The Origins of Soil
Formation of Parent Material
Soil forms from different parent materials; one such parent material is bedrock. As rocks become exposed at the Earth’s surface, they erode and become chemically and physically altered. The type of soil that forms depend on the type of rocks available, the minerals in the rocks, and how minerals react to temperature, pressure, and erosive forces. Parent materials that form in place from the weathering of rock in place are called residuum. The major types of rocks that weather to form residuum are igneous, sedimentary, and metamorphic.
Igneous Rocks
Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. All magma develops underground, in the lower crust or upper mantle, because of the intense heat there. Igneous rocks can have many different compositions, depending on the magma they cool from.
Extrusive Igneous Rocks
Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly.
Intrusive Igneous Rocks
Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet. Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface.
Metamorphic Rocks
Metamorphic rock, any of a class of rocks that result from the alteration of preexisting rocks in response to changing environmental conditions, such as variations in temperature, pressure, and mechanical stress, and the addition or subtraction of chemical components. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. Metamorphism comprises changes both in mineralogy and in the fabric of the original rock.
Sedimentary Rocks
Sedimentary rocks are formed on or near the Earth’s surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth. Sedimentary rocks are the most common rocks exposed on Earth’s surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks. Sedimentary rocks are produced by the weathering of preexisting rocks and the subsequent transportation and deposition of the weathering products.
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