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internal processes Flashcards

1
Q

what are the five ways in which we can collect informationof the internal earth?

A

drilling

geophysics

experiments

geochemistry

meteorites

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2
Q

how can geophysics be used to inform us of the internal earth

A

remote sensing

  • seismic
  • gravity
  • magnetics
  • prescence of melt
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3
Q

how does minerology provide information of the intenal earth?

A

certain minerals provide insight into pressure and temperature conditions

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4
Q

what is the importance of meteroites?

A
  • instrumental in determining the age of the earth
  • some meteroites are also believed to be good approximates of the bulk composition of the earth
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5
Q

what are the three types of metorites?

A
  • Stones – consisting largely of silicate minerals (similar to earth rocks)
  • Irons – alloys of iron and nickel
  • Stoney irons – have roughly equal proportions of iron- nickel and silicate
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6
Q

what is the rarest type of meteorite and why?

A

stony ones are less common because they look like any other rock and are more susceptible to weathering

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7
Q

define: igneous rock

A

igneous rock

a rock that has formed by the cooling of a liquid(magma, melt, lava)

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8
Q

define: magma

A

magma

a completely or partly molten natural substance that on cooling solidifies as a crystalline or glassy igneous rock

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9
Q

define: melt

A

melt

the liquid part of magma with no crystalline phases or inherited solid

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10
Q

define: lava

A

lava

a magma that due to volcanic activity is released on to the surface of the earth

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11
Q

define: plutonic rock

A

plutonic rock

is formed from magma that crystallised beneath the earths surface

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12
Q

what is igneous petrology?

A

igneous petrology

is the study of igneous rocks. it link mineralogy, geochemistry, field work, and physics

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13
Q

where is the largest magma production rate?

A

oceanic ridges

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14
Q

where is there greater magma production, volcanic or plutonic?

A

plutonic

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15
Q

what are the three most common elements in the crust? give general %

A

O - 46.6%

Si - 27.7%

Al - 8.3%

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16
Q

what are the most common minerals in crustal rocks and why?

A

Si and O are by far the most common elements thus silicates are the most common minrals in crustal rocks.

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17
Q

what is the building block of silicates?

A

SiO4 group = tetrahedron

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18
Q

how are silicate tetrahedra used to make minerals?

A

polymerisation of tetrahedral

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19
Q

we can combine the basic string of silicate tetrahedra together to form templates. how are these templates held together?

A

templates require cations (usually metals) to balance the charge.

metals are loacted in cavities

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20
Q

which minerals are single, double chain, and sheet silicates?

A
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21
Q

what three things cause melting?

A

heat

pressure changes

water

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22
Q

in partial melting is the melt more or less rich in silica than the starting material?

A

in partial melting the melt is richer in silica than the starting material.

e.g. partial melting of an ultramafic rock will yield a mafic melt.

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23
Q

what is the solidus?

A

P -T line where melting begins

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24
Q

what is the liquidus?

A

P - T line where melting is complete

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25
Q

what is anhydrous melting?

A

melting without water or no hydrous minerals

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26
Q

how is anhydrous melting brought about?

A

by increase in temperature or decrease in pressue (or both

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27
Q

is the composition of the liquid the same or different to the remaing solid in anhydrous melting?

A

the composition of the liquid is different to that of the remaining solid

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28
Q

what type of heat transfer is in the lithospher and the asthenosphere?

A

lithosphere - conduction

convection - asthenosphere

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29
Q

what is adiabatic decompression?

A

decompression where no heat is lost to the surrounding rock

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30
Q

what is hydrous melting?

A

melting with water or hydrous minerals

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31
Q

how does the addition of water change the solidus and liquidus?

A

moves them to the left and negative gradient

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32
Q

where does the anhydrous and hydrous melting curves converge?

A

anhydrous and hydrous melting converge at low pressure.

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33
Q

what is congruent and incongruent melting and which is more common?

A

congruent - melt produced has same composition as starting material e.g calcite –> carbonate

incongruent - the opposite

incongruent is the norm

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34
Q

the structure of quartz is entirely bulit from SiO4 units. why then is its chemical formula SiO2?

A

quartx is created from each tetrahedra being connected to four other tetrahedra. the tetrahedra share corners (e.g. each O atom is shared between two tetrahedra). this makes the ratio of Si to O 1:2 and hance the chemical formula of quartz SiO2

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35
Q

what is a solid solution? give examples of two important rock forming minerals that show solid solutions.

A

solid solutin refers to the range of chemical compositions possible for a given crystal structure.

solid solution involves the subsitution of one or more atoms or ions at specific sites in the crystal structure.

e.g. olvine can range from pure Mg2SiO4 to pure Fe2SiO4 with a continuos spread of solid solution compositions inbetween.

another example is plagioclase (anorthite to albite) and alkali feldspar (albite - K-spar)

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36
Q

suggest why parial melting of a mafic source rock is nlikely to yield a large body of felsic magma?

A

the first melt to form during partial metling of a mafic rock will tend to be intermediate in compositon.

however even if the first tiny fration of the melt was felsic, it would be too viscous to flow and thus cant group into large bodies.

as partial melting proceeded, the melt would become progressively less felsic, and the average composition of the melt would certainly be intermediate before enough melt can be collected to be able to rise freely.

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37
Q

what are the four properties of magma?

A

structure

temperature

viscosity

density

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38
Q

what are the 5 types of magma?

A

Melt only that generally contains only dissolved volatiles (single phase system)

Melt plus bubbles of volatile fluid (two phase system)

Melt plus crystals (two phase system)

Two immiscible melts of different composition (two phase system) like oil and water

Melt plus bubbles of volatile fluid and crystals of olivine and plagioclase (four phase system)

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39
Q

give examples of volatiles

A

water

carbon dioxide

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40
Q

what is the effect of changing pressure on volatiles?

A

changes in pressure cause volatiles to exsolve (undissolve) making gases

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41
Q

what effect does volatiles have rocks?

A

they break up the silica chains

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42
Q

which do volatiles have a greater effect on, felsic or mafic rocks? and why?

A

Volatiles have a greater effect on felsic rich as there are silica chains to disrupt.

On mafic rocks it has little effect due to no chains to disrupt.

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43
Q

describe the atomic structure of magma

A

SiO44- unit covalently bonded = remains as a unit during melting

Other bonds in minerals are weaker – they are the ones that break during melting

More felsic magmas have higher SiO4 = more links = polymerisation (creating links)

Network formers (SiO44-, MAlO44-) (M= metallic ions that keep the charge 4- and keeps the chemical structure) and network modifiers (cations, volatiles)

Felsic magmas have more links and thus remains long chains and has greatest changes.

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44
Q

which is more viscous out of felsic and mafic magmas and has the greatest changes? and why?

A

More felsic magmas have higher SiO4 = more links = polymerisation (creating links) thus more viscous.

Felsic magmas have more links and thus remains long chains and has greatest changes.

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45
Q

around what is the temperature of erupting magmas?

A

800oC to 1200oC

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46
Q

how can we use temperature to decide the composition of a magma?

A

Low temperatures = felsic compositions (rhyolite)

High temperatures = mafic compositions (basalt)

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47
Q

what is viscosity?

A

it is the Resistance to flow

Long polymerised chains hinder the flow

thus Felsic magmas more viscous than mafic

Volatiles disrupt polymerisation

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48
Q

what are the variables of viscosity?

A

temperature

time

pressure

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49
Q

how can a plume get blocked as magma raises to the surface? what happens if there is too much pressure?

A

As magma goes to the surface it lowers pressure thus volatiles exsolve. This increases viscosity and can then block the plume.

Thus an increase in pressure below can cause explosive eruptions.

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50
Q

what happens as lava flows away from their source or vent?

A

As lavas flow away from their source or vent, they cool and viscosity increases.

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51
Q

Viscosity of a mauna loa flow increased 2 fold over 20Km

Viscosity of a small flow from Mt. Etna increased 375-fold over 500m

why is there a difference?

A

Mt Etna is more felsic so it is much more viscous.

Thus it depends on the composition and temperature of the magma.

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52
Q

why does viscosity increase as it cools down?

A

As it cools down it has greater polymerisation so longer, more organised chains, so more viscous.

And the creation of crystals which creates an increase in viscosity.

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53
Q

describe this graph.

A

Crystals formed over a longer time will be larger thus interfere with melt flow.

If the same temp cooling is over a longer time it will be more viscous as the creation of crystals.

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54
Q

describe this graph

A

Increase in water content by one order of magnitude decreases viscosity by up to 6 orders.

water has no effect on mafic rocks as there are no silica chains to disrupt

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55
Q

does water have a greater effect on felsic or mafic rocks in terms of viscosity? why?

A

has a greater effect on felsic rocks as mafic rocks dont have any silica chains to disrupt thus adding water will have no impact,

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56
Q

why is viscosity important?

A

It affects how magmas are transported

It dictates the eruption style of volcanoes

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57
Q

what are the 7 textures of volcanic rocks?

A

Extrusive or volcanic rocks - rapid cooling - fine grained (< 0.25 mm)

Intrusive or plutonic rocks - slow cooling - coarse grained (> 2 mm)

Intermediate or hypabyssal rocks (dykes and sills) - medium grained (0.25 – 2 mm)

Volcanic glass or obsidian (fast cooling)

Phaneritic: mineral grains large enough to be identified by eye (> 1 mm)

Aphanitic: grains too small to be identified by eye (< 1 mm)

Porphyritic: two grain size populations (phenocrysts and groundmass) = initial slow cooling stage at depth followed by rapid cooling at or near the surface (can also be due to volatile exsolution which causes crystallisation thus we get or deposits from the volatiles

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58
Q

what is the nucleation centres texture?

A

Two stage crystallisation

If one mineral nucleates earlier or faster than others it may form large phenocrysts

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59
Q

describe the intergranular texture

A

Intergranular texture: when crystal nucleate and grow at the same rate.

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60
Q

what is formed when rapid cooling and supersaturation takes place?

A

Long slender crystals

Crystals that radiate from a common centre

Curved clusters of microlites

Skeletal crystals with hollow cores.

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61
Q

describe the porphyritic texture

A

Phenocrysts

Important for ore deposits

Eruption or volatile exsolution.

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62
Q

describe cumulate texture

A

Crystal settling

Layering

Layered intrusions

Coarse equidimensional crystals

Economic importance – lots of platinum.

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63
Q

give the three names to describe cumulate textures.

what do the names depend on?

A

they depend on the number of small crystals in it produced by the crystallisation of the intercumulus liquid.

left - orthcumulate

middle - mesocumulate

right - adcumulate

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64
Q

describe flow textures

A

Alignment of crystals in flowing magma

if feldspars are well aligned: trachytic

Glassy rocks may show flow delineated by vesicles parallel to the direction of flow.

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65
Q

how can we work out the order of crystallisation?

A

If a crystal surrounds another, it is younger

Early formed crystals are commonly euhedral, or at least more so than later formed ones

If larger and smaller crystals of the same mineral coexist, the larger ones began to grow first

BUT…

Interlocking grains that grow simultaneously may appear to cut each other

Included crystals may have crystallised later (e.g. exsolution)

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66
Q

what is the modal classification of felsic and mafic minerals?

A

Felsic: feldspar and silica = feldspar, feldspathoids, silica (light colour)

Mafic: magnesium and ferrous/ferric (ferromagnesian) = olivine, pyroxene, amphibole, opaques, accessory (dark colour)

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67
Q

what is the colour index?

A

Colour Index (CI): volume percentage of mafic or dark coloured minerals

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68
Q

for felsic, intermediate, mafic and ultramafic minerals what are the wt% SiO2 boundaries?

A

felsic > 65

intermediate - 52 - 65

mafic - 45 - 52

ultramafic <45

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69
Q

for felsic, intermediate, mafic and ultramafic minerals what are the colour index boundaries?

A

felsic 0 -35

intermediate 35 -65

mafic 65 -90

ultramafic 90 - 100

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70
Q

which mineral doesnt form a solid solution?

A

quartz

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71
Q

olivine cant coexist with?

A

quartz (as olivine is ultramafic and quartz it felsic)

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72
Q

earliest formed feldspars are _____________ rich

A

Earliest formed feldspars are calcium rich.

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73
Q

in a QAPF diagram what does QAPF stand for?

A

Q: quartz (tridymite, cristobalite)

A: alkali feldspars - orthoclase

P: plagioclase

F: feldspathoids or foids

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74
Q

before using a QAPF diagram what must we do?

A

we must minus the mafic proportion and then recalculate

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75
Q

when cant we use a QAPF diagram?

A

we cant use it when the grains are too fine

contains no matrix glass

if CI > 90

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76
Q

what are the wt% of major, minor and trace elements?

A

major elements > 1 wt%

minor elements 0.1 - 1.0 wt%

trace elements < 0.1 wt%

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77
Q

is the mineralogy of a rock dependent or independent of cooling rate?

A

mineralogy is independent of cooling rate e.g. gabbro, dolerite and basalt all have the same mineals and same composition.

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78
Q

what is a TAS diagram?

A

it is a classification of aphanitic and glassy volcanic rocks

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79
Q

what does TAS stand for?

A

TAS = Total Alkalis + Silica

alkalis = Na2O + K2O

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80
Q

as a magma cools what happens to the dense crystals?

A

as it cools the dense crystals leave the system

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81
Q

describe the Bowens reaction series

A

bowen suggested that the common minerals that crystallize from magma could be divided into a continuous reaction series and a discontinuous reaction series.

they dont crystallise one after another but instead they gradually change from one mineral to another.

plagioclase crystallises throughout the range however only calcium plagioclase crystallises first then it becomes more sodium rich.

mafic minerals crystallise first then felsic minerals

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82
Q

what is magmatic differentiation?

A

Differentiation is defined as any process by which magmas evolve and become more diversified

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83
Q

what are the two components of magmatic differentiation?

A

Two components:

Chemical

Physical

The chemical part establishes compositional differences in one or more phases (crystal growth)

The physical part allows the preservation of these differences by segregating or fractionating the phases so they can form distinctive rocks

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84
Q

the compositions of the rock forming minerals are _______ than the melts from which they form

A

The compositions of the rock-forming minerals are** simpler** than the melts (liquids) from which they form

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85
Q

the igneous minerals consist of a relatively small number of ____________ most of which have a ________ concentration in the crystal than in the melt

A

The igneous minerals consist of a relatively small number of essential constituents, most of which have a higher concentration in the crystal than in the melt.

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86
Q

as a crystal grows in the melt, the elements required by the mineral becomes more _________ in the melt

A

As a crystal grows in a melt, the elements required by the mineral becomes more depleted in the melt

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87
Q

Elements that are not required by the mineral are __________ in the diminishing volume or the residual (remaining) melt – these are termed ___________

A

Elements that are not required by the mineral are enriched in the diminishing volume or the residual (remaining) melt – these are termed incompatible

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88
Q

If the melt and solid are separated completely the final residual melt will have _________ composition from that at the beginning

Therefore when it finally crystallizes it will produce a rock ____________ composition

A

If the melt and solid are separated completely the final residual melt will have a very different composition from that at the beginning

Therefore when it finally crystallizes it will produce a rock of a very different composition

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89
Q

what does it mean if the distribution coefficient Kd is less than one?

A

Kd < 1 means the element is incompatible in the mineral (will be enriched in the melt)

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90
Q

what is the bulk distribution coefficient D?

A

Bulk distribution coefficient D is the sum of the individual Kd’s for a given element for all the minerals present in a magma:

Di = ∑Kd<span>i</span>xi (X is the percentage of element in mineral)

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91
Q

what is the bulk distribution D used for?

A

D can be used to model how a trace element changes in concentration in the melt as several minerals crystallize together

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92
Q

in terms of grains where does the melt form?

A

Melt forms at grain boundaries

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93
Q

how does the melt move around the grains?

A

Melt “wets” grains resulting in interconnectivity rather than drops

Melt in pockets and pockets connected by channels

Silicate melts are less dense than rocks of same composition

thus Melt less dense than solid so rises due to buoyency

Need 5% melting to overcome viscosity and surface tension

Cracks (dykes) allow magma to rapidly migrate created due to tectonic deformation

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94
Q

what scales do we need for significant migration through the crust?

A

need km sized bodies (30% melt)

this is due to the loss of heat to surrounding rocks and thus crystallising

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95
Q

give a way in which we can increase buoyancy of a melt?

A

Separation of crystals from melt + fractional crystallisation = melt becomes more felsic = more buoyant

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96
Q

Problem – mafic magma migration

  • Volatile undersaturated basaltic magmas have a density of > 2700 kg/m3
  • Continental crust average ~ 2700kg/m3
  • Basaltic magmas would not be expected to riseEvidence for appreciable underplating of the lower continental crust
  • Quartz- feldspar- rich (crustal) rocks therefore effectively serve as a density filter

So how do basaltic melt ever get to the surface?

A

If it is saturated in volatiles then its density decreases and thus there is a buoyancy force.

Also tectonic activity can compress the magma and thus moves upwards (like toothpaste)

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97
Q

what is underplating?

A

underplating

basaltic magma rises and then gets stuck under the continental crust.

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98
Q

what are diapirs?

A

are bodies of buoyant magma that push slowly through surrounding ductile, highly viscous country rock in the lower crust or mantle

Diapiric rise involves density driven rising of buoyant magma through denser country rock. Its mechanically easier for the magma to penetrate upwards as a thin dike rather than a large blob.

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99
Q

what is meant by assimilation?

A

Melting and incorporation of the melted wall rock into the magma in the pluton = assimilation

Basaltic magma ~12000C so can melt the rocks.

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100
Q

what is stoping?

A

At shallow level where country rocks are brittle, thermal and mechanical stresses associated with pluton emplacement can fracture rocks.

Relatively dense, detached blocks can sink into the deeper parts of the magma chamber.

With long exposure to the magma, xenoliths can partially melt and become disaggregated and so gradually lose their identity.

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101
Q

what are xenoliths?

A

found in igneous rocks

common near roof and walls

plucked from country rock during stoping

recrystallised fabric

partial assimilation can take place

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102
Q

what is meant by plutonic?

A

plutonic:

igneous rocks emplaced at depth below the surface of the earth

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103
Q

what is meant by hypabyssal?

A

hypabyssal:

pertaining to rocks whose type of emplacement is intermediate between plutonic and volcanic

often applied to rocks from minor intrusions, e.g. sills and dykes

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104
Q

what are plutons?

A

intruded magmas that crystallise at depth

coarse grained

exposed as a result of crustal thickening – erosion – isostatic uplift

collision zones, destructive margins

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105
Q

describe deep plutons

A

Mid-crustal (~20km)

Felsic/intermediate composition

Associated with gneisses and schists

Can form by partial melting of metamorphic rocks

Migmatite

Elongate masses of granite aligned with foliation

Granite foliated = granite gneiss

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106
Q

describe intermediate depth plutons

A

~10km

Single large pluton

Maybe associated with regional metamorphism

Edges parallel to foliation in country rock

Country rock hot and deformable

Diaper

Solidify in 105 years

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107
Q

describe shallow plutons

A

<10km

Discordant

Cold, brittle crust

Space created by fractures and faults

Magma moves up structures (cooling to form dykes)

right image

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108
Q

describe how space is made for shallow plutons.

A
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109
Q

describe a composite pluton

A

Many plutons are composite, being formed of more than one type of igneous rock. These may represent different batches of magma, or the same magma frozen at different stages of its evolution.

Using cross cutting relationships and radiometric dating allows us to determine the order of formation.

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110
Q

what are batholiths?

A

Successive pulses of magma

Combination of plutons

>100km2 in surface area

Take a long period of time to form

Batholiths are large, composite plutonic bodies. They can cover tens of thousands of square kilometers and be made of large number of individual plutonic bodies. They form in regions that had large magma supplies for long periods, such as subduction zones or slow continental rifts.

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111
Q

what are laccoliths?

A

Laccoliths are broadly concordant with sedimentary strata

Blister shaped

Sub- horizontal base

Elevated upper surface

especially thick sills that substantially bulge up their roofs

are usually formed from viscous magma emplaced at shallow depths.

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112
Q

what are lopoliths?

A

Lopoliths are large, spoon-shaped igneous rock bodies.

Most are mafic and have prominent igneous layering produced by crystal settling.

The largest known lopolith is the Bushveld complex in South Africa, which is 240km x 480km x 8km thick

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113
Q

what is a dyke?

A

a minor intrusion at shallow depth

Dyke = disconcordant (cut existing structures and strata)Fine or medium grain size

Dykes are tabular, thin intrusive bodies that crosscut layering.

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114
Q

what are sills?

A

minor intrusions - shallow depth

concordant with strata

Sills are thin, tabular intrusives that generally parallel rock layering.

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115
Q

describe the general shape of the magma chambers below volcanoes

A

Magma chambers aren’t round ‘blobs’ under volcanoes

Complex architecture = plumbing system

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116
Q

what controls the shape of magma chambers?

A

controlled by structures in the brittle crust

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117
Q

what are the three types of dyke associated with a volcano?

A

ring dyke

cone sheet

radiating dykes

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118
Q

what is a volcanic neck?

A

solidified volcanic conduit of the volcano

its where the volcano edges have been eroded away leaving the column of harder rock behind.

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119
Q

describe the differences between mafic and felsic dykes. why the difference?

A

mafic - thin, longer dykes due to

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120
Q

where do phenocrysts grow?

A

phenocrysts grow inside the magma

crystals thus have the same chemical composition as the magma

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121
Q

where do xenocrysts grow?

A

single crystal xenocryst broken from the dyke walls

different composition to the magma thus isnt the same as the magma.

xeno = foreign

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122
Q

where do xenoliths come from?

A

xenoliths are polycrystalline and broken from the dykes wall

xeno = foreign

lith = rock

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123
Q

explain why there are differing grain sizes in a dyke

A

grain size variations caused by differential cooling rates at different distances from the margin.

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124
Q

what margins are associated with dykes?

A

chilled margins

baked margins

very limited contact metamorphism

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125
Q

what is the chilled margin?

A

its the margin next to the metamorphosed country rock where the igneous dyke has cooled quickly giving a glass with or without phenocrysts

126
Q

what is pitchstone associated with?

A

associated with chilled margins

where it cools down quickly so is a glass

127
Q

what are radial dykes?

A

radial dykes radiate from a central point

128
Q

what is a parallel dyke?

A

where the dykes become parallel rather than radial.

start for central point

bottom image

129
Q

what is the difference in the creation of radial and parallel dykes?

A

Homogeneous crust = radial dyking
Heterogeneous crust and/or local stress regime = parallel dyking

parallel dykes come from tectonics putting a stress on radial dykes.
Can be local/regional in scale

130
Q

why dont we see parallel dykes on venus?

A

because there are no tectonics on venus we dont see parallel dykes

131
Q

how are ring dykes formed?

A

magma drains from a chamber

subsidence

large scale features

ignimbrite - associated rock

132
Q

describe cone sheets

A

generally thin (few ms thick)

characteristic 450 angle

133
Q

what is the general thickness of sills?

A

100s of metres thick

134
Q

which cools slower sills or dykes?

A

sills cool more slowly than dykes

likely due to being thick and underground

135
Q

what feeds sills?

A

dykes normally feed sills or are extended from intrusions

136
Q

where does cloumnar jointing come from?

A

Joints formed by shrinkage during cooling
•Joints form normal to the cooling surface

137
Q

how does columnar jointing form?

A

Joints formed by shrinkage during cooling

  • Thermal contraction stress exceeds brittle strength = fracture
  • Equidistant nucleation points
138
Q

what is the rate of columnar jointing?

A

it doesnt all happen in one go. it will go down till it reaches hotter material. then when more crystsllization has happened it will form another layer

139
Q

what shapes does columnar jointing form?

A

Regular polygons with 4, 5, 6, or 7 sides

140
Q

what is magma at the surface called?

A

lava

141
Q

what controls the shape of lava?

A

viscosity

142
Q

describe pahoehoe

A
  • “pa-hoey-hoey”
  • Low viscosity
  • 10-100 metres/hour
  • Rate of flow slow enough to allow the surface to chill and produce a solid but pliable crust
  • Lobate or “toey”
  • Chilled crust inflates to form a lobes/toe
  • “Breakouts” of new toes
143
Q

how is ropey pahoehoe form?

A

created due to drag on the underside of crust pleats the surface

144
Q

where does A’a get its name from?

A

due to the sharp edges. the word was created when they walked barefoot. thus when walking on this it would hurt so ahah

145
Q

which is more viscous A’a or pahoehoe?

A

A’a is more viscous than pahoehoe

146
Q

describe the process of creating A’a?

A

Fast flowing + viscous = high strain rate = surface fragmentation
thus Clinkery blocks

147
Q

describe the speeds of A’a

A

unpredictable speeds - the front will cool and stop,
and then a build up behind forces its way through so speed
increases again –> thus is more dangerous than pahoehoe

148
Q

what is the process of creating vesicles and thus amygdales

A

decrease in pressure at surface causes the exsolution of volatiles and thus the creation of bubbles

these bubbles then may be infilled later by secondary minerals such as quartz, calcite, and zeolites (amygdales)

149
Q

what are intermediate flows?

A

Too viscous for pahoehoe
•Surfaces break into slabs or blocks
•Blocky flows
•More viscous = larger blocks
10s of metres thick

150
Q

describe felsic flows

A

Too viscous to flow far
creates Domes

151
Q

what is meant by levee?

A

viscous andesite/ dacite - “toothpaste”

channel develops

central part flows faster than the outer part

walls of solidified

152
Q

what are lava tubes?

A

basaltic

surface of channelised flow solidifies

tube may be drained of lava

lava moves fast like a river

153
Q

describe underwater lava flows?

A

Basalts produce “pillow lavas” (diagnostic of submarine environment)
•Rapid quenching of skin
•Lobes up to 2 m
•Pillow grows when lava pressure ruptures skin, either lengthening the original pillow or “budding” a new pillow
•Spreading centres
•Intermediate and felsic lavas do not tend to form pillows

154
Q

describe pillow lavas

A
155
Q

what is petrogenesis?

A

the study of how and where rocks (particularly igneous rocks) form

156
Q

how much of the earths surface does oceanic crust cover?

A

70% of the earths surface

157
Q

what is the most abundant magmatism on earth?

A

constructive plate margins

158
Q

what do we find at destructive plate margins?

A

island arcs and continental arcs (depend on type of plates used in subduction)

159
Q

what are examples of intraplate volcanism?

A

mantle plumes

flood basalts

oceanic islands (Hawaii)

160
Q

what is a flood basalt?

A

A flood basalt is the result of a giantvolcanic eruption or series of eruptions that coats large stretches of land or the oceanfloor with basalt lava.

161
Q

what is an ophiolite?

A

section of oceanic crust that got sliced off and uplifted to the surface so we can see it

162
Q

what is the evidence of oceanic crust?

A

Geophysics,

grab sampling (dragging across the surface),

drilling

Ophiolite

163
Q

what is meant by obduction?

A

oceanic crust caught between colliding continents

164
Q

which mineral is formed due to the slow cooling of magma at boundary between diverging plates?

A

gabbro

165
Q

what is the differences in composition between basalt, dolerite, gabbro?

A

no difference. they have the same composition just different crystal size.

166
Q

what must be partial melted to form MORBs?

A

partial melting of ultramafic mantle is the only possible source to make a mafic basalt

167
Q

describe the origins of MORBs

A

Partial melting of ultramafic mantle (only possible source)

Diverging plates allow asthenospheric mantle to rise

Decrease in pressure

Anhydrous solidusMelting begins 90 (70-110) km depth

Decompression melting

168
Q

what are the two types of MORBs?

A

Most oceanic ridges produce tholeiitic basalts

Normal or N-MORB

MORB rich in trace elements (e.g. Iceland) = Enriched or E-MORB

169
Q

what are forsterite and fayalite?

A

they are the solid solution of olivine

170
Q

what does this graph show?

A

it shows how MORBs is in equillibrium with the mantle source

171
Q

why does MORB actually have a lower Mg# than is predicted?

A

Few MORB in equilibrium with mantle olivine

Fractional crystallisation and removal of olivine decreases Mg# of melt

Yields cumulates enriched in Mg

172
Q

what is fractional crystallisation?

A

Fractional crystallization is the removal and segregation from a melt of mineral precipitates.

except in special cases, removal of the crystals changes the composition of the magma.

173
Q

what is pyroxene replaced by due to hydrothermal activity at MORs

A

pyroxene is replaced by amphibole

174
Q

what is meant by metasomatism?

A

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids

175
Q

what gives black smokers their colour?

A

very hot reduced material reaches the cold oxidised water so precipitates instantly giving the black colour – sulphides

176
Q

what gives white smokers their colour?

A

White smokers are colder than black smokers and made of sulphates so is white in colour

177
Q

what is the economic importance of black and white smokers?

A

Volcanic-hosted massive sulfide (VHMS) ore deposits (mounds that are economic to mine as they contain Cu, Zn, Pb, Au, Ag

178
Q

what is the rate of pressure increase as you go below sea level?

A

1 atmosphere increase for every 10m below the sea level.

179
Q

what effect does pressure increase have on waters boiling point?

A

increase in pressure –> waters boiling temperature increases

180
Q

where do we find the oldest rocks?

A

We find the oldest rocks where there is no subduction zones and where the plate is very large so takes a long time to reach a subduction zone.

181
Q

what can you say of two ocean floors at similar depth below sea level?

A

Ocean floor of similar age have similar depth below sea level

182
Q

what is meant by the subduction trigger?

A

As plate moves away from ridge, oceanic lithosphere cools, volume decreases, density increases making it easier to subduct

thus sinks into asthenosphere

183
Q

what differences do you see when Oceanic lithosphere subducted beneath oceanic lithosphere and when Oceanic lithosphere is subducted beneath continental lithosphere?

A

oceanic under oceanic - island arcs (e.g. caribbean)

oceanic under continental - continental arcs on edges of continents (e.g. andes)

both will form submarine and subaerial volcanoes

184
Q

what does subduction do to the mantle?

A

it introduces water bearing sediments into the mantle and thus hydrated (altered) oceanic crust

185
Q

what rock types dominate above subduction zones?

A

Intermediate rock types dominate above subduction zones

186
Q

describe the origins of melts.

A

Intermediate rock types dominant above subduction zones

Pressure transmitted instantly but slab warms up slowly

Partial melting of wet oceanic crust and sediments

Water squeezed out from sediments and hydrous minerals are dehydrated by metamorphic reactions

Water moves upwards into “mantle wedge”

Hydrous melting of ultramafic mantle in wedge

If water reaches the base of crust it may generate felsic melts

Modification of melt composition by fractional crystallisation and assimilation

187
Q

what is the impact of hydrous melting?

A

solidus slopes the opposite direction thus increase in pressure leads to partial melting at moderate temperatures

188
Q

what do these graphs show?

A

The first three graphs have no sediment input thus has no 10Be. While the bottom two graphs do have readings due to sediment input.

189
Q

why is Be10 used as a tracer to find the origins of igneous rocks?

A

10Be is used as it is produced in the atmosphere that’s only seen at the surface and is radioactive as it melts it is released and then returns to the surface.

We can then use its half life(1.5 Ma) to calculate when the igneous melting of the rock happened.

thus an igneous rock with some 10Be in it will have been created due to melting of sediments from the surface

190
Q

what is the mantle wedge?

A

it is the corner flow of mantle as it reaches the subducting plate

191
Q

what is roll back?

A

when the slab falls down wards the top of the slab rolls backwards.

192
Q

what type of melts make up hotspots?

are hotspots dependent or independent of plate boundaries?

what is the name given to hotspots that occur in oceans?

what is the name given to hotspots that occur on continents?

A

what type of melts make up hotspots? basaltic melts

are hotspots dependent or independent of plate boundaries? independent

what is the name given to hotspots that occur in oceans? oceanic sea mounts

what is the name given to hotspots that occur on continents? intraplate volcanoes

193
Q

describe mantle plumes

A

Convective upwelling

Decompression melting

Initiate at thermal or density boundary

start from 660km upper/ lower mantle boundary

194
Q

what does the change in direction of hawaii island show?

A

change in direction due to the sollision of two plates and thus the change in the direction of plate.

hawaii arc shows us how plates move due to the stationary hot spot.

195
Q

what does the distance between sea mounts and hawaii show us?

A

age of seamounts proportional to distance from hawaii

rate of plate movement is constant

196
Q

where do flood basalts form?

A

intraplate volcanism

197
Q

what does flood basalts happen before usually?

A

often happen before continental rifting

198
Q

what are the average dimensions of flood basalts?

A

around 1km thick

can be continental in size

-deccan traps, india 1 million km3 in 0.5 million years

199
Q

what is meant by metamorphism

A

Changes in mineralogy and texture of a rock due to (primarily) changes in T and P

200
Q

what is meant by diagenesis?

A

is the change of sediment or existing sedimentary rocks into sedimentary rock during and after rock formation (lithification), at temperatures and pressures less than that required for the formation of metamorphic rocks.

201
Q

what are the boundaries of metamorphism

A

between diagenesis ~200c and partial melting ~800c

202
Q

what effect does increasing temp have on the grade of metamorphism?

A

higher temp= higher grade

203
Q

which is correct?

you have reactions between minerals to make new minerals

you have one mineral changing to other minerals

A

you have reactions between minerals to make new minerals

204
Q

complete the equation muscovite + quartz –>

A

muscovite + quartz –> alkali feldspar + sillimanite + water

205
Q

what is the general change to entropy in metamorphic reactions?

A

entropy increases

206
Q

what is the general change in molar volume in a metamorphic reaction?

A

molar volume generally decreases e.g. graphite –> diamond

207
Q

what generally happens to the amount of volatiles during a metamorphic reaction?

A

usually a loss of volatiles.

e.g. amphibole + calcite + quartz –> clinopyroxene + water + carbon dioxide

208
Q

what is meant by prograde reactions?

A

increasing metamorphic grade (decrease in order)

recrystallisation and new mineral growth removes the intergranular spaces in the rock making the rock impermeable

loss of volatile phases

reaction rate increases exponentially with increasing T

209
Q

what is meant by retrograde reactions?

A

**decreaseing metamorphic grade (increase in order) **

Sometimes you have to add water for lower grade. As it raises to the surface tectonics allow water to react (water is able to circulate).

Textural evidence - high-grade mineral partially replaced by lower grade mineral

Retrograde reactions usually require a fluid phase

210
Q

in the reaction of aragonite to calcite, water acts as a catalyst. how?

A

ions move rapidly in solution compared to solid

211
Q

what does this graph show?

A

it shows that for a small difference in temperature a reaction can differ from 10 years to a billion years

212
Q

why wont high grade metamorphics convert to low grade?

A

Loss of volatiles: unavailable for back reaction

thus Reaction rate limited by slow ionic diffusion

Reaction rates decrease with cooling à as it raises to the surface reactions are very slow

High-grade mineral assemblages “frozen in” and preserved [metagrade]

213
Q

what are the three Al2SiO5 polymorphs?

A

phases differ chemically and /or physically

andalusite, kyanite, sillimanite

214
Q

what is meant by metastability?

A

if the mineral is in a different field but is still there. e.g. kyanite in sillmanite.

thus we can have rocks with more than one mineral

there is only one point where all three can exist

215
Q

what does this show us?

what is it useful for?

A

shows under what conditions they are stable (stability field)

it is useful as the prescence of certain minerals can tell you what the conditions are.

216
Q

what is the petrogenetic grid?

A

is the P, T domains in which mineral assemblages are stable

has the key metamorphic minerals

grid lie pattern on a phase diagram obtained from superimposing different metamorphic reactions

thus E would have kyanite, muscovite and quartz

B would have andulusite, alkalifeldspar and Al2SiO5

217
Q

how does continantal crust generate heat?

A

generates radiogenic heat (K, U, Th are concentrated in the crust)

and the under thrust plate gets heated.

218
Q

what are the major points of continental collision?

A

compressional deformation of the over - riding continental crust

partial subduction of the continental crust on the lower plate

we get crustal thickening (e.g. himalayas 70 - 80km thick)

under thrust plate gets heated

partial melting –> I and S type granites

219
Q

what is the difference between S and I type granites?

A

melting of mud rich sedimentary rocks S type granites

melting of igneous rocks I type granites

220
Q

how can Mg and Fe can change easily between themselves?

A

they can change easily between themselves as they are very similar.

221
Q

what is meant by protolith?

A

means precursor rock

e.g.

sandstone –> quartzite

limestone –> marble

granite –> metagranite

222
Q

what is meant by and describe contact metamorphism?

A

thermal metamorphism from igneous intrusions

we get a metamorphic aureole (< few km)

there is an abscence of tectonic fabric (cleavage, schistosity)

irregular fracture due to mineral growth in random orientations

223
Q

what effects the mineralogy of the metamorphosed rock due to contact metamorphism?

A

composition of the source rock and its position in aureole.

224
Q

what is meant by index minerals?

A

An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith (parent rock), chemical reactions between minerals in the solid state produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. The higher the pressure and temperature in which the rock formed, the higher the grade of the rock.

225
Q

what is meant by hornfels?

A

any hard rock that results from baking (textural term)

226
Q

what are pophyroblasts?

A

larger crystals in a finer grain ground mass.

227
Q

what is meant by granoblastic texture?

A

equigranular texture in which crystals adopt a polygonal morphology with grain triple junctions of approximately 120 degrees. The formation of granoblastic textures occurs to minimise the combined surface energy of phases within a rock.

228
Q

what is the order of metamorphic grades?

A

slate

phyllite

schist

gneiss

229
Q

describe slate

A

fine grained rock with good rock cleavage containg microscopic clay minerals (low grade)

230
Q

describe phyllite

A

fine grained, foliated rock with a distinctive sheen on cleavage surfaces due to fine grained micas (wrinkled appearance)

231
Q

describe a schist

A

medium to coarse grained, foliated rock with mica crystals visible to the eye

232
Q

describe a gneiss

A

coarse grained, foliated (planar fabric) rock with distinctive light and dark banding due to segregation of felsic and mafic minerals.

coarser grain size = higher P,T conditions

does not contain large amounts of mica. (mica tends to react with quartz)

233
Q

describe a augen gneiss

A

special type of gneiss

looks like eyes

feldspar = brittle deformation

quartz and micas = ductile deformation

quartz and mica wrap around feldspar to give eye like structure.

234
Q

describe porphyroblasts and what type of metamorphism they form in.

A

large metamorphic crystals in a fine grained matrix

created in both contact and regional metamorphism

235
Q

what is foliation?

A

foliation is a planar fabric in a metamorphic rock

236
Q

what is the relationship between cleavage and foliation?

A

cleavage is preferential breaking of a rock along a foliation defined by the alignment of platy minerals (micas and clays)

237
Q

what is meant by schistosity?

A

schistosity is foliation in high grade rocks (visible with eye)

breaks along distinct lines that are spread out while cleavage in slates are very close thus can effectively break anywhere on the rock

238
Q

what is meant by crenulation fabrics?

A

its the superposition of a second cleavage

multiple phases of deformation

wavy lines

shown in structral earth

239
Q

what do we see from the recrystallisation of metamorphic rocks?

A

grain size coarsening

absence of foliation or lineation

we get a granular texture (sugary) = granoblastic

240
Q

why do volcanoes erupt in terms of buoyency?

A

magma is more buoyent than th surrounding rock so rises to the surface

241
Q

describe the ways in which overpressure can be caused.

A

violent expansion of dissolved gases from a magma explosion = driven by over pressure

it is significantly and quickly alter physical and or chemical conditions in a magma chamber by adding volatiles, increasing T, and decreases P

this is done by:

1) crystallisation = add volatiles
2) magma + external water = add volatiles
3) magma mixing = increase T
4) magma chamber unroofed = decrease P (by land slide)

often a combination of factors

242
Q

why do volcanoes erupt in terms of adding more volatiles?

A

if we add external water e.g. when volcanoes interact with underground water or

by partial crystallisation of an anhydrous minerals leaves high conc. of water left.

243
Q

what is a volcano?

A

edifice

structure built up from volcanic eruptions

diverse morphologies

244
Q

what % of volcanos are submarine?

A

80%

245
Q

how do volcanoes erupt?

A

through pathway (conduit) for magma to reach the surface due to buoyancy

violent expansion of dissolved gases from magma

explosion driven by overpressure of magma.

246
Q

what is meant by vesiculation and magma fragmentation?

A

the exsolution of dissolved volatiles (mainly water and carbon dioxide)

bubbles nucleate when gas reaches saturation

bubbles grow during ascent

fragmentation - transition from magma being the continuous phase (1-4) to gas being the continuous phase

247
Q

what does this graph show?

A

loss of water increases viscosity transforming the magma into an almost rigid foam

bubbles cannot grow and pressure builds within the bubbles, which eventually burst = magma fragmentation. this drives explosive eruptions

248
Q

describe hawaiian eruptions

A

emission of low viscosity basaltic lava

low level of gas

lava lakes and lava fountains

pahoehoe/ a’a lava

249
Q

describe stromolian eruptions

A

emission of medium low viscosity magma

medium level of gas

intermittent character, but frequent eruptions

lava fountains at regular intervals

scoria, bombs and splatter - cinder cones

<10km smoke height (eruption column)

250
Q

describe strombolian eruptions

A

repeated explosion of medium viscosity lava

medium volatile content

intervals of a few minutes to several hours between eruptions

large bombs falling with ballistic trajectory and high velocity

~20km eruption column

251
Q

what is meant by the eruption column?

A

it is the smoke that is given of from a volcanic eruption.

252
Q

describe sub-plinian/ plinian eruptions

A

violent and large volume

viscous (silicic) magma righ in gas

continuous fragmentation of magma

huge eruption columns (up to 55km)

253
Q

describe using the graphs how the shape of the eruption column is determined.

A

in the jet/ gas thrust phase velocity is able to propell the ash high even though it is more dense at this stage

in the convective phase the ash is less dense than the atmosphere so continues to rise

in the umbrella region the ash is the same density as the atmosphere so it stops raising and thus reaches it max height.

254
Q

whats the difference between convective and a collapsing column?

A

a convecting column is one that rises into the sky

it can ponly be sustained if it is a high enough rate.

when the convecting column becomes too heavy it collapses.

this can cause the deadly pyroclastic flows

255
Q

what does this graph show?

A

not all eruption columns collapse as long as the conditions stay in the convective column region

256
Q

what is meant by slug flow?

A

where bubbles get stuck together in groups

257
Q

what is meant by annular flow? and what sort of eruptions does it cause?

A

annular flow is where the bubbles are in the middle and magma on the outside

this causes eruptions that can be sustained for several years.

258
Q

what are the four types of magma flow?

A

bubbly flow

slug flow

annular flow

dispersed flow

259
Q

what is the difference between bubbly flow and dispersed flow? what kind of eruptions do they occur?

A

bubbly flow - lots of magma, small bubbles - pahoehoe

dispersed flow - little magma mostly bubble - highly explosive plinian eruptions

260
Q

describe pelean eruptions

A

violent and explosive

viscous magma rich in gas

eruption column <25km

dome collapse/ directed blast

almost always generate pyroclastic flows

very dangerous because focussed in one direction

261
Q

describe surtseyan volcanoes

A

magma + external water

typically basaltic

eruption column <20km

emergent seamounts/ submarine

262
Q

describe phreatoplinian volcaones

A

violent and exsplosive

more viscous (silicic) magma than surtsevan eruptions

eruption column >25km

263
Q

describe caldera collapse “supervolcano”

A

huge volume

very rare/ infrequent

yellowstone

where two phreatoplinian volcanos cause the overlying crust to collapse into the emptying chamber

264
Q

what is meant by pyroclast?

A

pyroclast = fragmented material formed by an explosion/ eruption

265
Q

what is meant by a tuff?

A

tuff = pyroclastic material that hardens to form a rock

266
Q

what are the three sizes of tephra?

A

block and bombs 64mm+

lapilli 2 - 64mm

ash <2mm

267
Q

whats the differences between blocks and bombs?

A

blocks = ejected as solid fragments with angular shape

bombs = ejected as incandescent lava fragments, aerodynamic shapes

268
Q

what is scoria?

how does it get its distinctive colour?

A

extremely vesicular basalt (black or red)

red due to iron and the fact that it has a high surface area due to bubbles so oxidises quickly

269
Q

what is a pumice?

A

froth of felsic volcanic glass - it floats on water

270
Q

what are the three mechanisms of transport and dispersion of pyroclastic flows?

A

fall - drops from the sky

pyroclastic flow - high density flow

surge - low density flow

271
Q

how can pyroclastic deposits be complicated?

A

due to having many cycles of eruptions

when erupting it can strip the underlying rock making it complicated to look at.

272
Q

describe the fall deposits of volcanos.

A

graded - average grain size decreases upwards

finest material settles out last and is dispersed furthest from source.

explosive eruptions decrease in intensity as they progress.

273
Q

what influences the deposition of fall deposits?

A

height of column

climatic conditions (wind speed, humidity)

274
Q

what two things can cause pyroclastic flows?

A

caused by collapse of lava dome (block and ash flow)

caused by collapse of eruption column (pumice/ ash flow)

275
Q

describe pyroclastic flows

A

high density mixture of gas, pumice, clasts and/or blocks

move at high speed under the action of gravity

due to being dense they hug the ground

chaotic deposits of pyroclasts

100 m/s

travel for 10s of km

276
Q

what is meant by a block and ash flow?

A

pryroclastic flow in which clasts are dense blocks

deposit does not contain pumice

collapse of an extruded lava dome

deposits are smaller in extent and volume than pumice/ ash flows

277
Q

what is meant by pyroclastic surge deposits

A

less dense than pyroclastic flow because it contains more gas

move close to the ground, very errosive

marked internal stratification with wavy structure and ripples

thin deposits (cms), not often preserved.

278
Q

describe a volcanic tuff/ ignimbrite

A

pyroclastic flow deposit

ash with clasts of pumice, crystals and lithic fragments

residual heat can weld material together

reverse grading - the flow process concentrates the larger clasts towards the top, although onl pumice reaches the top.

279
Q

describe ignimbrite

A

pumice/ ash flow deposit with flattened pumice clasts

welded/ non welded flow textures

very explosive felsic eruptions

thick and laterally extensive deposits

280
Q

describe a lahar

A

volcanic mudflow

ash deposits on steep slopes

melting snow

or tropical storms can mobilise ash and debris.

281
Q

describe micas chemical composition

A

sheet silicate

2 tetrahedral sheets sandwiching either a dioctahedral (Al3+) or a trioctahedral (Mg2+) sheet

282
Q

how does mica get its cleavage?

A

1 in 4 tetrahedra contain Al instead of Si

thus the sheets are negative charge

the sandwich structures are weakly held toghether by positive inter layer ions (K+)

we then get a perfect cleavage

283
Q

what is the difference in chemical composition between muscovite and biotite?

A

•Muscovite = K2Al4Si6Al2O204 = dioctahedral•Biotite = K2Mg6Si6Al2O204 = trioctahedral

284
Q

how is rock cleavage formed?

A

alignment of minerals at right angles to principal stress

285
Q

whats the difference between foliation and lineation?

A

foliation = alignment of platy minerals (mica)

lineation = alignment of elongate minerals (amphibole)

286
Q

what is metasomatism?

A

it is open system metamorphism (addition of elements to the rock)

the composition is modified by fluid (not isochemical)

287
Q

how is serpentinite formed?

A

Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of oceanic crust and underlying mantle.

288
Q

describe how skarns are formed

A
  • Calc-silicate rock derived from metasomatism of limestone
  • formed in Contact aureole
  • Hydrous fluids from intrusion + heated ground water

magmatic fluids interact with surrounding rock (often limestone or mafic igneous rock)

289
Q

describe regional metamorphism

A
  • Large-scale (continental collision)
  • Increase in T and P
  • Linear zones
  • Long duration (10s millions of years)
  • Pelitic rocks (mudstones and shales)
  • Zones characterised by a distinctive mineral
290
Q

why does regional metamorphism have a long period?

A

due to being large its total energy is more so takes a long time to cool by conduction.

291
Q

what are pelites?

A

original mineralogy: quartz, clays

original chemistyr: Si, Al, plus K, Fe, Mg, Ca

292
Q

what are the four Al- rich metamorphic mineral pelites?

A

kyanite, andalusite, sillimanite

steurolite

cordierite

garnet

293
Q

what colur is kyanite, andalusite, sillimanite?

A

blue

294
Q

what is the shape and colour of staurolite?

A

brown cross

295
Q

what colour is garnet? how is it easily seen?

A

usually red.

easily seen because its hard and the surrounding material is weathered away.

296
Q

what is meant by barrovian zones?

A

index minerals identify successive grades of metamorphism

there are six metamorphic zones

some minerals occur in multiple zones, others unique

P, T conditions determined experimentally

297
Q

what are the six barrovian zones?

A

•Chlorite: quartz-chlorite-muscovite-feldspar

•Biotite: quartz-biotite-muscovite-feldspar

  • Garnet: quartz-muscovite-biotite-almandine-feldspar
  • Staurolite: quartz-muscovite-biotite-almandine-staurolite-feldspar
  • Kyanite: quartz-biotite-muscovite-almandine-feldspar-kyanite
  • Sillimanite: quartz-biotite-muscovite-almandine-feldspar-sillimanite
298
Q

what are isograds?

A
  • Contact metamorphism = concentric mineral zones due to T
  • Regional metamorphism = variation in both T and P
  • In both cases points of first appearance of an index mineral can be mapped and linked by a line = isograd = equal metamorphic grade
  • Isograd is named after the index mineral of higher grade
299
Q

when are metamorphic zones not useful?

A

when we get different compositions results in different minerals forming at same P, T.

thus different index minerals at same P, T

when determined from observed mineral assemblages which can differ between rocks of different compositions

•e.g. in the Barrovian biotite zone you would find quartz-biotite-muscovite-feldspar in pelites but calcite-epidote-amphibole in coexisting carbonates; however, both rocks would be in the greenschist facies

300
Q

what does sandstone, limestone and pelites change to during metamorphism?

A

sandstone = quartzite

limestone = marble

pelites = range of minerals

301
Q

what are the 8 metamorphic facies?

A
  • zeolite: deeply buried sediments
  • prehnite-pumpellyite: very low grade regional metamorphism

•greenschist: low-grade regional metamorphism in orogenic areas

•amphibolite: high-grade regional metamorphism

•granulite: very high grade metamorphism / lower crust

  • hornfels: metamorphic aureoles (low P, high T)
  • blueschist: subducted oceanic crust
  • eclogite: subducted oceanic crust
302
Q

what does this graph show?

A

shows the conditions for different metamorphic facies.

303
Q

where would you find eclogite?

A

subducted oceanic crust

304
Q

what is metamorphic mineralogy dependent on?

A

chemical composition of the rock

temp and pressure.

305
Q

what is the general colour of eclogite?

A

red garnet in green pyroxene matrix

306
Q

in the formation of eclogite what does quartz change to?

A

quartz changes to coesite

still SiO2 but now with a compact structure.

307
Q

what gives the textural classification of a metamorphic rock?

A

grain size, foliation and banding

308
Q

how do we name a metamorphic rock?

A

rock name = prefix of porphyroblast and/or foliation mineral + textural type

e.g. garnet schist

309
Q

what are the two most common porphyroblasts?

A

garnet

staurolites (brown cross)

310
Q

which are the dark mafic minerals?

A

olivine

pyroxene

amphibole

biotite

magnetite

titanite

311
Q

define metamorphic facies.

A

A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures

312
Q

describe the igneous processes that occur at mid ocean ridges and the rock types that result.

A

plates pull apart

upwelling of asthenosphere

causing decompression melting.

magma has a basaltic composition

so we get gabbro at depth

then dolerite dykes feeding the magma to the surface where you get basaltic pillow lavas

geophysics (9 decks)

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