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• Metamorphic rocks are produced from preexisting igneous, sedimentary, or even other metamorphic rocks
• Metamorphism


  The transformation of one rock type into another
• Metamorphism


  • It leads to changes in the mineral content, texture, and sometimes the chemical composition of rocks
  • It takes place where preexisting rock is subjected to new conditions, usually elevated temperatures and pressures, that are significantly different from those in which it initially formed
• The intensity of metamorphism can vary substantially from one environment to another
• Low-grade metamorphic environments

  • Shale becomes slate
• In high-grade metamorphism, features like bedding planes, fossils, and vesicles that existed in the parent rock are obliterated
• When rocks deep in the crust are subjected to directed pressure, the entire mass may deform, producing large-scale structures, mainly folds
• During metamorphism the rock remains essentially solid—when complete melting occurs, we have entered the realm of igneous activity
• Agents of metamorphism

  Heat, pressure (stress), and chemically active fluids
• Heat as a metamorphic agent

  It provides the energy needed to drive the chemical reactions that result in the recrystallization of existing minerals and/or the formation of new minerals
• Heating promotes recrystallization of mineral grains and causes one or more minerals to become chemically unstable, leading to the formation of new minerals
• Source of heat


  Earth's internal heat from radioactive decay and thermal energy remaining from planet formation
• Environments where rocks may be carried to great depths and heated include convergent plate boundaries and large sedimentary basins
• Heat may also be transported from the mantle into the crust by igneous intrusions
• Confining pressure


  Pressure that increases with depth as the thickness of the overlying rock increases, causing the spaces between mineral grains to close and producing a more compact rock
• Differential stress


  Unequal forces in different directions that can fold and deform rocks
• In surface environments, rocks are brittle and tend to fracture when subjected to differential stress, while in high-temperature environments, rocks are ductile and their mineral grains tend to flatten and elongate
• Chemically active fluids

  Hot fluids expelled from dehydrating minerals that promote recrystallization and can transport mineral matter over considerable distances
• When substantial chemical change accompanies metamorphism, the process is called metasomatism
• Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed, except for the possible loss or acquisition of certain volatile components
• Recrystallization of clay minerals


  1. Dissolving and transporting ions from one site in the crystal structure to another

  2. Fluids become more reactive in hotter environments
• Transport of mineral matter by hot fluids

  1. Expelled from a cooling and solidifying magma body

  2. Exchange of ions between fluids and host rocks
• Metasomatism


  Substantial chemical change that accompanies metamorphism
• Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed, except for the possible loss or acquisition of volatiles such as water (H2O) and carbon dioxide (CO2)
• When magma forces its way into surrounding rock

  High temperatures and hot fluids may alter the host rock
• If the host rock is composed of unreactive minerals

  Any alterations will be confined to a narrow zone next to the pluton
• When the host rock is reactive (e.g. limestone)

  The zone of metamorphism may extend far from the intrusion
• Glacial ice is a metamorphic rock that exhibits ductile flow much like hot rocks buried deep within Earth's crust
• Glacial ice is "hot", relative to its melting temperature, and therefore gradually flows downslope in response to the force of gravity
• Texture


  The size, shape, and arrangement of grains within a rock
• Foliation


  A planar (nearly flat) arrangement of mineral grains or structural features within a rock
• Development of foliation


  1. Rotation of platy and/or elongated mineral grains into a parallel or nearly parallel orientation

  2. Recrystallization that produces new minerals with grains that exhibit a preferred orientation

  3. Mechanisms that change spherically shaped grains into elongated shapes that are aligned in a preferred orientation
• Rock cleavage


  Closely spaced, flat surfaces along which rocks split into thin slabs when hit with a hammer
• Slaty cleavage


  The excellent splitting property of slate
• Development of rock cleavage in slate

  1. Platy grains are kinked and bent, generating microscopic folds

  2. Old grains break down and recrystallize preferentially in the direction of the newly developed orientation
• Schistosity


  A planar or layered structure exhibited by rocks containing large enough platy minerals (e.g. mica, chlorite) to be discernible with the unaided eye
• Gneissic texture


  A banded appearance resulting from the segregation of dark and light minerals
• Nonfoliated metamorphic rocks


  Metamorphic rocks that do not exhibit foliation, typically developing in environments with minimal deformation and parent rocks composed of equidimensional mineral grains
• Porphyroblasts


  Unusually large grains surrounded by a fine-grained matrix of other minerals, developing during recrystallization in a wide range of metamorphic environments
• Foliated


  Metamorphic rocks that have a planar arrangement of mineral grains