Meatamorphic Processes and Rocks Flashcards
what is metamorphism?
Change that occurs when pre-existing rocks (protoliths) experience conditions different from those in which they formed
during metamorphism what changes in the rock?
Texture and/or Mineralogy/Composition
What controls metamorphic processes?
Composition of parent rock
Fluids (-> composition changes)
Temperature
Pressure
what are the 2 main types of metamorphism?
(1) Contact metamorphism (local)
2) Regional metamorphism (large-scale
what is Contact metamorphism?
Magma intrusions bake (T increase) surrounding rock
what is Regional metamorphism?
Associated with plate interactions (usually involves P & T increases, depth increases)
where does contact metamorphism occur?
magma chambers of volcanoes
where does regional metamorphism occur?
volcanoes and mountains at subduction zones
mid-ocean ridges
how does pressure change occur to metamorphic rocks?
rock is subducted
how does temp. change occur to metamorphic rocks?
magma rises and heats rocks
how does composition change occur to metamorphic rocks?
fluid gets into rock fractures
how does pressure and temp. change occur to metamorphic rocks?
Orogens (e.g., continent-continent collision)
Crustal thickening
Rocks move up/down P and/or T
what does compression lead to?
thickening, burial (or uplift)
How do high-P high-T metamorphic rocks get to the surface?
Uplift
Erosion
what are the drivers of metamorphism?
(1) Pressure
(2) Temperature
(3) Fluids
what is Confining Pressure (uniform stress)?
the load of the overlying rock= 1 kilobar for 3 km of burial
what is Directed Pressure (differential stress)?
unequal pressure from plate tectonics (compression, tensional, shear) -> distortion
what do Increases in Pressure favour?
dense, compact crystal structures
what is foliation?
Under directed pressure (stress), rocks develop a foliation due to the alignment of minerals
Minerals realign at right angles to P
Equidimensional minerals get squashed
New minerals crystallise in the preferred orientation
give some examples of metamorphic rocks from those formed in lower pressure, lower temps. to those formed in higher pressures, higher temps
Shale Slate Phyllite Schist Gneiss
temp. increases with…
depth (geothermal gradient)
geothermal varies with…
tectonic setting
temp in the crust is (also) influenced by…
intrusion of magma bodies
what do increases in temp. promote?
reactions to form new minerals, lower density phases, increased ductility
what happens during recrystallisation?
Pre-existing grains may recrystallise into larger crystals of the same mineral
e. g. limestone -> marble
e. g. quartz sandstone -> quartzite
give some examples of rocks from least metamorphic grade to most?
slate phyllite blueschist/schist gneiss migmatite
what does the mineralogy of metamorphic rocks depend on?
bulk composition, P–T conditions (& action of hot fluids)
Metamorphic minerals…
Nearly all minerals in igneous and sedimentary rocks can occur in metamorphic rocks but some minerals occur exclusively in metamorphic rocks
Metamorphic Index Minerals form at…
distinct P and T conditions
e.g. Andalusite (Al2SiO5), Sillimanite (Al2SiO5), Kyanite (Al2SiO5) all have the same formula but they form at different pressure and temp.
which minerals are not useful as index minerals?
ones with a wide stability range e.g. quartz, feldspar
what is an example of fluid metamorphism?
Hydrothermal metamorphism at mid-ocean ridges
basalt -> greenstone
H2O is added
when H2O is added what does pyroxene turn into?
Chlorite
Epidote
Actinolite
