Metamorphic Rocks: Transformation Through Heat Pressure and Chemical Processes
Metamorphic Rocks: Transformation Through Heat Pressure and Chemical Processes
Metamorphic rocks are formed when existing rocks, either igneous, sedimentary, or older metamorphic rocks, are transformed by heat, pressure, or chemically active fluids without melting completely. The word metamorphism comes from Greek roots meaning change of form, and metamorphic rocks record the changes that occur when rocks are subjected to conditions different from those under which they originally formed. Metamorphic rocks provide important information about the tectonic processes that deform the Earth’s crust, the conditions at depth within mountain belts, and the thermal history of specific regions. Many economically important minerals, including garnet, graphite, and talc, are found in metamorphic rocks. This guide explores the processes of metamorphism, the classification of metamorphic rocks, and their significance for understanding Earth dynamics.
The Conditions of Metamorphism
Metamorphism occurs when rocks are subjected to conditions of temperature and pressure that are different from those under which they originally formed. The temperatures of metamorphism range from about one hundred fifty degrees Celsius, where diagenesis ends, up to about nine hundred degrees Celsius, where rocks begin to melt. The pressures of metamorphism range from low pressures near the surface to very high pressures in deep subduction zones.
The conditions of metamorphism are determined by the tectonic setting. Regional metamorphism occurs over large areas in mountain belts, where rocks are buried deeply and subjected to high pressures and temperatures associated with plate collision. Contact metamorphism occurs when hot magma intrudes cooler surrounding rocks, baking them in a zone around the intrusion called an aureole.
Types of Metamorphism
Regional metamorphism is the most widespread type and is associated with convergent plate boundaries and mountain building. As tectonic plates collide, rocks are buried to depths of ten to thirty kilometers, where they experience both high pressure and high temperature. The resulting metamorphic rocks are typically foliated and show evidence of deformation. The Barrovian sequence of metamorphic zones, defined by the appearance of index minerals including chlorite, biotite, garnet, and sillimanite, records increasing metamorphic grade in regional metamorphic terrains.
Contact metamorphism occurs adjacent to igneous intrusions and is dominated by the effects of heat rather than pressure. The zone of contact metamorphism, known as the aureole, may extend from a few centimeters to several kilometers from the intrusion depending on its size and temperature. Contact metamorphic rocks are typically non-foliated and include hornfels and marble.
Foliation and Metamorphic Texture
Foliation is the most distinctive texture of metamorphic rocks and results from the alignment of mineral grains under directed pressure. Platy minerals including mica and chlorite rotate and recrystallize perpendicular to the direction of maximum stress, producing a planar fabric. The development of foliation records the deformation history of the rock.
Different types of foliation characterize different metamorphic grades. Slaty cleavage is the fine foliation characteristic of slate, formed at low metamorphic grades. Schistosity is a coarser foliation with visible mica crystals, characteristic of schist. Gneissic banding is a compositional layering with alternating light and dark mineral bands, characteristic of gneiss. The type and intensity of foliation provide information about the conditions of metamorphism.
Classification of Metamorphic Rocks
Metamorphic rocks are classified based on their texture and mineral composition. Foliated metamorphic rocks include slate, which has extremely fine-grained foliation; phyllite, which has a silky sheen from larger mica crystals; schist, which has coarse, visible mica crystals; and gneiss, which has distinct compositional banding. The progression from slate to gneiss represents increasing metamorphic grade.
Non-foliated metamorphic rocks lack aligned mineral grains and typically form under conditions of uniform pressure or from rocks composed of equant minerals. Marble forms from metamorphism of limestone and is composed of recrystallized calcite. Quartzite forms from metamorphism of sandstone and is composed of recrystallized quartz. Hornfels forms from contact metamorphism of various rock types and is typically fine-grained and hard.
Metamorphic Grade and Index Minerals
Metamorphic grade describes the intensity of metamorphism, ranging from low grade at low temperatures and pressures to high grade at high temperatures and pressures. Index minerals are particular minerals that form only under specific conditions of temperature and pressure, allowing geologists to determine the metamorphic grade of a rock.
The classic Barrovian sequence of index minerals begins with chlorite at low grade, followed by biotite, garnet, staurolite, kyanite, and sillimanite at increasing grades. Each index mineral appears at a specific temperature and pressure, marking isograds, lines on a map that separate zones of different metamorphic grade. Mapping isograds reveals the thermal structure of metamorphic belts and the tectonic processes that produced them.
Metamorphic Rocks and Plate Tectonics
The distribution of metamorphic rocks reflects plate tectonic processes. Regional metamorphic belts are associated with convergent plate boundaries, where continental collision produces high pressures and temperatures. The metamorphic rocks of the Himalayas record the collision of India with Asia. The Franciscan Complex of California preserves high-pressure, low-temperature metamorphic rocks that formed in a subduction zone.
The combination of metamorphic grade and tectonic setting allows geologists to reconstruct past tectonic events. Blueschist, a metamorphic rock containing the blue mineral glaucophane, forms only at high pressures and low temperatures characteristic of subduction zones. Eclogite, a dense metamorphic rock composed of garnet and pyroxene, forms at very high pressures in deeply subducted crust. The presence of these rocks at the surface today reveals where ancient subduction zones existed.
Frequently Asked Questions
What is the difference between metamorphic and igneous rocks? Metamorphic rocks form from the transformation of existing rocks without complete melting. Igneous rocks form from the cooling of completely molten magma or lava.
Can metamorphic rocks contain fossils? Generally no. The heat and pressure of metamorphism would destroy fossils. However, very low-grade metamorphic rocks may retain some fossil evidence.
What is the difference between foliation and bedding? Bedding is a primary sedimentary structure formed during deposition. Foliation is a secondary metamorphic structure formed by the alignment of minerals under directed pressure. They can be distinguished by their relationship to other structures and the minerals involved.
What happens if metamorphic rocks melt? If metamorphic rocks are heated enough, they begin to melt, producing magma. Partial melting of metamorphic rocks at high metamorphic grades can produce granitic magmas that rise and cool to form igneous rocks.
Conclusion
Metamorphic rocks record the dynamic processes that transform the Earth’s crust in response to changing temperature and pressure conditions. The study of metamorphic rocks provides essential information about the tectonic forces that build mountains, the thermal structure of the crust, and the conditions at depth within the Earth. Understanding metamorphism is essential for interpreting the geological history of deformed terrains, finding mineral resources associated with metamorphic processes, and understanding the large-scale dynamics of the Earth system.