Metamorphic rocks are geological formations that represent a profound transformation within the Earth’s crust. They originate from existing igneous, sedimentary, or even other metamorphic rocks, which undergo significant changes without melting. This process alters their physical features, including their mineral composition and texture. Studying these rocks helps geologists understand Earth’s history.
The Transformative Process
Metamorphism involves subjecting a protolith, or original rock, to new conditions of heat, pressure, and chemically active fluids. These changes occur deep within the Earth’s crust, often associated with tectonic plate collisions or hot magma intrusions. Temperatures typically exceed 150 to 200 °C and pressures exceed 100 megapascals.
During this transformation, the rock remains in a solid state. The conditions cause existing minerals to recrystallize, where atoms rearrange into new configurations or grow into larger grains. New minerals can also form through chemical reactions, a process called neocrystallization. Chemically active fluids, often superheated water, facilitate these changes by dissolving and transporting ions, altering the rock’s composition.
Key Physical Features
Foliation is a planar arrangement of mineral grains or features in metamorphic rocks. This texture arises from directed pressure, which squeezes and aligns platy or elongated minerals, such as mica, perpendicular to the applied force. Slaty cleavage is the simplest form of foliation, where fine-grained minerals align to allow the rock to split into thin, flat sheets.
As metamorphic intensity increases, schistosity develops, characterized by the parallel alignment of larger, visible platy minerals like mica, giving the rock a layered or flaky appearance. Gneissic banding signifies an even higher degree of metamorphism, where minerals segregate into alternating light and dark layers, often coarse-grained and sometimes folded, creating a striped pattern.
Not all metamorphic rocks display foliation. Non-foliated rocks form under uniform pressure, where stress is equal in all directions, or from parent rocks composed of minerals that do not readily align, such as quartz or calcite. These rocks appear granular or crystalline, with interlocking grains. New minerals like garnet, staurolite, and kyanite often grow as larger crystals called porphyroblasts. Grain sizes often increase as metamorphism progresses, resulting in more coarsely crystalline structures.
Common Metamorphic Rock Examples
Slate is a fine-grained, foliated metamorphic rock that forms from the low-grade metamorphism of shale. It exhibits slaty cleavage, allowing it to split into thin, durable sheets. Its minerals, primarily microscopic clay and mica, are aligned perpendicular to the stress.
Marble is a non-foliated metamorphic rock derived from limestone or dolomite. It consists of recrystallized calcite or dolomite grains, which often interlock, giving it a crystalline texture. Marble lacks foliation because its constituent minerals do not align in a planar fashion.
Schist is a medium-grained, foliated metamorphic rock, often originating from mudstone or shale. It features schistosity, where visible platy minerals, particularly micas, are aligned parallel, creating a shimmering, layered texture. This alignment allows schist to split easily into thin flakes.
Gneiss is a high-grade metamorphic rock, coarse-grained and characterized by gneissic banding. These bands consist of alternating light-colored layers, rich in quartz and feldspar, and darker layers, containing minerals like biotite and amphibole. Gneiss can form from the metamorphism of various rocks, including granite or schist.
Quartzite is a non-foliated metamorphic rock formed from quartz sandstone. During metamorphism, the quartz grains recrystallize and interlock, creating a hard and durable rock. Like marble, its lack of platy minerals prevents the development of foliation.