Metamorphic rocks represent one of the three fundamental rock types, alongside igneous and sedimentary rocks. They are defined simply as existing rocks that have been transformed by intense heat and/or pressure within the Earth’s crust. This process, known as metamorphism, involves profound physical or chemical changes to the original material. The resulting rocks provide geologists with a detailed record of the extreme conditions deep inside the Earth.
How Parent Rocks Are Transformed
The transformation of the protolith is driven by three main agents: heat, pressure, and chemically active fluids. The initial composition of the protolith—which can be any type of rock, including igneous, sedimentary, or even another metamorphic rock—strongly dictates the resulting rock’s characteristics. For instance, a protolith rich in aluminum, such as shale, will produce very different metamorphic minerals than one composed primarily of quartz, like sandstone.
Heat often raises the rock’s temperature far above 200°C, which is necessary to destabilize the original minerals and encourage recrystallization in the solid state. This heat can come from the deep burial of rocks or from contact with hot, intruding magma bodies. Pressure, the second agent, can be categorized into two types.
Confining pressure, also known as lithostatic pressure, is uniform and equal in all directions, caused by the weight of overlying rock. This uniform pressure causes the rock to become denser, but it typically does not lead to a layered appearance. Directed pressure, or differential stress, is unequal pressure applied mostly from one direction, often associated with tectonic processes like continental collisions.
Chemically active fluids, predominantly hot water and carbon dioxide, act as a third agent, migrating through the rock’s pores and fractures. These fluids accelerate the metamorphic reactions by transporting dissolved chemical components, allowing new minerals to form. This fluid-assisted change in chemical composition is a process known as metasomatism.
Textural Characteristics: Foliated and Non-Foliated
Metamorphism dramatically alters the rock’s texture, which refers to the size, shape, and arrangement of its mineral grains. The texture is the most immediate way to classify a metamorphic rock, separating them into two main groups: foliated and non-foliated. Foliation describes a layered or banded appearance that results from the alignment of platy or elongated minerals.
This distinct layering is a direct consequence of directed pressure, which physically squeezes and rotates minerals like mica into parallel planes. As the intensity of metamorphism increases, the foliation becomes more pronounced, progressing from the microscopic layers in slate to the visible light and dark bands in gneiss. Gneissic banding, for example, features segregated layers of granular minerals (like quartz and feldspar) alternating with layers of platy minerals.
Non-foliated rocks lack this layered structure because they typically form where pressure is uniform or very low, such as during contact metamorphism near a magma intrusion. These rocks are characterized by a granular texture where mineral crystals are interlocked and randomly oriented. Quartzite and marble are common examples, as their primary minerals—quartz and calcite, respectively—do not naturally form elongated shapes, preventing alignment even under directed pressure.
Mineralogical Characteristics and Index Minerals
Metamorphic rocks form new, stable minerals through recrystallization. As the protolith is subjected to higher temperatures and pressures, its original minerals become unstable and chemically break down. The constituent atoms then recombine to form minerals that are stable under the new conditions, a process sometimes called neocrystallization.
The resulting mineral assemblage is a direct reflection of the pressure and temperature the rock experienced. Geologists use specific minerals, known as index minerals, to pinpoint the minimum pressure and temperature conditions required for their formation. The presence of an index mineral, such as chlorite, indicates low-grade metamorphism, while the appearance of garnet or staurolite signifies a much higher metamorphic grade.
Mapping the first appearance of these index minerals, like the transition from chlorite to biotite or garnet, allows geologists to define metamorphic zones. These zones, bounded by lines called isograds, provide a tool for reconstructing the ancient pressure and temperature distribution within a region. Index minerals provide evidence of the thermal history of the Earth’s crust.
Familiar Metamorphic Rock Types
Marble is a well-known non-foliated rock that forms from the metamorphism of limestone or dolostone, resulting in a mass of recrystallized calcite. Quartzite is another non-foliated example, derived from quartz-rich sandstone, where the quartz grains fuse together to create an exceptionally hard rock.
Among the foliated types, slate is a fine-grained rock formed from the low-grade metamorphism of shale. It exhibits a property called slaty cleavage, allowing it to be split into thin, flat sheets. Schist is a medium-to-coarse-grained rock characterized by a wavy foliation and often contains visible, sparkling flakes of mica and other index minerals like garnet.
Gneiss represents a high-grade metamorphic rock, showcasing the most extreme form of foliation through its distinctive, visible banding. It is often derived from granite or volcanic rocks and demonstrates how a rock’s physical structure can be completely reorganized under immense tectonic stress.