Metamorphic rocks are rocks that have undergone significant transformations from their original state. These changes occur deep within the Earth’s crust, altering a rock’s physical and chemical properties without melting it entirely. This process results in new rock types with unique characteristics, distinct from their parent materials.
The Forces of Change
The formation of metamorphic rocks is driven by three primary forces: heat, pressure, and chemically active fluids. Heat often originates from the Earth’s geothermal gradient, where temperature increases with depth, or from the intrusion of hot magma. This elevated temperature causes existing minerals to recrystallize into larger grains or form new minerals stable under the new conditions.
Pressure plays a crucial role in metamorphism. Confining pressure, exerted equally from all directions by the weight of overlying rocks, compacts the rock and reduces pore space. Directed pressure, applied unequally from one direction (often associated with tectonic forces like mountain building), causes mineral grains to align perpendicular to the maximum stress, leading to a distinctive fabric.
Chemically active fluids, primarily water with dissolved ions, facilitate metamorphic reactions. These fluids permeate rock pores and grain boundaries, acting as catalysts. They dissolve existing minerals and transport components, leading to the precipitation of new minerals. This process, known as metasomatism, can significantly alter the rock’s overall chemical composition.
Different Metamorphic Environments
Metamorphism occurs in various geological settings, each characterized by specific combinations of heat, pressure, and fluid activity. Regional metamorphism is the most widespread type, affecting vast areas of the Earth’s crust during mountain-building events or deep burial. This process involves both high temperatures and intense directed pressures, leading to the formation of foliated metamorphic rocks like schist and gneiss over large regions.
Contact metamorphism occurs when existing rocks come into direct contact with or are close to an intrusion of hot magma. This type of metamorphism is primarily driven by heat, which bakes the surrounding rocks, causing recrystallization and the formation of new minerals. The effects are generally localized, creating a metamorphic “aureole” or halo around the igneous intrusion, where intensity decreases with distance from the heat source. Rocks like marble and quartzite can form in these environments.
Dynamic metamorphism, also known as fault-zone metamorphism, takes place along active fault lines where rocks are subjected to intense directed pressure and grinding forces. Friction along these zones can generate significant heat, and the intense shearing forces can pulverize existing minerals. This process often results in the formation of fine-grained, highly fractured rocks like mylonite, where minerals may recrystallize under the extreme stress.
What Changes Inside the Rock
During metamorphism, rocks undergo profound internal transformations, affecting both their mineral composition and physical texture. Mineralogical changes occur as existing minerals become unstable under new conditions and transform into new, more stable mineral phases. For example, clay minerals in shale can recrystallize into mica minerals like muscovite or biotite under increasing heat and pressure. Quartz sand grains in sandstone can recrystallize and interlock to form the much harder rock, quartzite.
Textural changes are also prominent, particularly the development of foliation, a planar arrangement of mineral grains or structural features within the rock. This alignment is a direct result of directed pressure, causing platy minerals like mica to grow perpendicular to the applied stress. Examples of foliation include the fine layering in slate, the wavy appearance of schistosity in schist, and the distinct light and dark banding of gneiss. These textures provide clues about the pressure conditions during metamorphism.
Some metamorphic rocks exhibit non-foliated textures, meaning their mineral grains do not show a preferred orientation. This often occurs when confining pressure is the dominant force or when the rock is composed primarily of equidimensional minerals like quartz or calcite. In these cases, minerals recrystallize without developing a planar fabric, such as in marble (from limestone) or quartzite (from sandstone). The concept of a “protolith,” or parent rock, is fundamental because its original chemical composition heavily influences the types of minerals that can form during metamorphism.
Familiar Metamorphic Rocks
Many common rocks are products of metamorphic processes. Slate, a fine-grained metamorphic rock, forms from the low-grade regional metamorphism of shale or mudstone. Its characteristic cleavage allows it to split into thin, flat sheets. The platy clay minerals in the shale recrystallize into tiny mica flakes, aligning perpendicular to the applied pressure.
Marble is a widely recognized metamorphic rock, originating from the metamorphism of limestone or dolostone. This transformation typically occurs under contact or regional metamorphic conditions, where the calcite or dolomite minerals recrystallize into a denser, interlocking mosaic of coarser grains. This recrystallization often erases the original sedimentary textures, giving marble its characteristic sugary appearance.
Quartzite forms when quartz-rich sandstone undergoes metamorphism, either through regional or contact metamorphism. The individual quartz grains in the sandstone recrystallize and interlock, forming a much harder and more durable rock that breaks across grain boundaries rather than around them. This makes quartzite highly resistant to weathering.
Schist is a medium-to-coarse-grained metamorphic rock that commonly forms from the regional metamorphism of shale, slate, or basalt. It is characterized by its prominent schistosity, a flaky or scaly foliation caused by the parallel alignment of platy minerals like mica, chlorite, or talc. The specific minerals present in schist depend on the protolith and the metamorphic grade.
Gneiss represents a higher grade of regional metamorphism, often forming from the transformation of granite, schist, or other igneous and sedimentary rocks. It is distinguished by its distinct banding, known as gneissic banding, where light-colored felsic minerals (like quartz and feldspar) are segregated into layers separate from darker mafic minerals (like biotite and amphibole). This banding is a result of extreme temperatures and pressures causing mineral segregation.