⛏️Intro to Geology Unit 7 – Metamorphic Rocks: Processes & Types

What Are Metamorphic Rocks?

• Form from pre-existing rocks that have undergone changes in physical and/or chemical conditions
• Changes occur in solid state, meaning the rock does not melt
• Require specific conditions of heat, pressure, and/or chemically active fluids to form
• Metamorphism alters the mineral composition, texture, and structure of the original rock
• Can form from igneous, sedimentary, or other metamorphic rocks
• Classified based on their texture and composition, which reflect the conditions of metamorphism
• Examples include marble (metamorphosed limestone) and quartzite (metamorphosed sandstone)

Key Metamorphic Processes

• Recrystallization involves the growth of new mineral grains from pre-existing ones without melting
  • Driven by changes in temperature, pressure, and/or chemical environment
  • Results in a new mineral assemblage that is more stable under the new conditions
• Metamorphic reactions involve the breakdown of unstable minerals and the formation of new, stable minerals
  • Controlled by the principle of chemical equilibrium
  • May involve the introduction or removal of chemical components (open system)
• Deformation occurs when rocks are subjected to stress, causing them to change shape or volume
  • Can result in foliation (alignment of mineral grains) or lineation (linear features)
  • Ductile deformation occurs at higher temperatures and pressures, allowing rocks to bend or flow
  • Brittle deformation occurs at lower temperatures and pressures, causing rocks to fracture or break
• Metasomatism is the chemical alteration of a rock by the introduction of fluids
  • Involves the exchange of chemical components between the rock and the fluid
  • Can lead to significant changes in the mineral composition of the rock

Types of Metamorphism

• Regional metamorphism occurs over large areas and is associated with mountain-building events
  • Caused by the collision of tectonic plates, resulting in high pressures and temperatures
  • Produces regionally extensive metamorphic rocks with distinct mineral assemblages and textures
• Contact metamorphism occurs when magma intrudes into surrounding rock, causing localized heating
  • Limited to the area immediately surrounding the intrusion (aureole)
  • Degree of metamorphism decreases with distance from the intrusion
• Hydrothermal metamorphism involves the interaction of rocks with hot, chemically active fluids
  • Often associated with volcanic activity or deep crustal processes
  • Can result in the formation of economically important mineral deposits (ore bodies)
• Burial metamorphism occurs when rocks are deeply buried and subjected to increased pressure and temperature
  • Typically affects sedimentary rocks in sedimentary basins
  • Degree of metamorphism increases with depth of burial
• Dynamic metamorphism occurs in localized zones of intense deformation, such as fault zones
  • Characterized by the development of highly deformed and recrystallized rocks (mylonites)
  • May involve significant changes in texture and mineral composition

Common Metamorphic Rocks

• Slate is a fine-grained, foliated metamorphic rock formed from shale or mudstone
  • Characterized by a well-developed slaty cleavage (ability to split into thin sheets)
  • Commonly used as a roofing material and for decorative purposes
• Schist is a medium-grained, strongly foliated metamorphic rock with visible mineral grains
  • Contains abundant mica minerals (biotite and/or muscovite), which define the foliation
  • May also contain other minerals such as quartz, feldspar, and garnet
• Gneiss is a coarse-grained, banded metamorphic rock with alternating light and dark mineral layers
  • Formed at high temperatures and pressures, often from the metamorphism of igneous or sedimentary rocks
  • Banding may be straight, wavy, or highly contorted, reflecting the deformation history
• Marble is a metamorphic rock composed primarily of recrystallized calcite or dolomite
  • Formed from the metamorphism of limestone or dolostone
  • Prized for its beauty and durability, often used in sculpture and as a building material
• Quartzite is a hard, compact metamorphic rock composed almost entirely of quartz
  • Formed from the metamorphism of quartz-rich sandstone
  • Highly resistant to weathering and erosion, often forming prominent ridges and peaks

Textures and Structures

• Foliation is the planar arrangement of mineral grains or aggregates in a metamorphic rock
  • Develops perpendicular to the direction of maximum compressive stress
  • Types include slaty cleavage (slate), schistosity (schist), and gneissic banding (gneiss)
• Lineation is a linear feature in a metamorphic rock, such as mineral alignment or stretching
  • Develops parallel to the direction of maximum stretching or shear stress
  • Can provide information about the deformation history and stress field during metamorphism
• Porphyroblasts are large mineral grains that grow within a finer-grained matrix during metamorphism
  • May contain inclusions of the surrounding matrix, preserving a record of the metamorphic history
  • Examples include garnet, staurolite, and kyanite
• Reaction rims are thin layers of minerals that form around pre-existing grains during metamorphism
  • Result from chemical reactions between the grain and the surrounding matrix
  • Can provide insights into the pressure-temperature conditions and chemical environment during metamorphism
• Metamorphic differentiation is the segregation of chemical components into distinct layers or zones
  • Can occur during metamorphism due to differences in chemical potential or mechanical properties
  • Results in the formation of compositional banding or segregations, such as quartz veins or calc-silicate layers

Metamorphic Facies and Grades

• Metamorphic facies are groups of metamorphic rocks that form under similar pressure-temperature conditions
  • Each facies is characterized by a distinct mineral assemblage that reflects the conditions of metamorphism
  • Examples include greenschist, amphibolite, and granulite facies
• Metamorphic grade refers to the intensity or degree of metamorphism a rock has undergone
  • Low-grade metamorphism occurs at relatively low temperatures and pressures, producing rocks like slate and phyllite
  • Medium-grade metamorphism occurs at intermediate temperatures and pressures, producing rocks like schist and gneiss
  • High-grade metamorphism occurs at high temperatures and pressures, producing rocks like granulite and migmatite
• Pressure-temperature (P-T) paths describe the changes in pressure and temperature a rock experiences during metamorphism
  • Can be determined by studying the mineral assemblages and textures of metamorphic rocks
  • Provide insights into the tectonic setting and burial/exhumation history of metamorphic terranes

Real-World Applications

• Metamorphic rocks are used as building materials, such as marble for sculpture and slate for roofing
  • Durability and aesthetic qualities make them valuable for construction and decoration
  • Examples include the Parthenon (marble) and historic buildings in Europe (slate)
• Metamorphic rocks can host valuable mineral resources, such as gold, copper, and zinc
  • Hydrothermal metamorphism can concentrate these elements into economically viable ore deposits
  • Examples include the gold deposits of the Mother Lode (California) and the copper deposits of the Andes (Chile)
• Metamorphic rocks provide insights into the tectonic history and evolution of the Earth's crust
  • The pressure-temperature conditions and deformation history recorded in metamorphic rocks can be used to reconstruct past tectonic events
  • Examples include the Himalayan mountain range (collision of India and Asia) and the Appalachian mountains (ancient collisional orogeny)
• Metamorphic reactions can be used to study the cycling of elements and the global carbon cycle
  • The formation and breakdown of carbonate minerals during metamorphism can influence atmospheric CO2 levels over geologic time scales
  • Examples include the metamorphism of limestone and the release of CO2 during contact metamorphism

Key Takeaways

• Metamorphic rocks form from pre-existing rocks that have undergone changes in physical and/or chemical conditions
• Key metamorphic processes include recrystallization, metamorphic reactions, deformation, and metasomatism
• Types of metamorphism include regional, contact, hydrothermal, burial, and dynamic metamorphism
• Common metamorphic rocks include slate, schist, gneiss, marble, and quartzite
• Textures and structures in metamorphic rocks, such as foliation and lineation, provide insights into the conditions and deformation history during metamorphism
• Metamorphic facies and grades reflect the pressure-temperature conditions under which metamorphic rocks form
• Metamorphic rocks have important real-world applications, including their use as building materials, hosts for mineral resources, and records of Earth's tectonic history
• Understanding metamorphic processes and the resulting rocks is crucial for unraveling the complex evolution of the Earth's crust and the cycling of elements over geologic time