Parent material


Parent material

Parent material, in soil science, means the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically get a great deal of structure and minerals from their parent material. Parent materials are made up of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering.

Residual

Non-transported parent material is sometimes called residual parent material. This often refers to bedrock.

Transported

Parent material is classified by its last means of transport. So material that was transported to a location by glacier, then deposited elsewhere by streams, that material is classified as stream transported parent material, or glacial fluvial parent material.

Glacial till refers to material dragged with a moving ice sheet. Because it is not transported with liquid water, the material is not sorted by size.

Water transported

Parent material transported by water is called alluvium. Within water transported parent material there are several important types.

Lake deposited parent material is called lacustrine parent material. Beach ridges may be present where glacial lakes once washed up sand. Lacustrine material is well sorted and fine textured, having finer silts and clays. Soils formed from lacustrine parent material have low permeability in part because of this high clay content.

Climate and weathering

Climate is generally considered the most important factor influencing physical and chemical weathering processes.

Physical weathering is especially important during the early stages of soil development. Rock can be disintegrated by changes in temperature which produces differential expansion and contraction. Changes in temperature can also cause water to freeze. The forces produced by water freezing can be as great as 2.1 * 10^5 kPa which can split rocks apart, wedge rocks upward in the soil, and heave and churn soil material.

Chemical weatheringThe principal agent is percolating rainwater charged with carbon dioxide from the atmosphere. Parent material becomes hydrolyzed by the acidic solution to produce minerals and to release cations.

See also

* Saprolite
* Soil production function


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