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Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering and the mode by which the materials were most recently transported.
Parent materials that are predominately composed of consolidated rock are termed residual parent material. The consolidated rocks consist of igneous, sedimentary, and metamorphic rock.
Parent material is classified by its last means of transport. Material that was transported to a location by glacier, then deposited elsewhere by streams, is classified as stream-transported parent material, or glacial fluvial parent material.
Within water transported parent material there are several important types.
Parent material transported by streams is called alluvium of which there are three main types. Floodplains are the parts of river valleys that are covered with water during floods. Due to their seasonal nature, floods create stratified layers in which larger particles tend to settle nearer the channel and smaller particles settle nearer the edges of the flooding area. Alluvial fans are sedimentary areas formed by narrow valley streams that suddenly drop to lowlands and widen dramatically. Sedimentary in these types of deposits tend to be larger closer to the uplands and finer near the edges of the fan. Delta deposits, the third of type of alluvium, are finer sediments that are discharged from streams into lakes and eventually settle near the mouth of the river.
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.
Ocean deposited parent materials, called marine sediments, are collections of material that have been carried by rivers and streams to the ocean and eventually sink to the bottom. Such materials can vary in texture.
Parent materials can also be transported by wind, which includes loess and wind-blown (aeolian) sand.
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 × 105 kPa, which can split rocks apart, wedge rocks upward in the soil, and heave and churn soil material.
Chemical weathering: the 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.