In this article we shall look at the most significant way in which geologists classify rocks. The reader should recall from the article on minerals that a rock is an aggregate of one or more minerals or mineraloids.
Igneous, sedimentary, and metamorphic rocks
There are all sorts of ways that we might classify rocks. We might, for example, divide them up according to chemistry: and indeed the distinction between silicates and carbonates is a useful one. We might also classify rocks according to whether they contain felsic or mafic minerals, and as we shall see this is a good way to classify certain rocks. But the most fundamental way in which geologists classify rocks is to label them as igneous, sedimentary, or metamorphic.
- Igneous rocks are rocks formed by the cooling and setting of molten rock.
- Sedimentary rocks are formed by sediment (for example, sand or mud) turning into rock (such as sandstone or mudrock).
- Metamorphic rocks are formed when rocks are subjected to heat, to pressure, to chemical reactions, or to any combination of these three, in such a way as to change the properties of the original rock in some way.
The rock cycle
The three types of rock can be converted on into the other by geological processes. Metamorphism can turn igneous and sedimentary rocks metamorphic; erosion can turn igneous and metamorphic rocks into sediment which can then become sedimentary rock; and sedimentary and metamorphic rocks can be melted down into molten rock which can then cool to form igneous rock.
The relation between the various kind of rocks can be summarized by a diagram of the rock cycle. One representation of it is shown to the right.
How do we know?
At this point we are beginning to touch on the main theme of this textbook. For to classify rocks as igneous, sedimentary, or metamorphic is implicitly to classify them not by their directly observable properties such as color or density or chemical composition, but by their history as inferred from their present-day properties. The reader will therefore want to know how this history is inferred. This question will be answered in separate articles on igneous, sedimentary, and metamorphic rocks.
In the meantime, let us observe how intrinsically historical geology is. After we've got past the most basic of considerations such as defining a mineral and defining a rock, we are plunged into historical considerations. And this is not just because this course is about historical geology: it would be the same if it was an introduction to how to find oil. Geology is so intrinsically a historical science that if we tried to do without historical inferences we might as well classify rocks by how pretty they are for all the good it would do us.