Metamorphic Rocks

Question 1

What are metamorphic rocks?

Answer 1

• A parent rock (protolith) that changes form

- Parent rock is either igneous or sedimentary

Question 2

What causes metamorphism?

Answer 2

• protoliths that are exposed to heating, squeezing, and hot fluids
• causes rocks to change form (size, shape, texture, colour, and/or mineralogy)
• Is a solid-state change

Question 3

Over which temperatures and pressures does metamorphism occur?

Answer 3

Occurs over a set range of P and T conditions.

The higher the P and/or T, the higher the degree of metamorphism or metamorphic grade

Question 4

Through burial, compaction, and cementation, sediments turn to stone (lithify) at

Answer 4

temperatures below 200C

Question 5

At low pressures in the shallow crust, metamorphism occurs between

Answer 5

~200C to ~850C

Question 6

At high pressures deeper in the crust, metamorphism can occur at

Answer 6

at much lower temperatures

Question 7

Above ~850C, rocks melt creating

Answer 7

igneous rocks

Question 8

What are the 3 types of metamorphism?

Answer 8

Heating, hot fluids, and squeezing

Question 9

Heating Metamorphism

Answer 9

• the avg (normal) geothermal gradient in the crust is ~
  25C/km
• Under normal conditions, rocks at >8km depth are >200C and will metamorphose
• However, metamorphism can also happen at shallower depths

Question 10

What could cause the temperature of the crust to rise above normal?

Answer 10

• Magma intrusions because they are unusually hot, magma is above 850C
• Tend to occur near the surface at mid-ocean ridges
• The high magma body will “bake” the surrounding country (protolith) rock resulting in contact metamorphism

Question 11

What is the contact aureole?

Answer 11

The zone of contact metamorphism that is narrow (mm to 10s of m) and completely surrounds the magma intrusion

Question 12

Water exists everywhere in the crust and in magma, therefore:

Answer 12

hot water is involved in nearly all metamorphic rxns, it also speeds up rxn time, end result may be the same

Question 13

Hot fluids

Answer 13

• Hydrothermal fluids often contain dissolved elements rejected from normal silicate minerals that form as magma solidifies
• Fluids migrate into cracks in the surrounding rocks. Changes in P/T cause minerals to precipitate out of the solution.
• Gold is not a component of silicate minerals, so it often gets flushed into cracks as magma cools and solidifies.
• Pegmatite formation is likely also linked to hydrothermal fluids. Pegmatites are igneous rocks but are often found in veins and dikes.
• Hot, mineral-rich water is involved in nearly all metamorphic rxns.

Question 14

Hydrothermal metamorphism

Answer 14

When water is the main agent of metamorphism, dominant near mid-ocean ridge magma

Question 15

Squeezing

Answer 15

• Pressure also increases with depth due to the increasing mass of the overlying rock
• The pressure is ~equal in all directions and is called lithostatic stress
• Directed stress: stress that is higher in one direction than other

Question 16

What processes can produce directed stress in the crust?

Answer 16

• The crust adjacent to convergent plates experiences directed stress. Compression causes crust to thicken, producing mountains.
• Directed stress results in large areas of regional metamorphism
• Directed stress also deforms rock, so regionally metamorphosed rock is often folded

Question 17

The distinction b/w types of metamorphism are often blurred:

Answer 17

subduction zones are prone to contact, hydrothermal, and regional metamorphism

Question 18

What is regional metamorphism?

Answer 18

occurs when rocks are buried deep in the crust. This is commonly associated with convergent plate boundaries and the formation of mountain ranges.

Question 19

How does metamorphism affect composition?

Answer 19

• usually the same as the protolith from which it formed.
• Elements are not added or removed, just rearranged
• Exception: hot fluids can (but not always) introduce/remove elements
• Elements rearrange themselves b/c minerals that were stable at the surface of the Earth become unstable as P/T rise.

Question 20

Temperature

Answer 20

• Some minerals are stable at low T, some at high T

- Minerals are stable at higher T when rocks are under pressure

Question 21

Pressure can cause

Answer 21

atoms in minerals to become more closely packed

- Metamorphic minerals tend to be dense

Question 22

In limestone, calcite crystals can grow:

Answer 22

larger, due to significant pressure over long period of time

Question 23

Neocrystallization

Answer 23

• new minerals grow that were not present in the protolith
• At low P/T, clay minerals become unstable, break down, and reform into chlorite.
• As P/T continues to rise, the newly formed chlorite becomes unstable and breaks down to form micas, then garnet, then kyanite, then sillimanite

Question 24

Index minerals only appear under specific P/T conditions. The presence of a given index mineral can tell us how much:

Answer 24

heat and pressure a rock experienced, or the metamorphic grade

Question 25

How does metamorphism affect texture?

Answer 25

• Some will break down and form new minerals (neocrystallization)
• Some will grow larger and become interlocked (recrystallization)
• As a result, most metamorphic rocks have a crystalline texture
• Grains in a metamorphosed sandstone undergo recrystallization, but some remnant clastic texture may still be visible
• This is called remnant clastic texture. The rock may appear granular or sugary, but grain won’t rub off. Sedimentary structures can be preserved.

Question 26

Additional metamorphic textures:

Answer 26

• Non-foliated (not layered)

- Foliated (layered)

Question 27

When pressure is equal in all directions, minerals can grow in any orientation, developing a:

Answer 27

non-foliated crystalline texture, often result of deep burial or contact metamorphism

Question 28

Directed stress causes platy or elongated minerals to grow:

Answer 28

perpendicular to main stress direction

Shale is the parent rock of most foliated rocks

Question 29

In which tectonic environment do we expect to find foliated metamorphic rocks?

Answer 29

• At continent-continent convergent boundary
• Slate will form at relatively shallow depth
• Gneiss will form deep in the core of mountains

Question 30

Metamorphism at continent-continent convergent boundary in order of least to most foliated (outermost to innermost):

Answer 30

Slate, phyllite, schist, gneiss

Question 31

The stability of a mineral is a function of:

Answer 31

P, T, and fluids present

Question 32

Metamorphic: if protolith is mudstone then is either

Answer 32

Slate, phyllite, schist, or gneiss

Question 33

Naming Schists

Answer 33

HIGHEST-GRADE MINERAL then MOST-ABUNDANT MINERAL schist

Question 34

Non-foliated metamorphic: if protolith is quartz sandstone

Answer 34

Quartzite

Question 35

Non-foliated metamorphic: if protolith is conglomerate

Answer 35

Metaconglomerate

Question 36

Non-foliated metamorphic: if protolith is carbonate rock

Answer 36

Marble

Question 37

Non-foliated metamorphic: if protolith is mafic igneous rock

Answer 37

Amphibolite