Introduction

If the Earth’s water were removed, most of its surface would be covered by sediments and sedimentary rock. Like icing on a cake, however, this thin veneer merely covers a voluminous and equally interesting interior: hot deep crust, a slowly convecting mantle, rising magmas, and sinking oceanic plates. Using round numbers, the silicate Earth makes up 84% of the planet’s volume. Because the sedimentary cover is so thin, and only a minute fraction of the upper mantle and crust are molten at any given moment, and because deformation and other solid state processes have modified most older sedimentary and igneous rocks, most of our planet is made of metamorphic rock.