METAMORPHISM

• Metamorphism


  Geologic process in which rocks change in form, composition, and structure due to intense heat and pressure and sometimes with the introduction of chemically active fluids
• Factors involved in Metamorphism


  • Temperature
  • Pressure
  • Hydrothermal fluids
• Temperature


  • Affects the rock's chemical composition, mineralogy, and texture
  • During burial metamorphism, at a depth of about 8 to 15 kilometers from the surface of the crust, metamorphic reactions begin
  • Rocks adjust to the new temperature causing their atoms and ions to recrystallize and form new arrangements thereby creating new mineral assemblages
  • During recrystallization, new crystals grow larger than the crystals in the original rock
• Pressure


  • Changes the composition, mineralogy, and texture of rocks
  • Metamorphism in the subduction zone is characterized by high-pressure metamorphism
  • Collision zone between two continental crusts is marked by moderate-pressure metamorphism
  • Vertical stress/Confining pressure - the stress or pressure exerted on the rock by the weight of overlying material such as in burial metamorphism, the same in all directions and makes the rocks to fracture or deform
  • Directed/Differential pressure - imposed by a force in a particular direction, dominant at convergent boundaries where plates move towards each other and collide thus exerting force and cause rocks to deform, pressure causes rocks to form folds in a particular direction as directed by the pressure, thus directed pressure guides the shape and orientation of the new crystals formed as minerals recrystallize under the influence of both heat and pressure- results in a textural change such that the minerals are elongated in the direction perpendicular to the directed stress and this contributes to the formation of foliation
• Foliation


  A set of flat or wavy parallel cleavage planes produced by deformation under directed pressures
• Hydrothermal fluids


  • The dissolved minerals in the fluids react with rocks causing changes in chemical and mineral compositions and sometimes completely replacing one mineral with another without changing the textures of the rocks
  • This type of metamorphism is known as metasomatism in which the alteration process is caused by fluids passing through the rock and catalyzing chemical reactions
  • When the heat of the intrusive igneous body heats up the groundwater containing dissolved minerals, convection of water forms flowing through the surrounding rocks and penetrating through them, reactions occur among chemicals in the rocks and in the water resulting in significant changes in the mineralogy of the rock, an example is the alteration of feldspars to clays, and deposition of quartz, calcite, and other minerals in fractures or cracks and other open spaces forming veins
• Types of Metamorphism


  • Regional Metamorphism
  • Contact Metamorphism
  • Shock Metamorphism
  • Burial Metamorphism
• Regional Metamorphism


  • Forms foliated metamorphic rocks such as Gneiss and Schist due to high temperature and pressure imposed on large parts of the crust
  • Mostly occurs within the continental crust
  • The confining and directing pressures by some tectonic forces onto rock formations cause new alignment of minerals (foliation) during recrystallization
• Contact Metamorphism


  • Prominent in areas where surrounding rocks are exposed to heat coming from magma intrusion within the layers of the rocks
  • Marble, quartzite, and other granoblastic rocks with large visible crystals of minerals may be formed through contact metamorphism
  • When the quartz-rich sedimentary rock encounters enough heat from the presence of igneous intrusions to trigger recrystallization, all sedimentary structures are destroyed and quartz grains in the sandstone recrystallize to form an interlocking mosaic of crystals giving it a granoblastic texture, the resulting rock is white or pale grey known as metaquartzite, the size of the crystals is larger near to the contact with the igneous intrusion and smaller when further away from the contact where temperatures are not as high
  • When limestone encounters heat from igneous intrusion, the calcite minerals including the fossils found in the limestone are destroyed and form a marble with an interlocking mosaic of crystals, a limestone made of pure calcite minerals would transform into pure white marble
• Shock Metamorphism


  • Takes place when the heat and shock waves from meteor or asteroid impact transform rocks immediately around the impact site
  • Examples are the transformation of mineral graphite into ultra-high-pressure polymorph diamond and the conversion of quartz minerals into coesite under high shock pressures
• Burial Metamorphism


  • Occurs at lower temperature and pressure which transforms sedimentary rocks that had undergone diagenesis into low-grade metamorphic rocks through relatively low temperature and pressure
  • Partial alteration of the mineralogy and texture may occur while other sedimentary structures are usually preserved
  • In subduction zones, between two converging plates, high-pressure metamorphism occurs, metamorphic rocks created through these processes are rarely found on the Earth's surface as they were formed in such a great depth, some metamorphic rocks formed through these types of metamorphism contain bits of microscopic diamonds, an indication that the rock was formed with great pressure, an example of rock made through this type of metamorphism is the eglcosite which has transformed with pressure greater than 28 kbar at a depth of approximately above 80 km