Metamorphic rocks are formed when any pre-existing rock—whether igneous, sedimentary, or another metamorphic rock—undergoes significant transformation. The term “metamorphism” literally means “change in form,” reflecting the profound alteration these rocks experience without completely melting into magma. This solid-state change involves a rearrangement of mineral components, resulting in new mineral compositions and textures that are stable under the new environmental conditions. These transformations occur deep within the Earth’s crust, often where tectonic plates interact.
The Driving Forces: Heat, Pressure, and Fluids
Heat is a primary agent of metamorphism, causing chemical reactions and recrystallization of minerals within the rock. One source of this heat comes from the geothermal gradient, which is the natural increase in temperature with depth inside the Earth. A second major heat source is the intrusion of hot magma into cooler surrounding rock, which “bakes” the rock closest to the intrusion.
Pressure acts in two distinct ways. Confining pressure (lithostatic pressure) is the immense weight of the overlying rock pressing equally in all directions, resulting in a denser, more compact rock. Differential stress is directed pressure, such as the forces found during a continental collision, which deforms the rock and aligns mineral grains.
Chemically active fluids, usually hot water rich in dissolved ions, also play a significant role. These hydrothermal fluids enhance the mobility of chemical components, allowing for the dissolution of old minerals and the growth of new ones. This chemical change, sometimes referred to as metasomatism, can dramatically alter the overall composition of the rock.
Key Environments of Metamorphism
The environment where metamorphism occurs leads to two main types of formation. Contact metamorphism is a localized process driven primarily by heat from a magmatic intrusion. As the magma heats the surrounding rock, it creates a metamorphic zone called an aureole, where changes are greatest near the contact point and diminish further away. This type of metamorphism occurs under low-pressure conditions and results in a rock that is “baked” but not heavily deformed.
Regional metamorphism affects rocks over a massive area and is associated with the large-scale forces of mountain building and continental collisions. This setting involves both high temperatures and intense differential stress, as rocks are buried and squeezed between colliding tectonic plates. The combination of heat and directed pressure produces the majority of metamorphic rock found on Earth’s surface.
Structural Classification of Metamorphic Rocks
Metamorphic rocks are structurally classified based on the resulting texture. Foliated rocks possess a layered or banded texture, which occurs because differential stress forces platy or elongate mineral grains to align perpendicular to the direction of the greatest pressure. This alignment gives the rock a sheet-like structure, with examples ranging from fine-grained slate to coarsely banded gneiss.
Non-foliated rocks exhibit a massive, non-layered texture. This structure forms when metamorphism is dominated by heat, such as in contact metamorphism, where the pressure is confining and not directed. It can also occur when the parent rock is composed of minerals that are not easily flattened or aligned, such as the quartz in quartzite or the calcite in marble. In these cases, the rock recrystallizes into a dense mosaic of interlocking grains.