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USGS Geology in the Parks

Visual Glossary

A cross-section through typical soil horizons
Nashville-series soil horizon, image from USDA Natural Resource Conservation Service website. For more information click here.

Soil is more than just dirt!

Soil is all loose, unconsolidated earth and organic materials above bedrock that support plant growth.

Given enough time, solid rock exposed at the Earth's surface is broken down into rock and mineral fragments by weathering. That's why most of the land surface is covered by a layer of loose material that geologists call surficial materials, but is often called dirt.

Soil is more than just dirt! While a layer of rock and mineral fragments is the prime ingredient, it takes decayed plant and animal remains (organic matter), and water to turn rocky debris into soil. Changing the proportions of these key ingredients produces a remarkable variety of soil types.

Rock, time, and climate

Every soil starts from a rocky parent (the technical term is parent material) and the type of parent rock has a strong influence on soil formation. Different types of rocks have different chemical makeups. As rock and minerals are broken down by weathering, chemical reactions between minerals, water, and air may produce new minerals such as clays. Disintegrating rock and weathering products accumulate to begin the soil-forming process.

Time, as always in geology, is another critical factor in soil formation. The characteristics of a particular soil depend largly on the lenght of time it's had to develop. When soils first begin forming, the nature of the parent material overwhelmingly determines the nature of the soil. Over time, parent material becomes more weathered and climate plays an increasingly important role in the soil-forming process.

Climate is perhaps the most important ingredient in soil formation. The amount of rain and other forms of precipitation and temperature patterns influence everything from the rate of rock weathering to the types of plants that grow on soils. As a consequence, desert, rain forest, and grassland soils develop distinctly different characteristics.


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