Physical Geology
Metamorphism and Metamorphic Rocks
Metamorphism
- - transformation of pre-existing rx into other rx by heat, pressure, and chemically-active fluids.
- - can metamorphose igneous, sedimentary, or even other metamorphic rocks.
- - changes are textural as well as mineralogical.
- keep in mind that rocks are capable of plastic deformation
- - like silly putty
- - takes great heat & pressure & time.
- - easily done in lab experiements.
- when huge areas of rx are affected during mountain building, it's called regional metamorphism. When 2 plates collide.
- when a hot igneous body pushes up into the crust, the crust is affected by contact metamorphism. Forms halo (aureole) of metam. rx around intrusion.
1) Heat is the most important factor
- - increases with depth. Can reach >750C.
- - causes minerals which are in equilibrium at the surface at surface temps to melt and recrystallize or just change their crystal structure to form another mineral which is stable at the new higher temp.
- - specific minerals form at different temps (kyanite, andalusite, sillimanite), so we can tell how hot a rock got by which mins it contains.
2) Pressure also increases with depth.
- - changes the texture of a rock by aligning & the mineral grains to form foliation.
- - micas and chlorite do this
- - forms a book structure
- - makes the rock deform plastically, allowing rocks to fold as they are subject to forces.
3) Chemically-active fluids
- - water is the most important agent, especially if it is contaminated with various elements.
- - as temps increase, this water aids in chemical reactions during mineral recrystallizations.
- - water is found in the spaces between grains in many rx (pore fluid).
- - water is a medium of transport to take away unwanted elements as minerals change into other minerals.
- - pressure forces the water out of the pore spaces, taking the unwanted elements with it.
FOLIATION
- - slaty cleavage
- - fine-grained mica minerals aligning to give the entire rock a layered look, like slate.
- - schistosity
- - medium-grained mica xls align to form rx with a shiny scaly appearance.
- - gneiss
- - high temps and press cause light and dark mins to segregate into separate layers.
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