metamorphism and metamorphic rocks (LEC 5)

Question 1

metamorphism is changing in rock composition or texture due to _____,
_____________, and _______________________

Answer 1

heat, pressure, and action of fluids

Question 2

The composition of the parent rock generally controls the ____________ of the metamorphic rock

Answer 2

chemistry

Question 3

what are the sources of heat during metamorphism (2):

Answer 3

• magmas: contact metamorphism
• earth’s internal heat (geothermal gradient)

Question 4

what does pressure (stress) do in metamorphism

Answer 4

responsible for changing the physical characteristics of the rock

Question 5

what are the two types of pressure and describe them

Answer 5

• Confining pressure (uniform): the pressure is equal (uniform) in all
  directions
• Direct pressure (differential): the pressure is not equal (non-uniform) in all directions

Question 6

what does increasing the temp do in metamorphism (2)

Answer 6

• transforms minerals into new forms that are thermodynamically
  stable
• influences mineral mobility and reactivity of chemically active
  fluids

Question 7

describe confining pressure (uniform stress) (4 things)

Answer 7

• 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

Question 8

describe directed pressure (differential stress) (2 things)

Answer 8

*results in distortion of the rock
*associated with horizontal shortening and stretching of the earth’s
crust

Question 9

three basic types of directed pressure

Answer 9

*compression
*tensional
*shear stress

Question 10

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

Answer 10

brittle; fracture
ductile; flow

Question 11

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

Answer 11

ductile deformation

Question 12

what is foliation

Answer 12

foliation refers to layers or sheet-like shape as a result of high pressure

Question 13

factors influencing the development of foliation (3)

Answer 13

*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

Question 14

how does confining pressure and differential pressure affect the texture

Answer 14

*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

Question 15

name 3 sources of the chemically active fluids:

Answer 15

*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

Question 16

at a temperature of _____°C, the metamorphism begins
and at a temperature of _____°C, the melting begins

Answer 16

200°C
700°C

Question 17

what are shields?

Answer 17

shields are flat, large expanses of metamorphic rocks and igneous
rocks that exist within stable continent interiors

Question 18

what are index minerals

Answer 18

metamorphic minerals that form and remain stable within narrow, well defined temp and pressure ranges

Question 19

most metamorphic rocks exposed to differential stress go through
ductile deformation. describe ductile deformation

Answer 19

during ductile deformation, mineral grains flatten and elongate, aligning to be parallel to the direction of minimum stress

Question 20

describe what metamorphic grade is

Answer 20

is the intensity of metamorphism experienced by a rock