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Intro to Earth Science > New From Old: Metamorphism - Lec 11 > Flashcards

New From Old: Metamorphism - Lec 11 Flashcards

1
Q

What does metamorphism do?

A

Changes the texture and/or mineral assemblage in rocks in the solid state

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2
Q

What are the cause of metamorphism?

A

Changes in temperature and/or pressure, differential stress (compression or shear) plus hydrothermal fluids may also be involved

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3
Q

What does the composition of a protolith control?

A

What minerals form

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4
Q

What are the processes involved in forming metamorphic rocks? (5)

A 
Recrystallisation
Phase Change
Neocrystallisation
Pressure solution
Plastic Deformation
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5
Q

Where does metamorphism occur and how can it be identified?

A

In various settings

On the basis of the mineral assemblage

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6
Q

How does contact metamorphism occur?

A

By an increase in temperature

Heating by an intrusion creates an aureole surrounding it

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7
Q

How does regional metamorphism occur?

A

Involves both Temperature and Pressure and is associated with orogenesis

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8
Q

How does subduction metamorphism occur?

A

Characterised by low temperature and high pressure

Occurs at subduction zones

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9
Q

Name 3 other types of metamorphism

A 
Shock metamorphism (asteroid impact)
Burial metamorphism (as sediments accumulate the succession passes from diagenesis to metamorphism) 
Dynamic metamorphism (localised deformation via ductile behaviour along faults)
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10
Q

How do prograde and retrograde metamorphism occur?

A

As temperature and pressure increase, and decrease, respectively

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11
Q

What is metamorphism?

A

Pre-existing rock (protolith) undergoes solid state (no change of state) change as response to modification of its environment (change in temp/pressure etc)

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12
Q

What metamorphism is associated with igneous intrusion?

A

Contact metamorphism

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13
Q

What are porphyroblasts?

A

Outsized large crystals that grow in a metamorphic rock

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14
Q

What is foliation?

A

The alignment of minerals

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15
Q

Are there textures in hand specimens that are diagnostic of metamorphism?

A 
Metamorphic minerals (indicative of Temp/Pressure conditions)
Metamorphic textures (foliation - garnet porphyoblasts cross-cutting foliation)
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16
Q

What does metamorphism often result in?

A

Foliation

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17
Q

What is planar foliation?

A

The parallel alignment of platy minerals (clays/micas)
Minerals oriented perpendicular to compression
Sometimes with alterations of differently coloured layers

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18
Q

List the different types of foliation from very fine foliation to fine foliation to medium scale foliation to banding

A

Slaty Cleavage (individual minerals not visible)
Phyllitic foliation
Schistosity (minerals clearly visible)
Gneissic Banding (alternating bands of light and dark coloured minerals)

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19
Q

In relation to texture, list 3 types of metamorphic rocks from low grade metamorphism (fine) to high grade metamorphism (coarse)

A

Slate
Schist
Gneiss

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20
Q

What is non-foliated texture?

A

No preferred orientation of the minerals in rocks

Occurs when deformation is low and clay/mica content is low (quartzite has poor foliation at best)

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21
Q

What is involved in recrystallisation?

A

Changes shape and size of grains
No change in mineral identity
(Protolith to metamorphic rock: quarts sand to quartzite)

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22
Q

What is involved in phase change?

A

One mineral transformed into another with same composition but different crystal structure
(Andalusite: low t and p, kyanite: low t high p, sillimanite: high t and p)

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23
Q

What is involved in neocrystallisation?

A

Growth of new minerals different from protolith

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24
Q

What is involved in pressure solution?

A

Mineral grains dissolve where their surfaces are pressed against other grains
Ions produced migrate
Precipitation where grains under less pressure
Can shorten grains in one direction and lengthen them in another
Also occurs under non-metamorphic conditions

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25
What is involved in plastic deformation?
Rock sheared/squeezed at high temps/pressures Minerals change shape without breaking: plastic behaviour Can occur without change to composition or crystal structure Can occur when metamorphic reactions are happening
26
Role of heat in metamorphism
Atoms vibrate Chemical bonds between them stretch and break Atoms detach Results in rearrangement of atoms within grains and migration of atoms into/out of grains Results in recrystallisation and/or neocrystallisation (product depends on protolith)
27
Role of pressure in metamorphism
Greater pressure = closer packing of atoms Density increases Results in phase changes and/or neocrystallisation (product depends on protolith)
28
What is differential stress?
Stress applied unequally in all directions
29
What is normal stress?
Applied perpendicular to a surface
30
What do compression and tension do to a material?
Compression: flattens a material Tension: stretch a material
31
What is shear stress?
Moves one particle sideways relative to another
32
How does preferred orientation form?
Develops via alignment of inequant grains (at elevated temps rock changes shape without breaking - internal texture changes sympathetic to stresses applied) Inequant grains may be pre-existing or form during the metamorphic episode
33
What is equant?
Roughly the same in all dimensions
34
What is inequant?
Dimensions not all the same
35
Name and describe to types of inequants
Elongate (cigar-shaped) : one dimension notably longer than the other 2 Platy (pancake-like): one dimension notably shorter than the other 2
36
How does preferred orientation form in relation to pressure solution?
Occurs on faces perpendicular to direction of compression (grains shorten in that direction)
37
How does preferred orientation form in relation to neocrystallisation?
Precipitation occurs where rock is stretching
38
How does preferred orientation form in relation to plastic deformation?
At high temp accompanied by differential stress
39
How does preferred orientation form in relation to shear stress?
It flattens grains and rotates them into alignment | As rock undergoes flattening, rigid inequant grains in 'soft matrix' rotate and flatten as overall rock changes shape
40
How does preferred orientation form in relation to compressive stress?
It flattens grains into alignment | As rock undergoes flattening, rigid inequant grains in 'soft matrix' rotate and flatten as overall rock changes shape
41
What are 3 hydrothermal fluids?
Hot water Steam Supercritical fluids
42
What are supercritical fluids?
Fluids under high temp and pressure with characteristics of fluid and gas. They are very pervasive
43
What is the role of hydrothermal fluids in metamorphism?
Accelerate reactions (atoms migrate faster through fluids than solids) Provide water that becomes absorbed by minerals during metamorphic reactions Metasomatism
44
What is metasomatism?
A metamorphic process that by which the chemical composition of a rock is altered by introduction and/or removal of chemical components as a results of its interaction with aqueous fluids
45
What are sources of water?
Groundwater (lots of rocks contain water at great depths) Released from rising magmas Products of metamorphic processes
46
Where does burial metamorphism occur?
Deep burial in a basin
47
Where does dynamic metamorphism occur?
Shearing in a fault zone
48
Where does contact (thermal) metamorphism occur?
Hating by an intrusion
49
Where does hydrothermal metamorphism occur?
Alteration by hot water leaching
50
Where does regional metamorphism occur?
Pressure and Temp change due to orogenesis
51
Where does subduction metamorphism occur?
High pressure and low temperature alteration
52
Where does shock metamorphism occur?
Extreme high pressure from a bolide impact
53
Burial metamorphism
Pressure and temp increase as sediments are buried in a basin due to the weight of overburden and the geothermal gradient (20-40 degrees per km) Typical metamorphism starts at temp >200 degrees - before that only get diagenesis Below depths of 8-15km,metamorphic changes replace diagenesis
54
Dynamic metamorphism
Breakage of rock by shearing within a fault zone At shallow depths (up to 15km) rocks are brittle, crushing to form fault breccia At deeper levels rocks are ductile, rocks in fault zones smear to form mylonite
55
Contact (Thermal) metamorphism
Thermal - develops in response to heat without change in pressure Contact - position adjacent to an intrusion Magma intrusion affects 'country rock' - heat transferred and hydrothermal fluids circulated - metamorphic aureole generated
56
Give an example of contact metamorphism in Ireland
Leinster granite (400mya)
57
Give an example of hydrothermal metamorphism in Ireland
Lithium deposits in S. Wexfird Pb deposits in N. Wicklow Avoca Gold (400 mya) - formed in association with volcanic arc
58
What are mineral deposits an indication of in relation to metamorphic rocks?
Hydrothermal metamorphism - indicates circulation of hydrothermal fluids
59
Hydrothermal metamorphism at mid-ocean ridges and example
Cold ocean water seeps into fractured oceanic crust Heated by magma, hot aggressive water chemically alters the basalt Hot water from black smokers provides mineral deposits and life Serpentinite: (480 mya) Altered mantle from ancient ocean in Clew Bay
60
Regional Metamorphism
Pressure and temp change due to orogensis At continent-continent convergent boundary Protolith subjected to increased temp (geothermal gradient and igneous activity), greater pressure (weight of overburden) and more compression and shearing
61
What is orogenesis?
Mountain building
62
Give an example of regional metamorphism
Grampian orogeny 470mya (ductile deformation of old rocks) - Connemara to Donegal
63
Subduction metamorphism
At subduction zones Trenches and accretionary prisms (wedges shaped build up of sediments scraped off descending plates) Subduction introduces colder rocks to depth so low geothermal gradient These conditions produce a unique low temp high pressure mineral assemblage called blueschist
64
Give an example in Ireland of subduction metamorphism
Achill
65
Shock metamorphism
Impact of comet/asteroid Extremely high pressure and temp High pressure minerals (eg. stishovite - highly dense polymorph of SiO2)
66
What are metamorphic facies?
A set of mineral assemblages indicative of certain pressure and temp conditions
67
What is metamorphic grade?
A measure of the intensity of temp and pressure conditions that lead to alterations
68
What is metamorphic grade reflected in?
What minerals are formed - protolith exerts control
69
What is prograde metamorphism?
Temp and pressure progressively increase As grade increases, reactions release water Therefore, high grade rocks tend to be 'drier' (lack minerals with -OH in their chemical formula)
70
What is retrograde metamorphism?
Temp and pressure progressively decrease AND Hydrothermal fluids MUST enter the system to add water Retrograde reactions do not take place under 'cold and dry' conditions Thus high grade rocks can be exhumed and visible on surface today

Intro to Earth Science (13 decks)

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