Metamorphic Petrology

Question 1

What is another word for metamorphic textures?

Answer 1

Metamorphic fabrics

Question 2

What are the characteristics of slaty cleavage?

Answer 2

Splits into thin sheets along cleavage planes. Fine grained.

Question 3

What are some examples of rocks with slaty cleavage?

Answer 3

Slate and phyllite

Question 4

How is slaty cleavage formed?

Answer 4

By low grade metamorphism. Flat, platy minerals like clays and micas align at 90° to the direction of pressure and they’re usually parallel to the axial planes in folds

Question 5

What does slaty cleavage represent?

Answer 5

Low grade metamorphism

Question 6

How is schistosity formed?

Answer 6

Regional metamorphism. Alignment of flat platy minerals. 90° from direction of metamorphic pressure.

Question 7

What is schistosity?

Answer 7

Thin parallel beds and shiny appearance where surface of micas visible (micaceous sheen)

Question 8

What does schistosity indicate?

Answer 8

Higher grade metamorphism

Question 9

What is the difference between slaty cleavage and schistosity?

Answer 9

Slaty cleavage: low grade. Fine grained.
Schistosity: higher grade. Coarser grained

Question 10

What does gneissose banding look like?

Answer 10

Clearly distinguishable light and dark bands.

Question 11

What makes up the light bands of gneissose banding?

Answer 11

Quartz and feldspar.
Usually granoblastic and equidimensional

Question 12

What are the dark bands made of in gneissose banding?

Answer 12

Biotite and Mafic material. Foliated

Question 13

Are bands in gneissose banding always straight?

Answer 13

No. They may be contorted or folded at 90° to maximum pressure

Question 14

What are porphyroblasts?

Answer 14

Large metamorphic crystals within a finer matrix

Question 15

What is the difference between porphyroblasts and phenocrysts?

Answer 15

Porphyroblasts are metamorphic.
Phenocrysts are igneous and formed by two stage cooling

Question 16

How are porphyroblasts formed?

Answer 16

Both contact and regional metamorphism

Question 17

How can porphyroblasts show the order of metamorphic events?

Answer 17

If they are pre, syn or post-kinematic

Question 18

What are some examples of porphyroblasts?

Answer 18

Cubic pyrite in slate.
Garnet in schists

Question 19

What is meant by granoblastic?

Answer 19

Unfoliated fabric formed by contact metamorphism. Randomly oriented equidimensional crystals

Question 20

What is granoblsstic sometimes called?

Answer 20

Sugary texture

Question 21

What rocks can be seen with a granoblastic texture?

Answer 21

Hornfels, marble, metaquartzite

Question 22

What is crenulation cleavage?

Answer 22

Several foliations in different directions

Question 23

How is crenulation cleavage formed?

Answer 23

Regional metamorphism. Two or more stress directions

Question 24

How does crenulation cleavage make the rock look?

Answer 24

Gives it a wavy appearance

Question 25

What rocks can have crenulation cleavage?

Answer 25

Phyllite, schist, and some gneisses

Question 26

How do metamorphic minerals form?

Answer 26

As mountain buildings happen, metamorphic minerals form due to high temps. High temps increase rate ions diffuse (as a solid) between minerals.

Question 27

How does water effect the formation of new metamorphic minerals?

Answer 27

Mineral formation takes millions of years but water speeds up this reaction as it allows ions to diffuse more rapidly

Question 28

How may we identify parent rock from a newly formed metamorphic mineral?

Answer 28

They may grow over sedimentary structures/features

Question 29

What happens if you rapidly apply stress to a rock?

Answer 29

Mineral grains form brittle fractures and are cursed and ground down mechanically at fault zones (cataclasis). Forms range of fabrics.

Question 30

What happens when stress is applied slowly to a rock?

Answer 30

Results in low strain rates. Rock doesn't fracture but gradually deform. Later stages of deformation may produce crenulation cleavage which can be used to determine stages of metamorphism

Question 31

What is a pre-kinematic mineral?

Answer 31

Mineral was formed before deformation. Mineral will be affected by deformation

Question 32

What is a syn-kinematic mineral?

Answer 32

Form at the same time as deformation.

Question 33

What is a post-kinematic mineral?

Answer 33

Formed after the deformation

Question 34

What are index minerals (metamorphic)?

Answer 34

Specific minerals that help to identify the metamorphic grade

Question 35

Who discovered index minerals?

Answer 35

George barrow

Question 36

What are the index minerals discovered by Barrow?

Answer 36

Chlorine (shale + phyllite) Biotite (phyllite + schists) Garnet (schists) Kyanite (schists) Sillimanite (gneiss) Chlorine found furthest from mountain centre

Question 37

How are Al2SiO5 Polymorphs formed?

Answer 37

Regional metamorphism

Question 38

Where is Kyanite in Al2SiO5 polymorphs?

Answer 38

Top left in diagram. Found in gneisses. Found in high grade metamorphism. High pressure, low temperature

Question 39

Where is Sillimanite in Al2SiO5 polymorphs?

Answer 39

Right on the diagram. High grade regional metamorphism. High temps and high pressure. Usually gneisses

Question 40

What temperatures and pressures is sillimanite formed in Al2SiO5 polymorphs?

Answer 40

Maximum temp of 700°C. Maximum pressure 700MPa. Pressure exists at depths about 25km. Geothermal gradient of 28°C/km

Question 41

Where is andalusite in Al2SiO5 polymorphs?

Answer 41

Bottom of the diagram. Low pressure and high temps. Contact metamorphism

Question 42

What is it called when all the points meet in an Al2SiO5 polymorph diagram?

Answer 42

Triple point

Question 43

What types of metamorphism are contact, regional and dynamic?

Answer 43

Prograde metamorphic processes

Question 44

What happens during prograde metamorphism?

Answer 44

Fluids such as water and CO2 are driven off. These fluids are necessary to form the hydrous minerals that are stable at earth's surface

Question 45

What is retrograde metamorphism?

Answer 45

Involves mineral hydration, carbonation and oxidation as metamorphic rocks are returned to the surface. Chemical reactions take place more slowly as temp decreases