metamorphism and metamorphic rocks (LEC 5) Flashcards
metamorphism is changing in rock composition or texture due to _____,
_____________, and _______________________
heat, pressure, and action of fluids
The composition of the parent rock generally controls the ____________ of the metamorphic rock
chemistry
what are the sources of heat during metamorphism (2):
- magmas: contact metamorphism
- earth’s internal heat (geothermal gradient)
what does pressure (stress) do in metamorphism
responsible for changing the physical characteristics of the rock
what are the two types of pressure and describe them
- Confining pressure (uniform): the pressure is equal (uniform) in all
directions - Direct pressure (differential): the pressure is not equal (non-uniform) in all directions
what does increasing the temp do in metamorphism (2)
- transforms minerals into new forms that are thermodynamically
stable - influences mineral mobility and reactivity of chemically active
fluids
describe confining pressure (uniform stress) (4 things)
- squeezing the rock into the smallest possible volume without
change in shape - increases with increasing burial depth
- forces mineral grains to crowd together and become compact
- recrystallization but no internal deformation
describe directed pressure (differential stress) (2 things)
*results in distortion of the rock
*associated with horizontal shortening and stretching of the earth’s
crust
three basic types of directed pressure
*compression
*tensional
*shear stress
at LOW TEMPS, similar to the earth’s surface, rocks are _________
and ________ when differential stress is applied
at HIGH TEMPS, rocks are ______ and ______ when
differential stress is applied.
options:
1. ductile/fracture/brittle/flow
brittle; fracture
ductile; flow
most metamorphic rocks exposed to differential stress go through
___________ _____________ where mineral grains flatten and elongate,
aligning to be parallel to the direction of minimum stress
ductile deformation
what is foliation
foliation refers to layers or sheet-like shape as a result of high pressure
factors influencing the development of foliation (3)
*rotation of mineral grains to a new orientation
*changing the shape of equidimensional grains to be
elongated along the preferred orientation
*recrystallization of minerals to form grains that grow in
the direction of the preferred orientation
how does confining pressure and differential pressure affect the texture
*confining pressure might not affect the texture significantly
although the minerals have recrystallized.
*differential pressure may distort the rock and introduce a new texture
important: most of metamorphic rocks form under condition of differential pressure
name 3 sources of the chemically active fluids:
*water originally trapped in pores of sedimentary rocks during formation
*volatile fluid within magma
*dehydration of water-bearing minerals such as gypsum which contain water in their crystal structure
at a temperature of _____°C, the metamorphism begins
and at a temperature of _____°C, the melting begins
200°C
700°C
what are shields?
shields are flat, large expanses of metamorphic rocks and igneous
rocks that exist within stable continent interiors
what are index minerals
metamorphic minerals that form and remain stable within narrow, well defined temp and pressure ranges
most metamorphic rocks exposed to differential stress go through
ductile deformation. describe ductile deformation
during ductile deformation, mineral grains flatten and elongate, aligning to be parallel to the direction of minimum stress
describe what metamorphic grade is
is the intensity of metamorphism experienced by a rock
