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.