Metamorphic rocks are one of the three major rock types on Earth, formed through a process of profound transformation. They begin as existing igneous, sedimentary, or even other metamorphic rocks, which are then changed by conditions deep within the Earth’s crust. This transformation causes physical and chemical changes, resulting in a completely new type of rock.
The Process of Metamorphism
The transformation of an existing rock, known as the protolith, occurs when it is subjected to intense heat and pressure, often coupled with chemically active fluids. This process, called metamorphism, changes the protolith’s mineral composition and texture without fully melting the rock. If the rock melted entirely, it would become an igneous rock upon cooling.
Temperatures involved in metamorphism generally range from above 150°C up to 700°C, while pressures can exceed 100 megapascals. The immense weight of overlying rock layers and stresses from tectonic plate collisions create these high-pressure environments. Directed pressure, where force is applied unevenly, is particularly important because it causes mineral grains to reorient themselves.
Chemically active fluids, primarily hot water rich in dissolved ions, circulate through the rock’s pore spaces and fractures. These fluids facilitate the growth of new, stable mineral crystals by transporting chemical components. The combination of heat, pressure, and fluid activity causes the original minerals to recrystallize. This results in a denser and more stable rock structure that is in equilibrium with the new environmental conditions.
Categorizing Metamorphic Rocks
Geologists primarily classify metamorphic rocks based on their texture, which is the size, shape, and arrangement of the mineral grains. This textural classification provides insight into the type of pressure the rock experienced during its formation. The two main textural groups are foliated and non-foliated rocks.
Foliated rocks exhibit a layered or banded appearance, resulting from directed pressure forcing platy or elongated mineral grains to align parallel to one another. This alignment is perpendicular to the direction of maximum stress, giving the rock a preferred plane of weakness that allows it to be split into sheets. The degree of foliation often increases with the intensity of the heat and pressure applied.
Non-foliated rocks lack a distinct layered texture and tend to have a massive, crystalline structure. This absence of alignment occurs either because metamorphism happened under uniform pressure, such as deep burial, or because the protolith was composed of minerals not easily flattened, like quartz or calcite. In these rocks, the mineral grains simply grow and interlock without a preferred orientation.
Common Metamorphic Rock Examples
Foliated Examples
Slate is a fine-grained, low-grade metamorphic rock formed primarily from the metamorphism of shale. It exhibits a well-developed, flat foliation, called slaty cleavage, which allows it to be easily split into thin, durable sheets. The alignment of its microscopic clay and mica minerals gives it a dull luster and makes it useful for roofing tiles and floor surfaces.
Schist is a medium-grade rock that forms when slate is subjected to greater heat and pressure. It is characterized by a schistosity texture, where visible, platy minerals, such as muscovite or biotite mica, are aligned in layers. These aligned, larger mineral grains give the rock a distinctive, often shiny appearance.
Gneiss represents a high-grade metamorphic rock, often formed from the intense metamorphism of granite or volcanic rocks. It displays a coarse, banded texture where light-colored mineral layers (quartz and feldspar) alternate with dark-colored mineral layers (biotite and hornblende). This segregated banding, known as gneissic banding, results from the highest temperatures and pressures.
Non-foliated Examples
Marble is a non-foliated rock that forms from the metamorphism of limestone or dolostone, which are rich in calcite. During metamorphism, the original calcite crystals recrystallize and grow into a mosaic of interlocking, larger crystals. This process obliterates any original layering and produces a rock prized for its pure color and capacity to be polished.
Quartzite is another non-foliated rock, derived from the protolith sandstone, which is composed mainly of quartz grains. The intense heat and pressure cause the quartz grains to fuse and recrystallize, creating a rock that is significantly harder and more durable than the original sandstone. Because the quartz grains are interlocked, the rock will fracture straight through the grains rather than around them, making it exceptionally resistant to weathering.