Unit 8 - Weathering and Erosion

 

 

Weathering is the gradual breakdown of pre-existing rock due to natural processes. These processes can be very quick, but some may take thousands of years to occur.

Rocks are weathered by their exposure to the elements- like rain, freeze and thaw and acid rain!. They are broken into smaller pieces, and then can undergo chemical changes. Erosion is the process of moving these weathered materials.

A lot of incredible landforms are made by these processes. Click here to take a virtual landscape tour!

 

 Physical Weathering

 

Physical weathering is processes that break down the rock without changing the chemical composition.

 

Types of Physical Weathering:

 

Frost Action-

Frost action is the result of the unusual property of water: As water is cooled, it expands. Most other materials expand when they are heated, and contract as they are cooled. Water expands dramatically between the temperature of 4 and 0 degrees Celsius. When water turns to ice, its volume increases by 9%!!! This expansion can exert pressure on an object measuring hundreds of thousands of pounds per square inch.

Water seeps into cracks and pores of rocks. When the temperature drops, the water expands, and this repeated action can wedge the rock apart. This repeated action of freezing and thawing is called frost action.

Abrasion-

Rocks can also be broken by abrasion. Abrasion occurs when rocks are being transported by an agent of erosion- like moving in a stream or falling down a cliff. As these rock fragments bounce around and off of each other, they break down.

How do you tell which way a stream flows by looking at a rock in the stream? Rock that faces upstream are angular, the downstream side is smooth.

 

Exfoliation-

In certain rock types that contain the mineral feldspar, exfoliation can occur. Water that seeps into the cracks of the rocks bonds to the water, and clay is formed As this layer is weak, it eventually falls off, along with the rock layer above the clay. this tends to happen faster along the edges of the rock, so an exfoliated rock tends to have a rounded appearance.

   Half-dome in Yosemite Park is a great example.

 

Plant and Animal Action-

Animals also have an effect on rocks. The most common effect posed by animals is burrowing, or creating tunnels under ground. Ants, worms, weasels, mice, etc.. can all create an extensive tunnel system. These tunnels allow water and air to reach< points below ground, and begin the weathering process.

Humans have a dramatic effect on rocks. Human technology has allowed us to level mountains, cut out roads and tunnels, and also created problems such as acid rain. This acid rain can also weather rocks. (see chemical weathering)

Changes in temperature-

As a rock is heated by the sun, its temperature can increase dramatically, and at night it cools down. As this process occurs over and over again, the bonds of the rock molecules can break, leading to cracks in the rock. These cracks then allow a path for other agents of weathering.

 

 

 Chemical Weathering

 

 

 

Chemical weathering is the breakdown of rock through changes in its chemical makeup. These changes either break down the rock or weaken the rock to allow for physical weathering.

 

Types of Chemical Weathering:

 

Oxidation-

The atmosphere is composed of 21 percent oxygen. When oxygen combines with an iron rich rock, a process called oxidation occurs. The result is iron oxide. This reaction does not weather the rock. If, however, water is present at the time of oxidation, the result would be rust.

The result is much the same as an iron nail. When these bonds are rearranged, the bonds of the rock are weakened. If you compare a rusty nail to a regular nail, the unweathered nail is much stronger.

 

 Slick Rock in Utah are the favorite red rocks ride

of moutnain bikers. The rocks are rustier

than the bikes in this climate.

Dissolution-

Dissolving a rock in water is yet another form of chemical weathering. It is called solution. Solution occurs when compounds are dissolved into water. Halite, gypsum, and limestone are most susceptible to solution. Most compounds are at least slightly soluble in water, so water is called the universal solvent.

Carbonation-

Carbonic acid can devastate rocks made of calcite. Rocks such as limestone and dolomite can be entirely dissolved, leaving extensive caverns. In states with bedrock composed of calcite, such as Florida this process can cause extensive damage to structures above. Often, caverns are cut out of the rock, the structures on top are heavy, and everything on top collapses into the cavern, leaving what is called a sinkhole. This sinkhole can swallow many houses at a time.

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  Sea Caves form from Carbonation. A sea cave was featured in the movie Cast Away. For more on Sea Caves click here.

 

Take a Virtual Cave Tour to some of the finest examples of Carbonation at work.

 

Acid Rain-

Normal rain is at a pH of 6.2. Plants and animals love it. Pollution from cars, factories, and coal burning plants are creating rain so caustic that sometimes the rains at pH of 4. If an entire lake was that pH, it would be crystal clear but lifeless.

 

 

 

Factors Affecting Weathering

 

Climate, which is usually measured in terms of temperature and moisture, can drastically affect the rate and type of weathering. In an area with cold temperature, for instance, frost wedging would be the dominant formof physical weathering. Chemical weathering occurs most rapidly in a high temperature climate. In New York, the climate reaches both extremes. Hot moist summers and cold winters make for a climate that promotes weathering.

 

The particle size of the material can affect the rate of weathering. The smaller the particle size, the faster it will weather. The more surface area that is exposed, the more it will weather.

 

Rock that is expposed to the atmosphere tend to weather much quicker. Bed rock that is covered by soil and vegetation may not weather as quickly.

 

All chemical and physical properties are determined by the mineral composition of the rock. Mineral composition is more important in determining the rate of chemical weathering. Those minerals that are most reactive with acids, water, and air are weathered at a more rapid rate.

Mineral composition also affects physical weathering. Minerals that are soft (hardness lower thaan six) can be easily abraided and blasted.

 

Weathering is a slow process. When we speak of time in a geologic sense, millions of years are often used as the units. The bedrock of Greenland has barely been weathered, and its age is over 3.5 billion years. The longer that a rock has been exposed to the weather, the faster it weathers.

 

 

The Products of Weathering

 

Sediments-

The rock that soil comes from is called the parent rock, and soil that forms from the bedrock directly below is called residual soil. If the soil is from material that was transported, the soil is called transported soil. Transported soil often has a different composition that the bedrock directly below.

Cool landsforms like this one in Arches National Park Utah.

 

 

Erosion

 

Erosion is the movement of weathered rock. Streams, waves, floods, and winds are just a few of the agents of erosion.

 

Types of Erosion-

Mass Wasting-

Mass wasting is the downhill movements of material due to gravity. This type of movement is going on at all times on most hills, but at different rates . As the slope of the material increases, the rate increases. The steepest slope at which the material will remain stable is called the angle of repose. Size, density, and shape of particles determine the angle of repose. As the slope's angle increase past the angle of repose, the slope becomes unstable, and mass wasting occurs. If the angle is slightly above the angle of repose, mass wasting is slow, as in hillside creep. If the angle is great, then slides may occur.

Types of mass wasting-

Rockfall- rock falls occur when large fragments of rock either fall from a cliff, or bounce down the side of a hill

Rockslides- rock slides occur on slopes that are less steep when rocks slide down the entire slope, rather than falling through the air.

Slump- a process in which a mass of bedrock or soil slides down a slope in one large mass. The mass slides down a curved plane of weakness. The slump block rotates and comes to rest tilted toward the cliff.

Mudflow- mudflow is the rapid, downhill flow of a fluid mixture of rock, soil, and water. Mudflows generally occur when there are long periods of rainfall, and the water content of the soil builds up. The watery soil runs downhill, and thickens up as it travels downhill and picks up rocks and soil.

Erosion by Moving Water

All precipitation that is not absorbed by the ground is called runoff. Runoff can sometimes transport sediment, and often ends up in streams.

Streams are bodies of running water that flows under the influence of gravity. The running water cuts into the surface of the earth, building a channel. This channel is surrounded by a bank on either side.

Material that is carried by water is called the stream's load. The faster a stream, the larger the load. A slow moving stream does not have the energy to carry a large load. Large sediments are usually dragged or pushed, a process called traction. Saltation is when particles bounce along the stream bed. Suspension is the process of sediments picked up by the stream. When rocks are dissolved, they are said to be carried by the process of solution.

When the load of a stream is a resistant rock, it can abraid the surface of bedrock. This prolonged scratching can gouge rock, and eventually a canyon can be formed. The Grand Canyon was created in this manner.

 

Check out the virtual river flooding interactive lab

Ocean Erosion

The majority of the water in any stream will eventually end up on the earth's oceans. The ocean's waves can be very destructive. Formations such as sea cliffs, caves, stacks, and terraces may result. Oceanic waves cause longshore current, which can move sediments along the coast.

Wind Erosion

Deflation is the process of wind picking up sediment and transporting it. Abrasion is the process in which wind blown sediments collide and wear rocks down.

When deflation is concentrated, a deflation hollow is formed. There are thousands of such hollows on the Great Plains. In wet years, they fill in with grass or water. In dry years, the grass dies, exposing the hollow to even more deflation.

When deflation occurs in the desert, the water table may be exposed. Vegetation forms around the hollow, and an oasis is formed. After deflation occurs for a period of years, only the objects that could not be transported by wind are left, usually large particles. An area with only these particles left is called desert pavement. Large particles that are exposed to abrasion become ventrifacts. The rock has a flattenned surface where it was exposed to abrasion by wind.

As sand travels through the desert, being eroded by wind, dunes start to form. The gradual slope that sand is blown along is called the wind face. The steep slope that faces away from the wind is called the slip face.

 

Life Cycle of a Stream

 

Young Streams

When a stream is recently formed, it generally flows qiuckly. The stream wears a long v-shaped valley
into the ground.

Mature Streams

In a mature stream, the slope gets eroded over time, and the slope becomes much less steep. The water flows more slowly and sediment accumulates at the bottom of the valley. Water flows around obstacles, rather than over them, and the stream's path forms curving loops called meanders. If the river overflows, the water slows down, deposits its load, and a broad flat floodplain is formed.

Old Age Streams

In an old stream, a large peneplain is formed. This is the area that used to be the floodplain, that had the stream meander though it. This peneplain has oxbow lakes and cutoffs.

When a stream meanders, the outside of the meander is heavily eroded. Thats where you can find cliffs. The inside of the meander is where a sand beach can be deposited. The water moves faster toward the outside of the bend because of the water's inertia. Because the water can't "turn" it bashes into the far side. Meanwhile, the water on the inside of the bend is moving slower. Heavier sediments will fall out of the stream creating a great flat beach to camp on.

 

 

Glaciers

 

Glaciers form after long periods of snowfall, usually in the mountains. After many meters of snowfall, the snow at the bottom begins to recrystalize under the great pressure caused by the weight of the overlying snow. As the snow begins to crytalize into solid ice, the pressure continues to increase. This squeezing motion forces the ice to begin flow as a liquid.

 

As glaciers move along, the bottom melts, and refreezes. As this occurs, the rocks and sediment lying under the glacier are picked up and frozen into the ice. This process is called plucking. Sediments the size of boulders can be plucked and carried thousands of miles. Sometimes a plucked scraps on the bedrock as the glacier grids slowly onward. This causes deep grooves in the rock called glacial scratches.

 

As the glacier begins to recede, or melt, it deposits these sediments. Boulders that are not native to the area that they are deposited on are called Erratics. Large holes or depression that are plucked by glaciers are called Kettles. The sediment that is pushed by the glacier, similar to a bulldozer, is then deposited as the glacier recedes. This is called Morraine. Long Island is one long morraine deposit from a bulldozing glacier 11,000 years ago. Glacial deposited sediments are unsorted.

 

with thanks to Mr. O'Sullivan