Metamorphism is the sum of all transformations which cause a rearrangement of the elements of a rock at the atomic scale, leading to a recrystallization in the solid state (with or without a fluid phase), as a result of variations in temperature, pressure or of composition.

There are three categories of rocks- igneous, sedimentary and metamorphic. Igneous rocks have been formed by solidification of hot silicate melts known as magmas, which may be erupted at the Earth's surface as lavas from volcanoes. Sedimentary rocks form by a variety of processes at low temperatures near the surface, including the bottom of the seas and oceans. Metamorphic rocks, the third category, were originally either igneous or sedimentary, but their character has been changed by processes operating below the surface of the Earth. The constituent minerals of the rocks may be changed, or the shapes, sizes and mutual spatial relationships of the crystals may change. There are a variety of processes involved, and they are collectively described by the term metamorphism. <...>

Sedimentary rocks form at low temperatures and pressures at the surface of Earth owing to deposition by water, wind, or ice. By contrast, igneous and metamorphic rocks form mainly below Earth’s surface where temperatures and pressures may be orders of magnitude higher than those at the surface, although volcanic rocks eventually cool at the surface. These fundamental differences in the origin of rocks lead to differences in physical and chemical characteristics that distinguish one kind of rock from another. Sedimentary rocks are characterized particularly by the presence of layers, although layers are also present in some volcanic and metamorphic rocks, and by distinctive textures and structures. Many sedimentary rocks are also distinguished from igneous and metamorphic rocks by their mineral and chemical compositions and fossil content.

It is always easier to write a textbook the second time around, or so I thought. Writing my first textbook entitled “Earth’s Materials: Minerals and Rocks” (published in 2001 by Prentice-Hall) was difficult because I had to make some tough decisions about what materials to include and what to leave out. Based on my own thinking and input from colleagues and students since that time, I decided to write a textbook on petrology and leave out mineralogy altogether. The result is the present textbook. It was anything but easy to write this book.