Lecture 7: Metamorphic Rocks

Question 1

What is metamorphism?

Answer 1

Formed by the transformation of pre-existing rocks under the influence of high temperatures and pressures

Question 2

What is contact metamorphism?

Answer 2

Heat + uniform pressure EXAMPLE: shale (sedimentary) baked for a million years… becomes hornfels (metamorphic) with new mineral: andalusite (large crystals)

Question 3

What is regional metamorphism?

Answer 3

Heat + differential stress (uneven pressure)

Question 4

When does contact metamorphism commonly occur?

Answer 4

When a pre existing rock iis heated by igneous intrusion, forming a metamorphic halo or “aureole” in the adjacent rock- generally ____ in extent

Question 5

In contact metamorphism does the rocks composition basically remain the same?

Answer 5

Yes, (ions are just redistributed to make new minerals)

Question 6

What is foliation?

Answer 6

Regional metamorphism generally occurs in areas where 2 lithoshperic plates are pressing against one another, rocks are subjected to differential stress. In response to this stress, platy/elongate minerals line up to produce a foliated texture (folium =leaf)

Question 7

What is a foliated metamorphic rock?

Answer 7

Schist

Question 8

What is bedding?

Answer 8

(stratification) is a sedimentary characteristic, produced by the deposition of sediment in horizontal layers

Question 9

How do you see bedding direction?

Answer 9

is seen in the alternating light (quarts-rich) and dark (quartz poor) layers

Question 10

What is it called when rocks break in the direction of foliation?

Answer 10

(slaty cleavage) following the alignment of platy mica crystals

Question 11

T or F: In very few cases does a parent rock’s original texture or structure survive metamorphism

Answer 11

True

Question 12

What kind of rocks are typically foliated?

Answer 12

Rocks that are subject to regional metamorphism

Question 13

What does ______ result in?

Answer 13

an increased size of mineral grains, and the development of distinct types of foliation

Question 14

How does shale transform with increasing metamorphism?

Answer 14

Shale
Slate
Phyllite
Schist
Gneiss

Question 15

What is shale?

Answer 15

Parent rock—original rock before metamorphism. In this case is ____ (a sedimentary rock)… Dull, breaks into layers, may contain fossils

Question 16

What is slate?

Answer 16

Clay minerals from shale begin to transform into mica crystals that form slate. Development of very slight shininess due to slightly larger size of mica crystals relative to original clay particles.
Foliation Type: slaty cleavage

Question 17

What is phyllite?

Answer 17

Development of more distinct, silky sheen due to further growth of platy mica crystals
Foliation Type: phyllitic foliation

Question 18

What is schist?

Answer 18

Obvious sparkle due to much larger mirror like mica grains. In some cases mica crystals are observed to wrap around larger round crystals of minerals such as garnet (round crystals = porphyroblasts)
Foliation Type: schistose foliation (schistosity)

Question 19

What is gneiss?

Answer 19

Mica grains transform into more elongate crystals of amphibole or pyroxene, separated from light coloured minerals such as quartz (causing banding). Round porphyroblasts may be common
Foliation Type: gneissic foliation (gneissosity)

Question 20

What are non-foliated metamorphic rocks? Give an example.

Answer 20

Non-foliated texture caused by lack of platy/elongate minerals required to define foliation.
Quartzite (metamorphism of quartz sandstone)
Marble (metamorphism of limestone)

Question 21

What is migmatite? How does it complete the rock cycle?

Answer 21

Extreme temps and pressures cause non-ferromagnesian minerals to melt and produce felsic magma which produces migmitate which is now part metamorphic and part igneous
Felsic magma can then collect into magma chambers to produce igneous rocks.