Rocks, Weathering, and Soil Information

The Effects of Weathering

• All exposed rock is in the path of weathering and erosion.

• Weathering is the process that breaks down rock and other substances at the Earth’s surface.

• Heat, cold, water, ice, carbon dioxide, oxygen etc.. all contribute to weathering.

Weathering and Erosion• Weathering examples:

– Repeated freezing and thawing. – Rainwater dissolving minerals.

• Erosion is the movement of rock particles by wind water, ice, or gravity.

• Weathering and erosion work together to carry away the rocks at the surface.

• There are two types of weathering:– Mechanical Weathering– Chemical Weathering

Mechanical Weathering• Mechanical Weathering: rock is physically

broken into smaller pieces that have the same chemical composition as the rock they came from. – Examples:

• freezing and thawing,• release of pressure,• growth of plants, • actions of animals, and • abrasion (grinding away of rock by

particles in the wind and water).

Chemical Weathering• Chemical Weathering: the process that

breaks down rock through chemical changes and produces rock particles that have a different mineral makeup from the rock they came from. – Examples:

• Hot or soft spots in rock • Water• Oxygen• Carbon Dioxide• Living organisms• Acid Rain

Rate of Weathering• The most important factors that determine the rate

at which weathering occurs are the type of rock and climate .

• Type of Rock– Minerals will determine the rate.– Permeable rocks (full of tiny air spaces) weather faster.

• Climate:– Weathering occurs faster in wet climates and higher

temperatures.

Soil Formation• Over many years, weathering

and erosion will cause the formation of soil.

• Soil is the loose, weathered material on the Earth’s surface in which plants can grow.

• Soil forms as rock is broken down by weathering and mixes with other materials on the surface.

Soil Composition

• Soil is constantly being formed wherever bedrock is exposed.– Bedrock is the solid layer of

rock beneath the soil.

• Soil is a mixture of rock particles, minerals, decayed organic material, air, and water.

Soil Composition and Texture• The decayed organic

material in the soil is humus. – Humus is a dark-colored

substance that forms as plant and animal remains decay.

• Soil texture depends on the size of individual soil particles.– Soil texture is important for

plant growth.

Soil Horizons• A soil horizon is a layer of soil

that differs in color and texture from the layers above or below it.– Topsoil (A horizon) is a crumbly,

dark brown soil that is a mixture of humus, clay, and other materials.

– Subsoil (B horizon) usually consists of clay, and other particles washed down from the topsoil, but little humus.

– C horizon contains only partly weathered rock.

The rate at which soil forms depends on the climate and type of rock.

Life in Soil

• Some organisms mix the soil and make spaces in it for air and water.

• Other soil organisms make humus, the material that makes soil fertile.– Humus forms through decomposition.– Fertile soil is rich in nutrients that plants

need.

• As plants shed leaves, they form a loose layer called litter.

With and Without Worms

Soil Types in the U.S.

• Tundra Soils

• Northern Forest Soils

• Prairie Soils

• Mountain Soils

• Southern Forest Soils

• Desert Soils

• Tropical Soils

Soil Conservation• Soil is one of Earth’s most

valuable resources because everything that lives on the land depends directly on indirectly on soil.

• Fertile soil is valuable because there is a limited supply. – Less than 1/8th of the land on

Earth has soils well suited for farming.

Soil Damage and Loss• Soil can become

exhausted, or lose its fertility.

• Soil can also become lost to erosion by water and wind. – Water erosion can occur

wherever soil is not protected by plant cover.

– Wind erosion caused the Great Dust Bowl!!

The Dust Bowl• In the 1930s plowing removed the grass form

the Great Plains and exposed the soil. In times of drought, the topsoil quickly dried out, turned to dust, and blew away.

• This event helped people appreciate the value of soil.

• Check It Out!!

Soil Lost To The Seas!!

Soil Conservation• Soil Conservation is the

management of soil to prevent its destruction.

• Two ways that soil can be conserved include contour plowing and conservation plowing.

Contour Plowing• Contour plowing is the

practice of plowing fields along the contours of a slope.

• This helps slow the runoff of excess rainfall and prevents it from washing the soil away.

Conservation Plowing

• Conservation Plowing disturbs the soil and its plant cover as little as possible.

• Dead weeds and stalks of the previous year’s crop are left in the ground to help return soil nutrients.