Chapter 6: Metamorphism: Alteration of Rocks By Temperature and Pressure Flashcards
What is the parent rock of marble?
Limestone
What is the metamorphosed rock from limestone?
Marble
What is the parent rock of schist?
Shale
What is the metamorphosed rock from shale?
Schist
Exhume
To transport to the Earth’s surface
3 principal factors of metamorphism
Heat, pressure, fluid composition
Average rate that temp increases with depth in the crust
30 deg C/km
Geothermal gradient
The increase in temp with increasing depth in Earth’s interior
Geothermometer
A rock’s mineral composition, which determines the temperature at which the rock formed (because different minerals form at different temperatures).
2 plate tectonic mechanisms that form most metamorphic rocks
- subduction
* continent-continent collisions
2 kinds of pressure (stress) rocks are subjected to
- confining pressure
* directed pressure/differential stress
confining pressure
general force applied equally in all directions
directed pressure/differential stress
force exerted in a particular direction
Rate that pressure increases with depth in the crust
.3 to .4 kbars (kilobars) per km (300-400 bars/km)
geobarometers
Mineral assemblages that can be used as pressure gauges, to determine the pressure that the rock formed
metasomatism
A change in rock’s composition by fluid transport of chemical substances into or out of it
6 types of metamorphism
- regional
- contact
- seafloor
- high-pressure
- burial
- shock
regional metamorphism
- takes place where both high temps and pressures are imposed over large parts of the crust
- characteristic of convergent plate boundaries
- most widespread type
contact metamorphism
- heat from an igneous intrusion metamorphoses the rock immediately surrounding it, normally only affecting a thin zone of country rock
- heat is from intruding magma
- pressure only relevant where intrusion is at great depths
seafloor metamorphism
- associated with mid-ocean ridges
* hot lava heat sea water which circulates though the new ocean crust by convection; hot temperature alters the basalts
burial metamorphism
- affects a smaller amount of rock
- low-grade metamorphism caused by the progressive increase of pressure and temperature exerted by the growing layers of overlying sediments and rock
- begins at 6-10 km
high-pressure metamorphism
- pressures 8-12 kbars
- form at very low depths and take a long time to recycle to the surface
- most form in subduction zones as sediments scraped from oceanic crust and subducted to depths over 30 km
shock metamorphism
- occurs when a meteorite collides with Earth
* mass and velocity is transformed into heat and shock waves through the country rock
foliation
- a set of flat or wavy parallel cleavage planes produced by deformation of igneous and sedimentary rocks under directed pressure
- the most prominent textural feature of regionally metamorphosed rocks
as the grade of regional metamorphism increases, foliation becomes more or less pronounced
more
4 main criteria for classifying foliated rocks
- metamorphic grade
- grain (crystal) size
- type of foliation
- banding
progression of 5 foliated rocks (lowest grade to highest grade)
1 - slate 2 - phyllite 3 - schist 4 - gneiss 5 - migmatite
slate
- lowest grade of foliated metamorphosed rock
- formed from shales or volcanic ash deposits
- have strong foliation planes to split the rock
phyllite
- foliated rock
* similar in character and origin to slate, but have a more glossy sheen from mica and chlorite
schist
- foliated rock
- characterized by schistosity: coarse, wavy foliation of lighter and darker bands
- among the most abundant metamorphic rock types
gneiss
- foliated rock
- light colored rock with coarse bands of light and dark minerals throughout
- gneissic foliation is from segregation of light-colored quartz and feldspar from darker amphiboles and other mafic minerals
- poor foliation
migmatite
- foliated rock
- mixture of igneous and metamorphic rock
- higher temperatures begin to melt a part of the country rock, creating the igneous portions
granoblastic rocks
- nonfoliated metamorphic rocks that grow in equant shapes like cubes and spheres instead of platy or elongated
- form where directed pressure is absent, so no foliation occurs
- defined by mineralogy (because texture is homogeneous and granular)
hornfels
- granoblastic rock
- high-temperature contact metamorphic rock
- uniform grain size with little or no deformation
- formed from fine-grained sedimentary and other sliicate rocks
quartzite
- granoblastic rock
- some are homogeneous, some have bands of slate or schist
- formed from quartz-rich sandstones
marbles
- granoblastic rock
- some are pure white and uniform crystals, some have bands of impurities from silicates and other minerals in the limestone
- formed from limestone and dolomites
greenstones
- granoblastic rock
- many formed by seafloor metamorphism
- abundance of chlorite gives the greenish cast
- formed from volcanic rock
amphibolites
- granoblastic rock
- med- to high-grade metamorphism of mafic volcanic rocks
- formed from amphibole and plagioclase feldspar
granulite (granofels)
- granoblastic rock
- homogeneous medium- to coarse-grained texture
- formed from shale, impure sandstone, and many kinds of igneous rock
porphyoblasts
- large crystals formed in a much finer-grained matrix of other minerals
- formed by contact and regional metamorphism
zeolite minerals
- form at low temps and pressures
* contain water within crystal structures
metamorphic facies
grouping of rocks of various mineral compositions formed under different grades of metamorphism from different parent rocks
