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1A Earth > From Minerals to Rocks: How the Crust Works (Lectures 15-20) > Flashcards

From Minerals to Rocks: How the Crust Works (Lectures 15-20) Flashcards

1
Q

How are igneous rocks brought about?

A

Solidification from hot molten material (magma)

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2
Q

What evidence is there of bodies of magma at depth in the crust?

A

Active volcanoes

Hydrothermal activity

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3
Q

What is the major chemical differentiation that occurred inside the Earth?

A

Fe-S-Ni melts sank to the centre of the planet to form the core
Basaltic melts moved upwards to form the crust

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4
Q

For igneous rocks, which minerals are present at 40, 50, 60 and 70% silica content respectively?

A

40%: olivine + pyroxene
50%: low olivine, pyroxene, Ca-rich plag feldspar
60%: plagioclase feldspar, hornblende (amphibole)
70%: quartz, K-feldspar, Na-rich plag feldspar, biotite

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5
Q

What are the terms for the fine-grained and coarse-grained variants at 40, 50, 60 and 70% silica content?

A

40%: peridotite (coarse) and picrite (coarse)
50%: gabbro (coarse) and basalt (fine)
60%: diorite (coarse) and andesite (fine)
70%: granite (coarse) and rhyolite (fine)

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6
Q

How do Si-rich and Si-poor rocks differ in colour?

A

Si-rich are pale (leucocractic)

Si-poor are dark (melanocratic)

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7
Q

What are the three main ways to make the geotherm of the mantle intersect the solidus to produce melt?

A

Stretching
Hot spots
Add water

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8
Q

How does stretching induce melting of the mantle?

5 points

A

Movement of rigid plates on the surface sets up stresses within plates
Extensional stresses thin the plate
Reducing plate thickness pulls up the underlying mantle
Upwelling mantle too fast to cool conductively (behaves adiabatically)
Hotter than stable geotherm at any depth so melts

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9
Q

What happens if the stretching of a plate were to continue?

4 points

A

Continental crust ruptured
Formation of a new ocean basin
Mantle upwelling causes melting
The melt rises, solidifies and forms new oceanic crust

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10
Q

How do hot spots induce melting of the mantle?

2 points

A

Unsteady convection of the mantle causes narrow areas of hot upwelling material called plumes
Geotherm is hotter than usual and intersects the solidus at depth and causes extensive melting and volcano formation

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11
Q

How does adding water induce melting of the mantle?

6 points

A

The oceanic crust formed at mid-ocean ridges is in contact with seawater
Seawater penetrates upper few km of the hot crust
Hydration of pyroxene forms amphibole
Subducted ocean crust reverses hydration
Water released and flows into the overlying mantle
The melting point of the wet mantle is lower than that of dry mantle

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12
Q

What is peridotite made of?

A

Mostly olivine
Pyroxene
Minor spinel, plagioclase or garnet

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13
Q

How does basalt form from the mantle melt?

A

The mantle is modified during ascent and loses olivine

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14
Q

Why does melt migrate upwards towards the surface?

A

Buoyancy

Compaction of the hot and weak solid residue (squeezes out melt like a sponge)

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15
Q

When does rising melt stop?

A

At a level of natural buoyancy

OR a low-density material, i.e. granite, provides a barrier

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16
Q

What happens if mantle melts pond at the base of the crust?

3 points

A

Hottest magmas are basalts (1100-1300 degrees C)
Melting T of the crust is 700-900 degrees C
Hence the crust melts

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17
Q

Which processes take place in a magma chamber?

5 points

A

Magma stalled in the crust at its LNB forms a magma chamber
Cools by conduction and convection
Crystallisation occurs
Order minerals appear controlled by melt composition and pressure
Many crystallised first are denser than the magma and sink to the base to form cumulates

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18
Q

What does fractional crystallisation change?

A

The residual composition of the magma

Si % increases

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19
Q

What are plutonic rocks?

A 
Large bodies (>100's m) that cooled well below the Earth's surface
Coarse-grained (>5mm)
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20
Q

What does hypabyssal mean?

A 
Smaller intrusions (cm to 10's m) at shallow depths
Fine- to medium-grained (1-5mm)
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21
Q

What is a batholith?

A

A large body of magma (10-1000 km scale) emplaced deep in the Earth’s crust

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22
Q

When are batholiths revealed?

A

Substantial crustal uplift and erosion

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23
Q

What is a xenolith?

A

Fragments of rock from another source

Derived from the surroundings, different parts or earlier stages of igneous intrusion

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24
Q

How are country-rock xenoliths incorporates into the magma?

A

Process called stoping

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25
Q

What are dykes?

A

Steeply inclined or wall-like bodies formed by magma filling fractures

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26
Q

What does it mean for dykes to be discordant?

A

Dykes cut across bedding planes in the country rock

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27
Q

When are dyke swarms seen?

A

Stretching of the Earth’s crust produces many fractures

Large numbers of parallel or radiating intrusions occur

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28
Q

What are sills?

A

Sheet-like intrusions emplaced along bedding planes (concordant)

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29
Q

When do laccoliths form?

A

A concordant sheet of magma has bulged to dome the overlying country rocks

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30
Q

What does the behaviour of a volcano depend on?

A

Volatile content (gases)
The viscosity of the magma
Rate of lava emission
Environment (in water, under ice, on the surface)

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31
Q

What kind of volcanic eruption is seen in the intraplate setting?

A

Mainly basaltic
Low viscosity magma (fast-flowing)
Loses volatiles easily (not explosive)

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32
Q

What are the two types of basaltic lava flow?

A

Pahoehoe: fast-flowing, very low viscosity as it does not crystallise during flow, thin with wrinkled tops
aa: slower-moving, nucleates crystals throughout, viscosity increases, a mass of rubble

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33
Q

The crystallisation of the flow crust may result in what?

A

Formation of lava tunnels

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34
Q

What kind of volcano is formed in the intraplate setting?

A

Shield volcano

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35
Q

What are flood basalts?

A

Vast quantities of lava that spread out over enormous distances

36
Q

Outline intraplate volcanism underwater

A

At mid-ocean ridges
Basalt generally forms pillow lavas
Can be highly explosive in shallow water as steam is produced

37
Q

How is magma released at subduction zones?

A

Magma stalls and cools in the crust and fractionates towards Si-rich composition
Assimilation of melted country rock
Erupted magma is cooler and more Si-rich (andesite but rhyolites occur rarely)

38
Q

What occurs when gas is built up in lava domes?

A

Dome collapses to form a flowing cloud of gas and rock

39
Q

Outline the process of an explosion of a lava dome (including the build-up to)
(6 points)

A

Assimilation of continental crust and wet mantle source means high gas content
High viscosity so gases can’t escape
Decompression so bubbles grow
At 75~ % volume, bubbles touch, magma loses strength
Explosion triggered
The explosion shatters lava into rock fragments, mineral fragments and glass shards

40
Q

How are cinder cones produced?

A

Successive layers of small pyroclastic fragments

41
Q

What happens when explosive eruptions produce large amounts of fine ash, and surrounding air is entrained in the eruption column?
(4 points)

A

Powerful convection currents set up
Eruption columns high into the stratosphere
Widespread tephra dispersion occurs
Plinian eruption type

42
Q

What are lahars?

What can trigger them?

A

Super-fast mudflows

Abundant rainfall or snowmelt

43
Q

What can occur when the volcanic edifice collapses?

A

Tsunami from the collapse of volcanic islands

Magma upwelling triggers eruptions as volcano collapses from being too steep

44
Q

What is a pyroclastic flow?

How are they formed?

A

The base of nascent column collapses to form a cloud of hot gas and ash, pumice and rock debris that travels very fast across the ground
If the energy of the eruption can’t sustain the eruption column

45
Q

Define tephra

A

Collective term for all pyroclastic particles

Usually applied to airfall material

46
Q

What is the melting process for the mantle at mid-ocean ridges that forms basalt?
( 4 points)

A

Melting of mantle leaves a depleted rock of olivine and pyroxene
Picritic liquid forms by partially melting peridotite during adiabatic decompression
Moves upwards as it less dense than peridotite
Loses olivine during ascent and becomes a basalt

47
Q

What causes the partial melt of the mantle to move towards the mid-ocean ridges?

A

The flow of the mantle as the plates move apart creates a pressure gradient

48
Q

What evidence is there for the composition and structure of the ocean floor from the ocean floor itself?
(4 points)

A

Magnetic stripes preserved in oceanic crust
Seismic profiles - 4 distinct layers within the crust
Deep-sea drilling penetrates upper few km
Submersibles - observe volcanic activity

49
Q

What are ophiolites?

A

Slices of oceanic crust emplaced onto the margins of continents

50
Q

What do ophiolites indicate the structure of the oceanic crust is?

A

Top-down: sediments, pillow lavas, sheeted dyke complex, gabbros

51
Q

What processes take place at mid-ocean ridges?

3 points

A

Spreading causes extensional faults
Shallow, small earthquakes - thin brittle layer overlying hot, ductile layer
Hydrothermal circulation - cools crust to a depth of 2km, water removes metalliferous elements

52
Q

What is an accretionary prism?

A

Stacked repeated sequences of sea-floor sediments separated by low-angle faults

53
Q

Which metamorphic rocks were formed from basalt by intense deformation and high-pressure?

A

Eclogite - garnet and pyroxene

Blueschist - blue amphibole called glaucophane

54
Q

For a subducting slab the depth at which rocks change into another is not the same as the surrounding mantle, what does this do and why does it happen?

A

Olivine changes to wadsleyite at a lower depth
Wadsleyite is denser so this pulls the slab down
Olivine to wadsleyite is exothermic so happens at shallower levels

55
Q

How are ophiolites potentially formed?

A

Parts of young oceanic crust formed by back-arc spreading, thrust onto continents during continental collision as they were too hot and buoyant for subduction

56
Q

What is back-arc spreading?

A

Extension caused by slab roll-back

57
Q

What two possible outcomes are there for subducted slabs?

A

Penetrate down to the base of the mantle

Get bent and stuck at the upper/lower mantle boundary

58
Q

Where does the water come from in subducted slabs?

A

Water added by hydrothermal alteration at a mid-ocean ridge

Hydrated basalt carried down where water is released as the temperature rises

59
Q

What do volcanoes look like above subduction zones?

A

Steep sides and explosive eruptions

Andesite mostly, but basalt or basaltic andesite also common

60
Q

Outline the properties of andesite

3 points

A

Fine-grained intermediate rocks containing amphibole and biotite
Viscous magmas due to high silica %
Viscous because wet magmas crystallise when depressurised

61
Q

How does silicic magma form plutons and batholiths?

4 points

A

Wet granite melts are buoyant and gravitationally unstable
Granite plutons formed by magma rising along fractures and congregating in inflating chambers
Upwards movement of melt in dykes feeds large intrusions
Intrusions get bigger by assimilation and stoping

62
Q

What is metamorphism?

A

The process whereby new minerals and/or textures form in pre-existing rocks in the solid state, generally at increased P and T

63
Q

What does metamorphic assemblage depend on?

A

Bulk composition
Pressure
Temperature

64
Q

Why do metamorphic minerals often not revert to the to assemblages stable at lower P and T as they reach the surface (retrogression)?

A

Loss of water on metamorphism - must be re-introduced and is very hard to do so
Speed reactions occur is controlled by T, at lower T reactions are much slower

65
Q

If retrogression is very uncommon, what can be said about the mineral assemblages at the surface?

A

They are metastable

66
Q

What is the start of metamorphism?

A

Diagenesis

During which sediment is turned into a rock (lithification)

67
Q

What is contact metamorphism?

A

High T, low P metamorphism

Followed by rocks which are intruded by plutons

68
Q

Where does high P, low T metamorphism take place?
How are these conditions obtained?
When are these rocks seen?

A

Only in subduction zones
Rock is pushed down too rapidly to reach thermal equilibrium
Only see rocks from this if brought up rapidly before heating up i.e. by continental collision

69
Q

When does high T, high P metamorphism take place?

Where does it take place?

A

Heating rocks with burial
Occurs in zones 100’s of km wide and 1000’s of km long called orogenic belts
Called regional metamorphism
During continental collisions

70
Q

What is orogeny?

What is orogenesis?

A 
Orogeny = continental collision, the environment of regional metamorphism
Orogenesis = creation of mountains
71
Q

How are higher than normal temperatures generated for regional metamorphism?

A

Erosion: high mountains are rapidly eroded, brings deep, hot rocks nearer the surface and makes a shallower geotherm
Radioactive decay: continents contain abundant granite, which is enriched in U, Th and Rn. These decay and generate heat
The combination gets rocks away from the stable geotherm

72
Q

How is a rock fabric formed?

4 points

A

Prograde metamorphism involves a T increase, and generally a P increase
Increasing P involves large rock movement, so shearing stresses occur
Squashing/folding/stretching a rock affects the shape and orientation of minerals that are thermodynamically stable during the deformation
Deviatoric stress applied during mineral growth

73
Q

What are the two types of rock fabric?

A

Foliation: any planar feature in a rock (bedding usually not referred to as a foliation)
Lineation: any linear feature (e.g. intersection of two surfaces, or alignment of elongate minerals)

74
Q

Define cleavage

A

Secondary foliation

Generally cutting bedding

75
Q

What can be decoded from mineral shape and orientation?

A

Strength and orientation of the stress field

76
Q

What can be decoded from the mineral assemblage?

A

Where in PT space the deformation took place

77
Q

What are porphyroblasts?

A

Larger than average crystals in metamorphic rocks

78
Q

The relationship of porphyroblasts to the fabric tells us what?

A

When they grew relative to the fabric-forming deformation event

79
Q

What is the process of metamorphism for mudrock?

A

Mudrock -> slate -> phyllite -> schist -> gniess

80
Q

With contact metamorphism, what happens to T with distance? and what is a contact aureole?

A

Maximum T reached decreases away from the intrusion
A series of zones develop with minerals characteristic of high T found near the contact and lower T minerals further away

81
Q

When is a hornfels created?

A

In the hottest parts of the aureole

Recrystallisation is so intense it obliterates any pre-existing fabric

82
Q

What is the equation for the period of time for a pluton to cool?
What can the equation be approximated to?

A

τ = (a^2)/(π^2 x κ)
a is diameter
κ is thermal diffusivity
τ (years) = (a^2)/300 (a in m)

83
Q

When do these respective rocks form:

eclogite, amphibolite and blueschist?

A

Eclogite: high P, high T
Amphibolite: medium P, medium T
Blueschist: high P, low T

84
Q

Why is blueschist formed in subducting slabs?

A

The thermal anomaly of down-going slabs means rocks are metamorphosed at high P and low T
Steady shearing makes them strongly foliated

85
Q

What forms eclogite in a subducting slab?

A

Basalt of the oceanic crust at greater depths than blueschist forms

86
Q

Outline the Wilson cycle

A
  1. Embryonic stage: involves uplift and crustal extension of continents with rift valley formation
  2. Young stage: evolution of rift valleys into spreading centres resulting in a narrow, parallel-sided sea
  3. Mature stage: Widening of growing basin and development into a major ocean with the production of new oceanic crust along the ridge system
  4. Subduction stage: Expanding system becomes unstable and away from the ridge, the oldest lithosphere sinks into the asthenosphere (subduction)
  5. Terminal stage: Subduction outpaces new crust formation, the ocean contracts, island arcs collide and create young mountain ranges
  6. End stage: all oceanic crust has subducted, the continents converge along a collision zone with an active fold mountain belt. Plate boundary becomes inactive and becomes a new zone of weakness, restarts
87
Q

Give examples of each stage in the Wilson cycle

A
  1. Embryonic stage: East African Rift System
  2. Young stage: the Red Sea
  3. Mature stage: Atlantic Ocean
  4. Subduction stage: Pacific Ocean
  5. Terminal stage: the Mediterranean
  6. End stage: the Himalayas

1A Earth (15 decks)

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