The Scoop on Soil -- Background Information

Why Rocks Crack

It would be nice to age without changes occurring in our appearance. Unfortunately, human flesh deteriorates over time, and we witness this deterioration as alterations in our appearance. Changes in appearance can be seen in non-living things as well as living things. The earth has changed in its appearance over the passage of time. Like the flesh of the human body, the rocks that compose the earth are worn down and deteriorate with age. Many factors contribute to the breakdown of the rocks of the earth. The process of breaking down the rocks that compose the earth is called weathering. Weathering can be chemical or physical.

Physical and Chemical Weathering

Physical weathering changes the size of a rock but does not alter the composition of the rock. Temperature changes can cause physical weathering to occur. Rocks expand in the summer and contract in the winter. If you are unclear on the terms expansion and contraction, think about how tight your rings feel in the summer and how loose they feel in the winter. In other words, the fingers swell (expand) in hot weather and shrink (contract) in cold weather. This alteration of expanding and contracting in rocks weakens the rock and can cause it to crack and break into pieces. Roots from shrubs and trees can also grow between the openings in rocks, causing them to break apart. This also represents physical weathering.

Chemical weathering may change not only the size but also the composition of a rock. In this type of weathering, a new material is formed. Oxidation and carbonization are two forms of chemical weathering. Oxidation is the process in which combining oxygen combines with other materials to form new substances. Some rocks can appear to have a black, brown, or orange coloration because of oxidation. Iron oxide (rust) develops on these rocks when the iron mineral in the rock is exposed to the atmosphere. In effect, the rock has rusted. Carbonization is another agent of chemical weathering. Carbonization forms limestone caves when carbonic acid (formed from the union of rainwater and carbon dioxide in the atmosphere) reacts with limestone rock to dissolve it slowly. Over thousands of years a cave is formed. As time goes by, the cave will increase in size.

From Where Do We Get Soil?

What determines the speed and intensity of the weathering process? Factors include (1) the type of rock, (2) the type and hardness of the materials in the rock, and (3) the climate. Rocks that dissolve easily in water or acid weather much more quickly than insoluble rocks. Sedimentary rocks generally weather more quickly than igneous or metamorphic rocks. Sandstone weathers more quickly than granite. This is because sandstone is sand grains cemented together with natural materials and is more porous than granite. The more precipitation in an area, the more quickly the rock weathers. (The amount of precipitation can affect weathering.) A lot of weathering occurs in areas with hot, humid climates.

Weathering can create and shape structures. Mountains can be shaped through the weathering process. Weathering can also break down rocks into useful materials such as soil. Over time, weathering causes the bedrock of the earth to decompose. With the passage of years, soil is formed. The formation of soil occurs in soil horizons or layers. The uppermost layer (Horizon A) is composed of the nutrient-rich topsoil. It is here that plants are nurtured with essential minerals. Decayed plants and animals form humus, which enriches the topsoil. The layer beneath the topsoil is the subsoil (Horizon B). This layer has some nutrients, but certainly not to the extent found in the topsoil. Horizon B has an orange coloration due to the high content of clay particles. Water and most plant roots have great difficulty penetrating through the subsoil. Horizon C is the layer below the subsoil and just above the bedrock. It is composed of broken pieces of rock. When the process of soil formation is completed, all three soil layers exist. When rock has not been sufficiently transformed by weathering into all three layers, it is called immature soil. As time passes, weathering will change immature soil into mature soil (soil with all three layers present).

As weathering occurs, pieces of rock break off and fall to the ground. Some of these pieces remain in that location, and other pieces are carried away by natural agents such as wind or water. The transportation of these broken pieces of rock to other locations is called erosion. The four most common agents of erosion are running water, glaciers, wind, and gravity.

Erosion by Water

In most areas, running water is a dominant agent of erosion. Rivers and streams transport eroded material. All material transported by rivers and streams is called the stream load. The composition of the stream load is dependent on the speed and volume of water in the stream. Heavier particles of material bounce along the bottom of the stream, grinding and wearing down surfaces they strike. This action is called abrasion. Lighter particles become suspended in the water and are eventually dropped when the stream slows in velocity. The weight and composition of these particles influence the order in which the deposition of particles takes place. Running water can even be so powerful that it creates new landscapes. Rainwater carries particles down mountain slopes and forms river valleys between two mountains.

Erosion by Glaciers

Glaciers (mounds of sliding snow on top of ice) can cause erosion. The icy bottom of the glacier causes it to flow smoothly. It may move only an inch a day, but this is enough to carry along particles underneath it. Glaciers can transport small particles and large boulders, but they cover much less area than running water.

Erosion by Wind

Wind can pick up and move sediments easily because of the great velocity it can generate. These traveling sediments suspended in air rub against rocks and other surfaces. This can be very dangerous. If you have ever been in a wind gust on a sandy ball field or playground, you know that the whirling sand particles sting as they strike your body. Think what damage huge dust storms might do to plants, animals, and buildings. In the 1930s a huge dust storm occurred in the Great Plains. Long dry spells in that area had killed the vegetation and left the topsoil exposed to the air. Violent winds came along and picked up the topsoil and bombarded nearby crops and buildings. The abrasive action of these storms caused massive devastation.

The clay and silt particles in soil can damage some structures, but the most devastating culprit is sand particles. Sand grains are much larger and much more abrasive. They can damage structures and grind giant boulders and small rocks into shapes called ventifacts. Sand particles can wear away materials to form smooth surfaces called facets.

As wind carries particles of sand along, it will eventually lose energy. When the wind loses energy and slows, it drops the sand in mounds called sand dunes. These dunes can migrate over time, covering and destroying objects in their path.

Erosion by Gravity

Gravity is the last and most influential agent of erosion. This is so because gravity is the underlying force behind all erosion. Gravity causes water to run downhill, it causes glaciers to flow, and it produces winds by pulling heavy colder air beneath lighter warm air. Gravity is the factor responsible for phenomena such as landslides, rock slides, and snow slides.

Preventing Soil Erosion

What effect does erosion have on the surface of the earth? Valuable topsoil is carried away by erosion. This loss of topsoil reduces soil fertility and lowers crop production. This can cause severe environmental and economic problems in a region. This depletion of topsoil can be controlled by a variety of methods. Use of these methods is called soil conservation.

Farmers practice soil conservation techniques daily. They use vegetation to cover topsoil and prevent erosion. The roots of the vegetation extend down into the soil and hold the soil in place. When farmers do not have crops planted in their fields, they allow wild vegetation to grow there as a protective measure.

Lumber companies can practice wise soil conservation by replanting trees after cutting down trees for lumber. Selective cutting of mature trees, as opposed to clear cutting of all trees in an area, is a form of soil conservation.

Soil conservation is important to our future and our food supply. Some methods employed by farmers to conserve the topsoil include the following:

1. Planting windbreaks: Belts of trees are planted along the edge of a field to slow wind erosion.
   
   
2. Contour plowing: Crops are planted in rows parallel to the contour of the land to prevent rapid flow of water downhill.
   
   
3. Terracing: The slope of a hill is flattened by building small terraces or ridges that resist erosion.
   
   
4. Strip cropping: Alternating crops are planted next to one another, rather than having bare ground in between rows (an example is corn and alfalfa planted side to side with no uncovered soil in between).
   
   
5. No till: An area is plowed, planted, and fertilized all at once, so the ground is not disturbed again until harvest.

Even though the earth changes with age, humans can help to control some undesirable alterations. Uncontrolled erosion is a change in the earth that humans can help to slow. Soil conservation is important to our generation and future generations. Our food supply depends on it.

Excerpted from Hands-On General Science Activities with Real-Life Applications.