A rock is a naturally occurring solid substance composed of one or more minerals. Broadly speaking, there are three types of rock--sedimentary, igneous, and metamorphic--each of which is covered in greater detail later in this chapter.
Igneous rocks are those that either cool from magma (molten rock) or material expelled from volcanoes. The two main types of igneous rocks are plutonic (or, intrusive) and volcanic (or, extrusive).
Sedimentary rocks are formed from either broken fragments of older rocks, or are precipitated chemically from water.
Metamorphic rocks form from preexisting rocks (sedimentary, igneous, or other metamorphic rocks) that have been subjected to heating and pressure (but not melting), which results in changes to their mineral composition and crystal textures.
The Rock Cycle
The rock cycle, introduced at the end of this chapter, describes the many processes that produce rocks, while also illustrating differences between the rock types. One type of rock may be transformed into either of the other types, often with the help of other parts of the Earth system, such as plate tectonics, the water cycle, and biological processes, to name a few.