Meatamorphic Processes and Rocks

Question 1

what is metamorphism?

Answer 1

Change that occurs when pre-existing rocks (protoliths) experience conditions different from those in which they formed

Question 2

during metamorphism what changes in the rock?

Answer 2

Texture and/or Mineralogy/Composition

Question 3

What controls metamorphic processes?

Answer 3

Composition of parent rock
Fluids (-> composition changes)
Temperature
Pressure

Question 4

what are the 2 main types of metamorphism?

Answer 4

(1) Contact metamorphism (local)

2) Regional metamorphism (large-scale

Question 5

what is Contact metamorphism?

Answer 5

Magma intrusions bake (T increase) surrounding rock

Question 6

what is Regional metamorphism?

Answer 6

Associated with plate interactions (usually involves P & T increases, depth increases)

Question 7

where does contact metamorphism occur?

Answer 7

magma chambers of volcanoes

Question 8

where does regional metamorphism occur?

Answer 8

volcanoes and mountains at subduction zones

mid-ocean ridges

Question 9

how does pressure change occur to metamorphic rocks?

Answer 9

rock is subducted

Question 10

how does temp. change occur to metamorphic rocks?

Answer 10

magma rises and heats rocks

Question 11

how does composition change occur to metamorphic rocks?

Answer 11

fluid gets into rock fractures

Question 12

how does pressure and temp. change occur to metamorphic rocks?

Answer 12

Orogens (e.g., continent-continent collision)
Crustal thickening
Rocks move up/down P and/or T

Question 13

what does compression lead to?

Answer 13

thickening, burial (or uplift)

Question 14

How do high-P high-T metamorphic rocks get to the surface?

Answer 14

Uplift

Erosion

Question 15

what are the drivers of metamorphism?

Answer 15

(1) Pressure
(2) Temperature
(3) Fluids

Question 16

what is Confining Pressure (uniform stress)?

Answer 16

the load of the overlying rock= 1 kilobar for 3 km of burial

Question 17

what is Directed Pressure (differential stress)?

Answer 17

unequal pressure from plate tectonics (compression, tensional, shear) -> distortion

Question 18

what do Increases in Pressure favour?

Answer 18

dense, compact crystal structures

Question 19

what is foliation?

Answer 19

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

Question 20

give some examples of metamorphic rocks from those formed in lower pressure, lower temps. to those formed in higher pressures, higher temps

Answer 20

Shale
Slate
Phyllite
Schist
Gneiss

Question 21

temp. increases with…

Answer 21

depth (geothermal gradient)

Question 22

geothermal varies with…

Answer 22

tectonic setting

Question 23

temp in the crust is (also) influenced by…

Answer 23

intrusion of magma bodies

Question 24

what do increases in temp. promote?

Answer 24

reactions to form new minerals, lower density phases, increased ductility

Question 25

what happens during recrystallisation?

Answer 25

Pre-existing grains may recrystallise into larger crystals of the same mineral e. g. limestone -> marble e. g. quartz sandstone -> quartzite

Question 26

give some examples of rocks from least metamorphic grade to most?

Answer 26

``` slate phyllite blueschist/schist gneiss migmatite ```

Question 27

what does the mineralogy of metamorphic rocks depend on?

Answer 27

bulk composition, P–T conditions (& action of hot fluids)

Question 28

Metamorphic minerals...

Answer 28

Nearly all minerals in igneous and sedimentary rocks can occur in metamorphic rocks but some minerals occur exclusively in metamorphic rocks

Question 29

Metamorphic Index Minerals form at...

Answer 29

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.

Question 30

which minerals are not useful as index minerals?

Answer 30

ones with a wide stability range e.g. quartz, feldspar

Question 31

what is an example of fluid metamorphism?

Answer 31

Hydrothermal metamorphism at mid-ocean ridges basalt -> greenstone H2O is added

Question 32

when H2O is added what does pyroxene turn into?

Answer 32

Chlorite Epidote Actinolite