Download Teacher Background on Erosion, Weathering, Soil

Soil-The Top of the World
Teacher Background Information
Have you ever visited or seen Providence Canyon in Georgia? The Canyon apparently began
forming in the early 1800’s. It is thought that the forests in this area were clear cut for farming.
The natural vegetation was removed exposing the easily eroded Providence Sand. The erosion
continued at a high rate until it met a more resistant clay layer at the base of the Canyon. Visit
http://gastateparks.org/info/providence/ to see Providence Canyon.
Water has been shaping our Earth since the very first raindrop fell. The breaking up of rocks is
called weathering. Bits of rocks and soil are carried away in many ways. This movement is
called erosion. Rivers and streams can carry tons of soil and broken rocks. Frozen water in
glaciers slide across the Earth’s surface dragging along rocks and soil beneath them. Sand
carried by strong winds can wear away huge rocks. Just think of the process of sandblasting.
The moving grains of sand grind away bits of rock. This may take thousands of years, but one
day the rock will be worn away. Wind can cause grains of sand to travel hundreds of kilometers.
Dust in the air may actually be carried across whole continents.
The process of weathering is going on all the time. Mechanical weathering refers to the process
of breaking down rocks by physical means, such as by ice, wind, water, gravity, sunlight, plants,
and animals. In mechanical weathering, rocks break into smaller pieces but their chemical makeup stays the same. Think about scraping chalk against a chalkboard, as the particles are rubbed
on to the board, the tip becomes rounded and worn down. This occurs on Earth when rocks and
sediment grind against each other. Think about the smooth surface of river rocks that are worn
smooth by the constant flow of rushing water. Rocks heated by the sun expand and then
contract in the cool of darkness. Over time, this process can cause rocks to chip. Water in cracks
of rocks can freeze, expand, and cause the rock to break. Roots of trees can also break off parts
of rocks. Even animals digging burrows can assist in the weathering process. Physical
weathering can facilitate chemical weathering by breaking up rocks and exposing more of the
rocks to the elements.
Chemical weathering is the chemical breakdown of rocks into new substances. This is usually
caused by water, acids, air, and soil. Think about that piece of chalk mentioned earlier.
Dropping it in vinegar would cause it to eventually dissolve. Acid rain can cause rocks to
weather. Lichens growing on rocks produce organic acids that slowly weather the host rock.
Oxidation can cause rust. Scientists study rock strata for signs of oxidation to determine the
amounts of oxygen in Earth’s atmosphere at a particular time. So, in chemical weathering, new
minerals might be added or taken away from the rock. In oxidation, when the air or water
combines with the iron in rocks, we get the rust or iron oxide. Water changes the mineral
feldspar, found in many rocks, into clay.
Soil is made from bits of rock mixed with plant and decayed animal matter. Soil forms very
slowly. Wind, water, and temperature changes help make soil. Look at the rocks around you and
at the varied colors you see. Does that help you to understand why soils have different colors
too? In a million years from now, the rock you see may be soil. Soil varies from place to place.
S3E1c & 1d
16
GYSTC
Background
In some places, the soil is hundreds of meters thick. In other places it is only a few centimeters
deep. Sand, clay and loam are different kinds of soil.
Sand does not hold water very well. Just think of playing in sand at the beach. You have
experienced how fast water runs through sand. Clay is made up of certain kinds of rock dust and
bits of clay are packed tightly together. We in Georgia know the properties of clay well. It can
hold a great deal of water but when it dries, it cracks. Plants do not grow well in clay. Loam is a
mixture of sand, clay and humus (formed from decayed plants and animals). Loam is very good
for plant growth.
The uppermost layer of soil is called topsoil. Under the topsoil lies the subsoil, which tends to be
more clay and less or no organic matter. Beneath the subsoil are pieces of partially weathered
bedrock and then under that, solid bedrock. When thinking about soil forming factors, consider
parent material, climate, living organisms, topography, and time.
Students have, undoubtedly, experienced the fun of playing in the “dirt” or sand. However, they
probably have never given much thought to what it is or where it comes from. Help them to
realize that the handful of soil they may pick up in their hands has been on an incredible journey
through time. It is the basis of the ecosystem and supports life.
S3E1c & 1d
17
GYSTC
Background