Metamorphic rocks form when igneous or sedimentary rocks are subjected to extreme heat, pressure or chemical reactions. These forces alter the composition of the rocks, leading to changes in the rocks' density, appearance and structure.
Tectonic forces are a major driver of rock metamorphosis. As tectonic plates move against each other they create massive forces on their composite rocks. This pressure alters the grain of the rocks, flattening layers of rock into thin striations. Simultaneously, this pressure creates heat, which causes some components to change form. As a result, rocks near plate boundaries often have unique metamorphic structures.
Metamorphosis is not always a violent process of plates grinding together; slower interactions with chemical solutions also lead to gradual changes in rock structure. Water and steam that pass through rocks dissolve some materials in the rocks and carry minerals that interact with the rocks. These processes lead to recrystallization of the rock structure.
Volcanoes produce huge quantities of metamorphic rock because they expose rock to several concurrent metamorphic forces. The magma and lava of the volcano heat rock to extreme temperatures, causing contact metamorphosis. Marble is created through this form of metamorphosis as limestone is baked by lava. Volcanoes also move minerals in their flow, introducing new materials to the rocks as they are undergoing change from heat. This environment allows rapid chemical changes in the rocks.