Metamorphic Rock questions
Explain how a geologists use index minerals.
A geologist can identify different grades of metamorphism in the field based on the presence of specific minerals, called index minerals. certain minerals have limited pressure and temperature range. These index minerals record metamorphic grade. When these index minerals are identified, one can get a general idea of the metamorphic history of the rock. (index mineral maps, define metamorphic zones )
What is a metamorphic grade, and how can it be determined? How does grade differ from facies?
A metamorphic grade refers to a series of temperature and (to a lesser extent) pressure regimes under which metamorphism takes place. For example, high-grade metamorphism occurs under greater temperatures (and pressures) than does low-grade metamorphism. Metamorphic grade is usually assessed on the basis of the mineral assemblage making up the metamorphic rock, as well as its foliation and other textural clues (such as grain size). Facies is a more precise term used for a restricted range of temperatures and pressures defined by the presence of key minerals.
What is metamorphic foliation, and how does it form?
Foliation is the presence of parallel planar surfaces or layers in metamorphic rock. Under sufficiently differential stress, platy or elongate grains are broken down and regrown in a preferred orientation perpendicular to maximum compressive stress.
list the different type of metamorphic facies and the environments in which they form
Geothermal gradient metamorphism; heating by a plutonic intrusions burial metamorphism; increase pressure and temperature by deep burial in a basin Dynamic metamorphism: shearing in a fault zone subduction zone metamorphism: high pressure to low-temperature alternation, shock extremely high pressure attending a bolide impact regional metamorphism: pressure and temperature alternation due to orogenesis, metamorphism associated with large-scale mountain building contact metamorphism: changes in rock caused by the heat of a nearby magma body; also called thermal metamorphism hydrothermal metamorphism: (alternation by hot water leaching) chemical alterations that occur as hot, ion-rich water circulates through fractures in the rock impact metamorphism: metamorphism that occurs when meteorites strike Earth's surface
Why are hornfels nonfoliated?
Hornfels form through contact metamorphism, without the application of differential stress
Explain the difference between low grade and high-grade metamorphic rocks
Low grade metamorphic rocks are formed under temperatures of 400ºC and pressures of 400MPa, (low grade - slight) High grade metamorphic rocks are formed at temperatures above 400ºC and pressures higher than 400MPa. (high grade- intense)
What two features characterize most metamorphic rocks?
Metamorphic mineral assemblages (minerals uniquely produced under the temperature and pressure regimes of metamorphism) and metamorphic texture (grain arrangement, often involving foliation: a preferred alignment of platy grains or alternating light and dark mineral bands) are characteristic of most metamorphic rocks
explain the upper temperature limit of metamorphism
Metamorphic rocks do not melt. The upper temperature limit for metamorphism is 800ºC. Above this temperature, rocks begin to melt, which by definition, is no longer a metamorphic rock.
What is Metasomatism ?
Metasomatism is a type of metamorphism in which a rock's bulk composition changes only because of fluid transport of chemical substances into or out of the rock
What phenomena cause metamorphism?
Mountain building, plutonism, volcanism, faulting, meteoric impact, mantle convection, and water-rock interactions all lead to metamorphism.
how are metamorphic rocks different from igneous and sedimentary rocks?
Rocks differ in the way they form. Igneous rocks form when magma or lava cools and solidifies. Sedimentary rocks form when sediments become compacted and cemented. Metamorphic rocks form when existing rocks are changed by heat, pressure, or solution
How does slate differ from phyllite? How does phyllite differ from schist? How does schist differ from gneiss?
Slate and its characteristic slaty cleavage arise from the preferred orientation of clay minerals resulting from the relatively low-temperature and low-pressure metamorphism of a body of shale. Phyllite arises when significantly higher temperatures and pressures cause clay grains within slate to be recrystallized to form mica grains, which retain a preferred orientation. Unlike slate, which is rather dull, mica gives phyllite a silky luster. Schist differs from phyllite in that, as a result of greater heat and pressure, its mica grains are large, visible discrete plates, unlike the smooth sheen of tiny mica grains within phyllite. Gneiss is compositionally banded, with alternating bands or swirls of light- and dark-colored minerals, including additional minerals besides mica (quartz, feldspar, amphibole). (look at the diagram )
What is the difference between slaty cleavage and schistosity?
Slaty cleavage(foliation) is literally cleavage made of thin slates of rock. (slate is fine clay, low grade metamorphic ). Schistosity is when mica and chlorite crystals begin to grow. these platy materials are large enough to be discernible with the unaided eye and exhibit a layered structure they are said to exhibit schistosity. (medium to high grade metamorphism)
What are the factors that control metamorphism?
There are many factors that control metamorphism. These factors include temperature and pressure. The higher the temperatures and pressures, the higher the grade of metamorphism. The presence or absence of pore fluids has an affect on metamorphism. The direction and amount of stress present plays a role in the type and amount of metamorphism. Finally, the rate and duration of deformation affect the type of metamorphic rocks that will form.
What are the difference between blue-schist and greenschist?
blueschist is a metamorphic rock containing glaucophane (the characteristic mineral of this facies, it's blue, hence the name). while greenschist is a metamorphic rock formed at low temperature and pressure, often with an abundance of green minerals such as chlorite, serpentine, and epidote.
Non-foliated
parent rock (protolith) - metamorphic rock - Gabbro ( or Basalt) -> Amphibolith ( dominated by amphibole minerals) - mudstone -> hornfels (alternation by heating, associated w plutonic intrusion) - sandstone -> quartzite (sand grains in the protolith recrystallize and fuse ) -limestone -> marble ( coarsely crystalline calcite of dolumite, extensive recrystallization)
