when the glacier melted.
Mounds of till are called moraines. If it is at the front of a glacier it is called terminal moraine. It is called ground moraine if the till is dropped wherever it was when the glacier melted.
Till
Till is the material that is dropped as a glacier melts. It consists of boulders, sand and clay.
Striations
Abrading
Abrasion is the process of a glacier scouring the surface it is moving over. Rocks beneath the surface may be ground up or polished. Glaciers may leave long scratches or grooves called striations in the rock.
Plucking
Plucking is the process in which the glacial meltwater freezes again in rock cracks, expands and breaks the rock apart. The broken rocks are added to the glacier. This can hollow out the head of the valley where the snow has accumulated to start the glacier. This hollow area is called a cirque. If the cirques reach the summit they may form a steep peak called a horn.
Picture of a Cirque
Picture of a horn
Gravity
Erosion by gravity moves material from high elevations to lower elevations.
Mass Movement
Mass movement is the downhill slide of soil and /or rock due to gravity. Sometimes this is called mass wasting. This movement can be either fast or slow.
Fast Movement
Some mass movement happens quickly and suddenly like mudflows and rockslides.
Landslides and rockslides are rapid downhill movements of soil or rock in large blocks. Talus is the result of rockslides.
Slow Movement
Some mass movement happens slowly over a long period of time like creep and slump.
Creep is the slow downhill movement of soil on a hillside. This occurs over a long period of time and may account for leaning fence posts, tombstones and telephone poles.
Slump is the slow downhill movement of a weak layer of rock overlain by a more resistant layer. Slump leaves a curved scar where the resistant layer was pulled away.