In the context of modern plate tectonics theory, different rock types form in different geologic settings, as we discuss in succeeding chapters.Įach of the three groups contains many different individual rock types, distinguished from one another by physical characteristics. Metamorphic change occurs in the solid state, which means that it does not require melting. Using this approach, geologists recognize three basic groups: igneous rocks, which form by the freezing (solidification) of molten rock sedimentary rocks, which form either by the cementing together of fragments (grains) broken off preexisting rocks or by the precipitation of mineral crystals out of water solutions at or near the Earth’s surface and metamorphic rocks, which form when preexisting rocks change character in response to a change in pressure and temperature conditions. This scheme focuses on the origin (genesis) of rocks. By the end of the 18th century, most geologists had accepted the genetic scheme for classifying rocks that we continue to use today. Classification schemes help us organize information and remember significant details about materials or objects, and they help us recognize similarities and differences among them.
Beginning in the 18th century, geologists struggled to develop a sensible way to classify rocks, for they realized, as did miners from centuries past, that not all rocks are the same.
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