L11: Metamorphic Rock

Question 1

What is metamorphism?

Answer 1

• Occur when a rock is subjected to new temperatures and pressures
• Rocks change mineralogically and texturally until its in equilibrium with the new set of temperatures and pressures
• Changes the composition of a rock

Question 2

What are the 3 types of metamorphism?

Answer 2

• Contact
• Burial
• Regional

Question 3

Contact metamorphism

Answer 3

• Transformed by heat

- Hot magma rises to the surface

Question 4

Burial metamorphism

Answer 4

• Sediment is buried and accumulates under moderate temperatures and pressures
• Follows on from diagenesis

Question 5

Regional metamorphism

Answer 5

• Buried deep into the crust, undergo elevated temperatures and pressures
• Commonly takes place during mountain building and it occurs on scales much larger than simple intrusions
• Think of subduction and mountain building!!

Question 6

What do plate tectonics affect?

Answer 6

Greatly affects the metamorphic reactions because the process may move sediments and rocks from earth’s surface to interior, where the temperatures are higher

Question 7

As you go ______, temperature goes ______.

Answer 7

• Down

- Up

Question 8

What is a geothermal gradient?

Answer 8

• Increase in temperature with increasing depth
• Steeper where crustal thinning occurs
• Shallower where the continental lithosphere is old and thick

Question 9

What do geologists use to study composition of rocks?

Answer 9

geothermometer/geobarometer

Question 10

What is metasomatism?

Answer 10

Metamorphic process in which abundant fluids change the composition and texture of a rock

Question 11

What are metamorphic textures?

Answer 11

• Foliated
• Granoblastic (non-foliated)
• Porphyroblastic

Question 12

Foliated rocks

Answer 12

• Show alignment of minerals
• Have preferred orientation due to directed stresses
• Have clear texture
• Typically slate, phyllite, schist, gneiss
• Typical parent rocks = shale, sandstone

Question 13

Granoblastic rocks (non-foliated)

Answer 13

• No clear texture
• Grow in equant shapes (such as cubes or spheres)
• Form from metamorphism in which deformation is absent (such as contact metamorphism)
• Hornfel, quartzite, marble
• Parents: shale, basalt, limestone or dolomite

Question 14

Porphyroblastic rocks

Answer 14

• Large crystals set in a fine-grained matrix
• Surrounded by much finer grain matric
• Contact and regional metamorphism rocks
• Dissolved and reorganized
• Rock name: slate to gneiss
• Parent: Shale

Question 15

What else increases as the intensity of metamorphism does?

Answer 15

So does crystal size and coarseness of foliatio

Question 16

Examples of non-foliated rocks

Answer 16

• Marble
• Quartzite
• Hornfel

Question 17

Define metamorphic facies

Answer 17

• The assemblage of minerals formed during metamorphism of a rock of a given composition subjected to a given temperature and pressure
• Partly depends on the composition of the parent rock

Question 18

Zeolite minerals

Answer 18

• A class of silicates that contain interstitial water in cavities within the crystal
• AKA zeolite grade
• Basalt usually lowest grain minerals

Question 19

Greenschists

Answer 19

A higher grade of metamorphosed rocks; abundant minerals include chlorite

Question 20

Amphibolites

Answer 20

Contain amphibole and then the granulites (often contain pyroxene)

Question 21

Blueschist

Answer 21

• High pressure, moderate temperature

- Typically blue in color

Question 22

Ecologites

Answer 22

Produced in the highest temperature conditions

Question 23

What are the 2 essential points that characterize the concept of metamorphic facies?

Answer 23

1. Different kinds of metamorphic rocks form from parent rocks of different composition at the same grade of metamorphism
2. Different kinds of metamorphic rocks form at different grades of metamorphism from parent rocks of the same composition

Question 24

What are isograds?

Answer 24

Used to plot metamorphic grades over a regional metamorphic belt