Weathering has led to valuable mineral deposits and has provided materials for sedimentary rocks. Most important, weathering has helped form a priceless resource—Earth’s life-supporting SOIL.

A soil is formed as its parent material is weathered away. The rate of this weathering depends on the type of rock the climate of the location. Over time, organic material—decaying plant and animal remains—are mixed with the materials that have weathered away from the bedrock and a layer of soil covers the parent material. Average soil: 45% mineral, 5% organic material, 25% water, 25% air

Soil scientists can study soils by digging down to the parent material. The cross section of earth exposed by the digging is called the soil profile.

colorcomposition A-horizon (top soil) Usually dark to light gray Fine particles of weathered rock materials mixed with humus B-horizon (sub soil) Often red or brown Clay, iron oxides, and dissolved minerals washed down (leached) from A- horizon C-horizon (regolith) Dependent on parent rock Partially weathered rocks (fragments) (Unweathered bedrock)

The rock material in soil contains 3 noticeable parts: Sand (2mm-.05mm) Silt (.05mm -.002mm) Clay (<.002mm) The variety of combinations determines a soils texture. The amount of these materials in a particular soil can affect the soil’s ability to hold water and air which are necessary for plant growth. For example, soil that is very sandy holds water easily but dries out quickly. LOAM (a mix of sand, silt, clay, and organic material) is a soil texture with both the ability to hold water and allow drainage.

Time Parent material: Plants and animals Topography *CLIMATE (see next slide) Sandy soil is rich in quartz which is from weathered granite Clay-rich soil comes from basalt and is rich in feldspar

climateSoil characteristics ArcticMostly mechanical weathering breaks up the rock but little organic material, thus thin top soil Temperateb/c moisture and seasonal changes in temps cause high rates of both mechanical and chemical weathering, as well lots of organic material, these top soils are often thicker and fertile for farming. Tropicalb/c of climate, rock materials weather quickly and soil profiles are deep; b/c of heavy rain, top soils are thin Desertb/c of lack of moisture, vegetation is limited, contributing to wind erosion. Soils are usually sandy; top soil is thin

CLICK on this pyramid to see an ANIMATION of how this works