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
2
Q
how can geophysics be used to inform us of the internal earth
A
remote sensing
- seismic
- gravity
- magnetics
- prescence of melt
3
Q
how does minerology provide information of the intenal earth?
A
certain minerals provide insight into pressure and temperature conditions
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
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
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
7
Q
define: igneous rock
A
igneous rock
a rock that has formed by the cooling of a liquid(magma, melt, lava)
8
Q
define: magma
A
magma
a completely or partly molten natural substance that on cooling solidifies as a crystalline or glassy igneous rock
9
Q
define: melt
A
melt
the liquid part of magma with no crystalline phases or inherited solid
10
Q
define: lava
A
lava
a magma that due to volcanic activity is released on to the surface of the earth
11
Q
define: plutonic rock
A
plutonic rock
is formed from magma that crystallised beneath the earths surface
12
Q
what is igneous petrology?
A
igneous petrology
is the study of igneous rocks. it link mineralogy, geochemistry, field work, and physics
13
Q
where is the largest magma production rate?
A
oceanic ridges
14
Q
where is there greater magma production, volcanic or plutonic?
A
plutonic
15
Q
what are the three most common elements in the crust? give general %
A
O - 46.6%
Si - 27.7%
Al - 8.3%
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.
17
Q
what is the building block of silicates?
A
SiO4 group = tetrahedron
18
Q
how are silicate tetrahedra used to make minerals?
A
polymerisation of tetrahedral
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
20
Q
which minerals are single, double chain, and sheet silicates?
A
21
Q
what three things cause melting?
A
heat
pressure changes
water
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.
23
Q
what is the solidus?
A
P -T line where melting begins
24
Q
what is the liquidus?
A
P - T line where melting is complete
25
what is anhydrous melting?
melting without water or no hydrous minerals
26
how is anhydrous melting brought about?
by increase in temperature or decrease in pressue (or both
27
is the composition of the liquid the same or different to the remaing solid in anhydrous melting?
the composition of the liquid is different to that of the remaining solid
28
what type of heat transfer is in the lithospher and the asthenosphere?
lithosphere - conduction
convection - asthenosphere
29
what is adiabatic decompression?
decompression where no heat is lost to the surrounding rock
30
what is hydrous melting?
melting with water or hydrous minerals
31
how does the addition of water change the solidus and liquidus?
moves them to the left and negative gradient
32
where does the anhydrous and hydrous melting curves converge?
anhydrous and hydrous melting converge at low pressure.
33
what is congruent and incongruent melting and which is more common?
congruent - melt produced has same composition as starting material e.g calcite --\> carbonate
incongruent - the opposite
incongruent is the norm
34
the structure of quartz is entirely bulit from SiO4 units. why then is its chemical formula SiO2?
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
35
what is a solid solution? give examples of two important rock forming minerals that show solid solutions.
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)
36
suggest why parial melting of a mafic source rock is nlikely to yield a large body of felsic magma?
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.
37
what are the four properties of magma?
structure
temperature
viscosity
density
38
what are the 5 types of magma?
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)
39
give examples of volatiles
water
carbon dioxide
40
what is the effect of changing pressure on volatiles?
changes in pressure cause volatiles to exsolve (undissolve) making gases
41
what effect does volatiles have rocks?
they break up the silica chains
42
which do volatiles have a greater effect on, felsic or mafic rocks? and why?
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.
43
describe the atomic structure of magma
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.
44
which is more viscous out of felsic and mafic magmas and has the greatest changes? and why?
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.
45
around what is the temperature of erupting magmas?
800oC to 1200oC
46
how can we use temperature to decide the composition of a magma?
Low temperatures = felsic compositions (rhyolite)
High temperatures = mafic compositions (basalt)
47
what is viscosity?
it is the Resistance to flow
Long polymerised chains hinder the flow
thus Felsic magmas more viscous than mafic
Volatiles disrupt polymerisation
48
what are the variables of viscosity?
temperature
time
pressure
49
how can a plume get blocked as magma raises to the surface? what happens if there is too much pressure?
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.
50
what happens as lava flows away from their source or vent?
As lavas flow away from their source or vent, they cool and viscosity increases.
51
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?
Mt Etna is more felsic so it is much more viscous.
Thus it depends on the composition and temperature of the magma.
52
why does viscosity increase as it cools down?
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.
53
describe this graph.
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.
54
describe this graph
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
55
does water have a greater effect on felsic or mafic rocks in terms of viscosity? why?
has a greater effect on felsic rocks as mafic rocks dont have any silica chains to disrupt thus adding water will have no impact,
56
why is viscosity important?
It affects how magmas are transported
It dictates the eruption style of volcanoes
57
what are the 7 textures of volcanic rocks?
**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
58
what is the nucleation centres texture?
Two stage crystallisation
If one mineral nucleates earlier or faster than others it may form large phenocrysts
59
describe the intergranular texture
**Intergranular texture:** when crystal nucleate and grow at the same rate.
60
what is formed when rapid cooling and supersaturation takes place?
Long slender crystals
Crystals that radiate from a common centre
Curved clusters of microlites
Skeletal crystals with hollow cores.
61
describe the porphyritic texture
Phenocrysts
Important for ore deposits
Eruption or volatile exsolution.
62
describe cumulate texture
Crystal settling
Layering
Layered intrusions
Coarse equidimensional crystals
Economic importance – lots of platinum.
63
give the three names to describe cumulate textures.
what do the names depend on?
they depend on the number of small crystals in it produced by the crystallisation of the intercumulus liquid.
left - orthcumulate
middle - mesocumulate
right - adcumulate
64
describe flow textures
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.
65
how can we work out the order of crystallisation?
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)
66
what is the modal classification of felsic and mafic minerals?
Felsic: feldspar and silica = feldspar, feldspathoids, silica (light colour)
Mafic: magnesium and ferrous/ferric (ferromagnesian) = olivine, pyroxene, amphibole, opaques, accessory (dark colour)
67
what is the colour index?
**Colour Index (CI**): volume percentage of mafic or dark coloured minerals
68
for felsic, intermediate, mafic and ultramafic minerals what are the wt% SiO2 boundaries?
felsic \> 65
intermediate - 52 - 65
mafic - 45 - 52
ultramafic \<45
69
for felsic, intermediate, mafic and ultramafic minerals what are the colour index boundaries?
felsic 0 -35
intermediate 35 -65
mafic 65 -90
ultramafic 90 - 100
70
which mineral doesnt form a solid solution?
quartz
71
olivine cant coexist with?
quartz (as olivine is ultramafic and quartz it felsic)
72
earliest formed feldspars are _____________ rich
Earliest formed feldspars are calcium rich.
73
in a QAPF diagram what does QAPF stand for?
Q: quartz (tridymite, cristobalite)
A: alkali feldspars - orthoclase
P: plagioclase
F: feldspathoids or foids
74
before using a QAPF diagram what must we do?
we must minus the mafic proportion and then recalculate
75
when cant we use a QAPF diagram?
we cant use it when the grains are too fine
contains no matrix glass
if CI \> 90
76
what are the wt% of major, minor and trace elements?
major elements \> 1 wt%
minor elements 0.1 - 1.0 wt%
trace elements \< 0.1 wt%
77
is the mineralogy of a rock dependent or independent of cooling rate?
mineralogy is independent of cooling rate e.g. gabbro, dolerite and basalt all have the same mineals and same composition.
78
what is a TAS diagram?
it is a classification of aphanitic and glassy volcanic rocks
79
what does TAS stand for?
TAS = Total Alkalis + Silica
alkalis = Na2O + K2O
80
as a magma cools what happens to the dense crystals?
as it cools the dense crystals leave the system
81
describe the Bowens reaction series
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
82
what is magmatic differentiation?
Differentiation is defined as any process by which magmas evolve and become more diversified
83
what are the two components of magmatic differentiation?
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
84
the compositions of the rock forming minerals are _______ than the melts from which they form
The compositions of the rock-forming minerals are** simpler** than the melts (liquids) from which they form
85
the igneous minerals consist of a relatively small number of ____________ most of which have a ________ concentration in the crystal than in the melt
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.**
86
as a crystal grows in the melt, the elements required by the mineral becomes more _________ in the melt
As a crystal grows in a melt, the elements required by the mineral becomes more **depleted** in the melt
87
Elements that are not required by the mineral are __________ in the diminishing volume or the residual (remaining) melt – these are termed \_\_\_\_\_\_\_\_\_\_\_
Elements that are not required by the mineral are **enriched** in the diminishing volume or the residual (remaining) melt – these are termed **incompatible**
88
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
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
89
what does it mean if the distribution coefficient Kd is less than one?
Kd \< 1 means the element is incompatible in the mineral (will be enriched in the melt)
90
what is the bulk distribution coefficient D?
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 = ∑Kdixi (X is the percentage of element in mineral)
91
what is the bulk distribution D used for?
D can be used to model how a trace element changes in concentration in the melt as several minerals crystallize together
92
in terms of grains where does the melt form?
Melt forms at **grain boundaries**
93
how does the melt move around the grains?
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
94
what scales do we need for significant migration through the crust?
need km sized bodies (30% melt)
this is due to the loss of heat to surrounding rocks and thus crystallising
95
give a way in which we can increase buoyancy of a melt?
Separation of crystals from melt + fractional crystallisation = melt becomes more felsic = more buoyant
96
**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?
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)
97
what is underplating?
**underplating**
basaltic magma rises and then gets stuck under the continental crust.
98
what are diapirs?
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.
99
what is meant by assimilation?
Melting and incorporation of the melted wall rock into the magma in the pluton = assimilation
Basaltic magma ~12000C so can melt the rocks.
100
what is stoping?
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.
101
what are xenoliths?
found in igneous rocks
common near roof and walls
plucked from country rock during stoping
recrystallised fabric
partial assimilation can take place
102
what is meant by plutonic?
**plutonic:**
igneous rocks emplaced at depth below the surface of the earth
103
what is meant by hypabyssal?
**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
104
what are plutons?
intruded magmas that crystallise at depth
coarse grained
exposed as a result of crustal thickening – erosion – isostatic uplift
collision zones, destructive margins
105
describe deep plutons
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
106
describe intermediate depth plutons
~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
107
describe shallow plutons
\<10km
Discordant
Cold, brittle crust
Space created by fractures and faults
Magma moves up structures (cooling to form dykes)
right image
108
describe how space is made for shallow plutons.
109
describe a composite pluton
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.
110
what are batholiths?
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.
111
what are laccoliths?
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.
112
what are lopoliths?
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
113
what is a dyke?
a minor intrusion at shallow depth
## Footnote
Dyke = disconcordant (cut existing structures and strata)Fine or medium grain size
Dykes are tabular, thin intrusive bodies that crosscut layering.
114
what are sills?
minor intrusions - shallow depth
## Footnote
concordant with strata
Sills are thin, tabular intrusives that generally parallel rock layering.
115
describe the general shape of the magma chambers below volcanoes
Magma chambers aren’t round ‘blobs’ under volcanoes
Complex architecture = plumbing system
116
what controls the shape of magma chambers?
controlled by structures in the brittle crust
117
what are the three types of dyke associated with a volcano?
ring dyke
cone sheet
radiating dykes
118
what is a volcanic neck?
solidified volcanic conduit of the volcano
its where the volcano edges have been eroded away leaving the column of harder rock behind.
119
describe the differences between mafic and felsic dykes. why the difference?
mafic - thin, longer dykes due to
120
where do phenocrysts grow?
phenocrysts grow inside the magma
crystals thus have the same chemical composition as the magma
121
where do xenocrysts grow?
single crystal xenocryst broken from the dyke walls
different composition to the magma thus isnt the same as the magma.
xeno = foreign
122
where do xenoliths come from?
xenoliths are polycrystalline and broken from the dykes wall
xeno = foreign
lith = rock
123
explain why there are differing grain sizes in a dyke
grain size variations caused by differential cooling rates at different distances from the margin.
124
what margins are associated with dykes?
chilled margins
baked margins
very limited contact metamorphism
125
what is the chilled margin?
its the margin next to the metamorphosed country rock where the igneous dyke has cooled quickly giving a glass with or without phenocrysts
126
what is pitchstone associated with?
associated with chilled margins
where it cools down quickly so is a glass
127
what are radial dykes?
radial dykes radiate from a central point
128
what is a parallel dyke?
where the dykes become parallel rather than radial.
start for central point
bottom image
129
what is the difference in the creation of radial and parallel dykes?
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
why dont we see parallel dykes on venus?
because there are no tectonics on venus we dont see parallel dykes
131
how are ring dykes formed?
magma drains from a chamber
subsidence
large scale features
ignimbrite - associated rock
132
describe cone sheets
generally thin (few ms thick)
characteristic 450 angle
133
what is the general thickness of sills?
100s of metres thick
134
which cools slower sills or dykes?
sills cool more slowly than dykes
likely due to being thick and underground
135
what feeds sills?
dykes normally feed sills or are extended from intrusions
136
where does cloumnar jointing come from?
Joints formed by shrinkage during cooling
•Joints form normal to the cooling surface
137
how does columnar jointing form?
Joints formed by shrinkage during cooling
* Thermal contraction stress exceeds brittle strength = fracture
* Equidistant nucleation points
138
what is the rate of columnar jointing?
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
what shapes does columnar jointing form?
Regular polygons with 4, 5, 6, or 7 sides
140
what is magma at the surface called?
lava
141
what controls the shape of lava?
viscosity
142
describe pahoehoe
* “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
how is ropey pahoehoe form?
created due to drag on the underside of crust pleats the surface
144
where does A'a get its name from?
due to the sharp edges. the word was created when they walked barefoot. thus when walking on this it would hurt so ahah
145
which is more viscous A'a or pahoehoe?
A'a is more viscous than pahoehoe
146
describe the process of creating A'a?
Fast flowing + viscous = high strain rate = surface fragmentation
thus Clinkery blocks
147
describe the speeds of 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
what is the process of creating vesicles and thus amygdales
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
what are intermediate flows?
Too viscous for pahoehoe
•Surfaces break into slabs or blocks
•Blocky flows
•More viscous = larger blocks
10s of metres thick
150
describe felsic flows
Too viscous to flow far
creates Domes
151
what is meant by levee?
viscous andesite/ dacite - "toothpaste"
channel develops
central part flows faster than the outer part
walls of solidified
152
what are lava tubes?
basaltic
surface of channelised flow solidifies
tube may be drained of lava
lava moves fast like a river
153
describe underwater lava flows?
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
describe pillow lavas
155
what is petrogenesis?
the study of how and where rocks (particularly igneous rocks) form
156
how much of the earths surface does oceanic crust cover?
70% of the earths surface
157
what is the most abundant magmatism on earth?
constructive plate margins
158
what do we find at destructive plate margins?
island arcs and continental arcs (depend on type of plates used in subduction)
159
what are examples of intraplate volcanism?
mantle plumes
flood basalts
oceanic islands (Hawaii)
160
what is a flood basalt?
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
what is an ophiolite?
section of oceanic crust that got sliced off and uplifted to the surface so we can see it
162
what is the evidence of oceanic crust?
Geophysics,
grab sampling (dragging across the surface),
drilling
Ophiolite
163
what is meant by obduction?
oceanic crust caught between colliding continents
164
which mineral is formed due to the slow cooling of magma at boundary between diverging plates?
gabbro
165
what is the differences in composition between basalt, dolerite, gabbro?
no difference. they have the same composition just different crystal size.
166
what must be partial melted to form MORBs?
partial melting of ultramafic mantle is the only possible source to make a mafic basalt
167
describe the origins of MORBs
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
what are the two types of MORBs?
Most oceanic ridges produce tholeiitic basalts
**Normal or N-MORB**
MORB rich in trace elements (e.g. Iceland) = **Enriched or E-MORB**
169
what are forsterite and fayalite?
they are the solid solution of olivine
170
what does this graph show?
it shows how MORBs is in equillibrium with the mantle source
171
why does MORB actually have a lower Mg# than is predicted?
Few MORB in equilibrium with mantle olivine
Fractional crystallisation and removal of olivine decreases Mg# of melt
Yields cumulates enriched in Mg
172
what is fractional crystallisation?
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
what is pyroxene replaced by due to hydrothermal activity at MORs
pyroxene is replaced by amphibole
174
what is meant by metasomatism?
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids
175
what gives black smokers their colour?
very hot reduced material reaches the cold oxidised water so precipitates instantly giving the black colour – sulphides
176
what gives white smokers their colour?
White smokers are colder than black smokers and made of sulphates so is white in colour
177
what is the economic importance of black and white smokers?
Volcanic-hosted massive sulfide (VHMS) ore deposits (mounds that are economic to mine as they contain Cu, Zn, Pb, Au, Ag
178
what is the rate of pressure increase as you go below sea level?
1 atmosphere increase for every 10m below the sea level.
179
what effect does pressure increase have on waters boiling point?
increase in pressure --\> waters boiling temperature increases
180
where do we find the oldest rocks?
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
what can you say of two ocean floors at similar depth below sea level?
Ocean floor of similar age have similar depth below sea level
182
what is meant by the subduction trigger?
As plate moves away from ridge, oceanic lithosphere cools, volume decreases, density increases making it easier to subduct
thus sinks into asthenosphere
183
what differences do you see when Oceanic lithosphere subducted beneath oceanic lithosphere and when Oceanic lithosphere is subducted beneath continental lithosphere?
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
what does subduction do to the mantle?
it introduces water bearing sediments into the mantle and thus hydrated (altered) oceanic crust
185
what rock types dominate above subduction zones?
Intermediate rock types dominate above subduction zones
186
describe the origins of melts.
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
what is the impact of hydrous melting?
solidus slopes the opposite direction thus increase in pressure leads to partial melting at moderate temperatures
188
what do these graphs show?
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
why is Be10 used as a tracer to find the origins of igneous rocks?
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
what is the mantle wedge?
it is the corner flow of mantle as it reaches the subducting plate
191
what is roll back?
when the slab falls down wards the top of the slab rolls backwards.
192
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?
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
describe mantle plumes
Convective upwelling
Decompression melting
Initiate at thermal or density boundary
start from 660km upper/ lower mantle boundary
194
what does the change in direction of hawaii island show?
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
what does the distance between sea mounts and hawaii show us?
age of seamounts proportional to distance from hawaii
rate of plate movement is constant
196
where do flood basalts form?
intraplate volcanism
197
what does flood basalts happen before usually?
often happen before continental rifting
198
what are the average dimensions of flood basalts?
around 1km thick
can be continental in size
-deccan traps, india 1 million km3 in 0.5 million years
199
what is meant by metamorphism
Changes in mineralogy and texture of a rock due to (primarily) changes in T and P
200
what is meant by diagenesis?
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
what are the boundaries of metamorphism
between diagenesis ~200c and partial melting ~800c
202
what effect does increasing temp have on the grade of metamorphism?
higher temp= higher grade
203
which is correct?
you have reactions between minerals to make new minerals
you have one mineral changing to other minerals
you have reactions between minerals to make new minerals
204
complete the equation muscovite + quartz --\>
muscovite + quartz --\> alkali feldspar + sillimanite + water
205
what is the general change to entropy in metamorphic reactions?
entropy increases
206
what is the general change in molar volume in a metamorphic reaction?
molar volume generally decreases e.g. graphite --\> diamond
207
what generally happens to the amount of volatiles during a metamorphic reaction?
usually a loss of volatiles.
e.g. amphibole + calcite + quartz --\> clinopyroxene + water + carbon dioxide
208
what is meant by prograde reactions?
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
what is meant by retrograde reactions?
**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
in the reaction of aragonite to calcite, water acts as a catalyst. how?
ions move rapidly in solution compared to solid
211
what does this graph show?
it shows that for a small difference in temperature a reaction can differ from 10 years to a billion years
212
why wont high grade metamorphics convert to low grade?
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
what are the three Al2SiO5 polymorphs?
phases differ chemically and /or physically
andalusite, kyanite, sillimanite
214
what is meant by metastability?
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
what does this show us?
what is it useful for?
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
what is the petrogenetic grid?
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
how does continantal crust generate heat?
generates radiogenic heat (K, U, Th are concentrated in the crust)
and the under thrust plate gets heated.
218
what are the major points of continental collision?
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
what is the difference between S and I type granites?
melting of mud rich **_s_**edimentary rocks **_S_ type granites**
melting of **_i_**gneous rocks **_I_ type granites**
220
how can Mg and Fe can change easily between themselves?
they can change easily between themselves as they are very similar.
221
what is meant by protolith?
means precursor rock
e.g.
sandstone --\> quartzite
limestone --\> marble
granite --\> metagranite
222
what is meant by and describe contact metamorphism?
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
what effects the mineralogy of the metamorphosed rock due to contact metamorphism?
composition of the source rock and its position in aureole.
224
what is meant by index minerals?
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
what is meant by hornfels?
any hard rock that results from baking (textural term)
226
what are pophyroblasts?
larger crystals in a finer grain ground mass.
227
what is meant by granoblastic texture?
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
what is the order of metamorphic grades?
slate
phyllite
schist
gneiss
229
describe slate
fine grained rock with good rock cleavage containg microscopic clay minerals (low grade)
230
describe phyllite
fine grained, foliated rock with a distinctive sheen on cleavage surfaces due to fine grained micas (wrinkled appearance)
231
describe a schist
medium to coarse grained, foliated rock with mica crystals visible to the eye
232
describe a gneiss
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
describe a augen gneiss
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
describe porphyroblasts and what type of metamorphism they form in.
large metamorphic crystals in a fine grained matrix
created in both contact and regional metamorphism
235
what is foliation?
foliation is a planar fabric in a metamorphic rock
236
what is the relationship between cleavage and foliation?
cleavage is preferential breaking of a rock along a foliation defined by the alignment of platy minerals (micas and clays)
237
what is meant by schistosity?
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
what is meant by crenulation fabrics?
its the superposition of a second cleavage
multiple phases of deformation
wavy lines
shown in structral earth
239
what do we see from the recrystallisation of metamorphic rocks?
grain size coarsening
absence of foliation or lineation
we get a granular texture (sugary) = granoblastic
240
why do volcanoes erupt in terms of buoyency?
magma is more buoyent than th surrounding rock so rises to the surface
241
describe the ways in which overpressure can be caused.
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
why do volcanoes erupt in terms of adding more volatiles?
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
what is a volcano?
edifice
structure built up from volcanic eruptions
diverse morphologies
244
what % of volcanos are submarine?
80%
245
how do volcanoes erupt?
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
what is meant by vesiculation and magma fragmentation?
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
what does this graph show?
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
describe hawaiian eruptions
emission of low viscosity basaltic lava
low level of gas
lava lakes and lava fountains
pahoehoe/ a'a lava
249
describe stromolian eruptions
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
describe strombolian eruptions
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
what is meant by the eruption column?
it is the smoke that is given of from a volcanic eruption.
252
describe sub-plinian/ plinian eruptions
violent and large volume
viscous (silicic) magma righ in gas
continuous fragmentation of magma
huge eruption columns (up to 55km)
253
describe using the graphs how the shape of the eruption column is determined.
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
whats the difference between convective and a collapsing column?
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
what does this graph show?
not all eruption columns collapse as long as the conditions stay in the convective column region
256
what is meant by slug flow?
where bubbles get stuck together in groups
257
what is meant by annular flow? and what sort of eruptions does it cause?
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
what are the four types of magma flow?
bubbly flow
slug flow
annular flow
dispersed flow
259
what is the difference between bubbly flow and dispersed flow? what kind of eruptions do they occur?
bubbly flow - lots of magma, small bubbles - pahoehoe
dispersed flow - little magma mostly bubble - highly explosive plinian eruptions
260
describe pelean eruptions
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
describe surtseyan volcanoes
magma + external water
typically basaltic
eruption column \<20km
emergent seamounts/ submarine
262
describe phreatoplinian volcaones
violent and exsplosive
more viscous (silicic) magma than surtsevan eruptions
eruption column \>25km
263
describe caldera collapse "supervolcano"
huge volume
very rare/ infrequent
yellowstone
where two phreatoplinian volcanos cause the overlying crust to collapse into the emptying chamber
264
what is meant by pyroclast?
pyroclast = fragmented material formed by an explosion/ eruption
265
what is meant by a tuff?
tuff = pyroclastic material that hardens to form a rock
266
what are the three sizes of tephra?
block and bombs 64mm+
lapilli 2 - 64mm
ash \<2mm
267
whats the differences between blocks and bombs?
**blocks** = ejected as solid fragments with **angular shape**
**bombs** = ejected as incandescent lava fragments, **aerodynamic shapes**
268
what is scoria?
how does it get its distinctive colour?
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
what is a pumice?
froth of felsic volcanic glass - it floats on water
270
what are the three mechanisms of transport and dispersion of pyroclastic flows?
**fall** - drops from the sky
**pyroclastic flow** - high density flow
**surge** - low density flow
271
how can pyroclastic deposits be complicated?
due to having many cycles of eruptions
when erupting it can strip the underlying rock making it complicated to look at.
272
describe the fall deposits of volcanos.
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
what influences the deposition of fall deposits?
height of column
climatic conditions (wind speed, humidity)
274
what two things can cause pyroclastic flows?
caused by collapse of lava dome (block and ash flow)
caused by collapse of eruption column (pumice/ ash flow)
275
describe pyroclastic flows
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
what is meant by a block and ash flow?
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
what is meant by pyroclastic surge deposits
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
describe a volcanic tuff/ ignimbrite
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
describe ignimbrite
pumice/ ash flow deposit with flattened pumice clasts
welded/ non welded flow textures
very explosive felsic eruptions
thick and laterally extensive deposits
280
describe a lahar
volcanic mudflow
ash deposits on steep slopes
melting snow
or tropical storms can mobilise ash and debris.
281
describe micas chemical composition
sheet silicate
2 tetrahedral sheets sandwiching either a dioctahedral (Al3+) or a trioctahedral (Mg2+) sheet
282
how does mica get its cleavage?
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
what is the difference in chemical composition between muscovite and biotite?
•Muscovite = K2**Al4**[Si6Al2O20](OH)4 = dioctahedral•Biotite = K2**Mg6**[Si6Al2O20](OH)4 = trioctahedral
284
how is rock cleavage formed?
alignment of minerals at right angles to principal stress
285
whats the difference between foliation and lineation?
foliation = alignment of platy minerals (mica)
lineation = alignment of elongate minerals (amphibole)
286
what is metasomatism?
it is open system metamorphism (addition of elements to the rock)
the composition is modified by fluid (not isochemical)
287
how is serpentinite formed?
Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of oceanic crust and underlying mantle.
288
describe how skarns are formed
* 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
describe regional metamorphism
* 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
why does regional metamorphism have a long period?
due to being large its total energy is more so takes a long time to cool by conduction.
291
what are pelites?
original mineralogy: quartz, clays
original chemistyr: Si, Al, plus K, Fe, Mg, Ca
292
what are the four Al- rich metamorphic mineral pelites?
kyanite, andalusite, sillimanite
steurolite
cordierite
garnet
293
what colur is kyanite, andalusite, sillimanite?
blue
294
what is the shape and colour of staurolite?
brown cross
295
what colour is garnet? how is it easily seen?
usually red.
easily seen because its hard and the surrounding material is weathered away.
296
what is meant by barrovian zones?
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
what are the six barrovian zones?
•**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
what are isograds?
* 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
when are metamorphic zones not useful?
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
what does sandstone, limestone and pelites change to during metamorphism?
sandstone = quartzite
limestone = marble
pelites = range of minerals
301
what are the 8 metamorphic facies?
* **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
what does this graph show?
shows the conditions for different metamorphic facies.
303
where would you find eclogite?
subducted oceanic crust
304
what is metamorphic mineralogy dependent on?
chemical composition of the rock
temp and pressure.
305
what is the general colour of eclogite?
red garnet in green pyroxene matrix
306
in the formation of eclogite what does quartz change to?
quartz changes to coesite
still SiO2 but now with a compact structure.
307
what gives the textural classification of a metamorphic rock?
grain size, foliation and banding
308
how do we name a metamorphic rock?
rock name = prefix of porphyroblast and/or foliation mineral + textural type
e.g. garnet schist
309
what are the two most common porphyroblasts?
garnet
staurolites (brown cross)
310
which are the dark mafic minerals?
olivine
pyroxene
amphibole
biotite
magnetite
titanite
311
define metamorphic facies.
A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures
312
describe the igneous processes that occur at mid ocean ridges and the rock types that result.
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
