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. Classification schemes help us organize information and remember significant details about materials or objects, and they help us recognize similarities and differences among them. By the end of the 18th century, most geologists had accepted the genetic scheme for classifying rocks that we continue to use today. This scheme focuses on the origin (genesis) of rocks. 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. Metamorphic change occurs in the solid state, which means that it does not require melting. In the context of modern plate tectonics theory, different rock types form in different geologic settings, as we discuss in succeeding chapters.
Each of the three groups contains many different individual rock types, distinguished from one another by physical characteristics.
- Grain size: The dimensions of individual “grains” (here used in a general sense to mean fragments or crystals) in a rock may be measured in millimeters or centimeters. Some grains are so small that they can’t be seen without a microscope, whereas others are as big as a fist or larger. Some grains are equant, meaning that they have the same dimensions in all directions; some are inequant, meaning that the dimensions are not the same in all directions. In some rocks, all the grains are the same size, whereas other rocks contain a variety of grain sizes.
- Composition: A rock is a mass of chemicals. The term rock composition refers to the proportions of different chemicals making up the rock. The proportion of chemicals, in turn, affects the proportion of different minerals constituting the rock.
- Texture: This term refers to the arrangement of grains in a rock, that is, the way grains connect to one another and whether or not inequant grains are aligned parallel to each other. The concept of rock texture will become easier to grasp as we look at different examples of rocks in the following chapters.
- Layering: Some rock bodies appear to contain distinct layering, defined either by bands of different compositions or textures, or by the alignment of inequant grains so that they trend parallel to each other. Different types of layering occur in different kinds of rocks. For example, the layering in sedimentary rocks is called bedding, whereas the layering in metamorphic rocks is called metamorphic foliation.
Each distinct rock type has a name. Names come from a variety of sources. Some come from the dominant component making up the rock, some from the region where the rock was first discovered or is particularly abundant, some from a root word of Latin origin, and some from a traditional name used by people in an area where the rock is found. All told, there are hundreds of different rock names, though in this book we will introduce only about 30.