Geodynamics Flashcards
1
Q
What is geodynamics?
A
The study of the large scale evolution of the solid earth
2
Q
What is geodynamics?
A
The study of how the Earth operates thermo-mechanically on a large scale
3
Q
What is Earths first order shape?
A
An oblate spheroid
21.38km thicker at equator than the poles (radius) (equatorial bulge)
4
Q
What is earths second order shape?
A
The geoid - the gravitational equipotential surface relative to mean sea level
5
Q
What does gravitational anomalies suggest?
A
That earth is not homogeneous
6
Q
What are geoid anomalies?
A
Large scale variations in earths structure as reflected in density
7
Q
What are equipotential surfaces?
A
Surface that is always perpendicular to the gravitational potential direction
8
Q
What makes a positive and negative geoid anomaly?
A
Density excess= +ve anomaly
Density deficit= -ve anomaly
9
Q
Why is it useful to look at Geoid anomalies?
A
Tells is about earths deep structure
10
Q
How do we know that geoid anomalies are caused by deep earth?
A
Lack of correlation between geoid and the continent ocean transitions
11
Q
What is earths third order shape?
A
Topography - land surface above or below the ocean
Bimodal planet - oceans -4km, continents +8km
Largely caused by tectonics
12
Q
What are the compositional layers of earth?
A
Crust (accessible)
Mantle (sampleable)
Core (inaccessible)
13
Q
What are the mechanical layers of earth?
A
Lithosphere - rigid layer, crust and top mantle
Asthenosphere - very ductile layer, mantle
Lower mantle - moderately ductile, mantle
Outer core - liquid
Inner core - solid
14
Q
What allows the presence of a dynamic earth?
A
Combination of different density and strength is a major control on plate tectonics
(Mechanically layered)
15
Q
What is the crust made out of?
A
Oceanic (basalt)
Continent (andesite)
16
Q
How thick is the crust?
A
30-60km
17
Q
How thick is the mantle?
A
2850km
18
Q
How thick is the core?
A
3480km
19
Q
What does adiabatic mean?
A
A process in which het and mass is conserved.
Temperate increases and decreases as pressure changes.
20
Q
What is the density of the crust?
A
2900kg/m3
21
Q
What is the density of the upper mantle?
A
4400kg/m3
22
Q
What is the density of the outer core?
A
9900kg/m3
23
Q
What is the density of inner core?
A
12800kg/m3
24
Q
What is the density for the average earth?
A
5500kg/m3
25
What are primary controls on geodynamic processes?
Viscosity
26
What is the viscosity for the outer core?
Between water and honey
27
What are plates?
A discrete segment of lithosphere that behaves as a single unit and has a largely coherent vector
Crust and the upper part of the mantle
Oceanic crust or oceanic and continental crust
28
Where does the lithosphere sit?
Floats isostatically on the weak low viscosity layer (asthenosphere)
29
What are the physical attributes of the asthenosphere?
Flows as a solid
Low viscosity
30
What is MOHO?
Mohorovicc discontinuity between crust and mantle due to composition and density differences
31
How does the lithosphere transfer heat?
Conduction - transferred by direct contact between molecules ie spoon getting hot in tea
32
How does the asthenosphere transfer heat?
Convection - transferred by the movement of fluids
33
How does the continental crust behave?
Thicker
Lower part behaves ductile-y
34
What is stronger: oceanic or continental lithosphere?
Oceanic
35
What are the two theories for isostatic equilibrium?
Pratt theory - blocks of same mass, different densities. Explains oceans vs continents
Airy theory- blocks of same density, different masses. Examples continents
36
What makes up a normal isostatic equilibrium?
Continents 35km felsic crust, 65km mantle, 700m above sea level
Oceans 7km mafic crust, 93km mantle, 4.5km below sea level
37
What is abnormal isostatic equilibrium?
Continents 70-80km felsic crust (Himalayas), 20km felsic crust
Oceans 14km mafic crust
38
How many plates are there today?
7-8 major plates
Lots of microplates
39
What are the fastest moving plates?
Australian-Indian plate
Pacific plate
Nazca plate
40
How has the Indian plate changed from 50Ma to today?
Slowed from 200mm/yr to 35mm/yr
41
What are the three types of plate boundaries?
Divergent - plates moving apart, new ocean formation
Convergent - plate collision, subduction or orogeny
Transform - lateral, extension or compression
42
What happens at an ocean-ocean divergent boundary?
Seafloor spreading
43
What happens at continent-continent divergent boundaries?
Rifting
44
What happens at ocean-ocean and ocean-continent convergent boundaries?
Subduction
45
What happens at continent-continent convergent boundaries?
Continental collision
46
What are features at divergent margins?
Mid ocean ridges - oceanic divergence, mantle upwelling, decompression causes melting, basaltic ie Atlantic pacific
Continental rifting -continental divergence forms rift valleys, may evolve to new ocean floors, graben structures, high heat flow, volcanic and geothermal activity, mafic igneous rocks ie East African rift
47
What evidence do we have for seafloor spreading?
Mapping of seafloor mountain ranges and trenches
Magnetic reversals in seafloor - symmetrical stripes either side of the mid ocean ridge. Basalt cools forming magnetite that locks in the ambient magnetic field at that time
48
What is the Curie point?
570 degrees c
49
What does the width and height of magnetic stripes tell us?
Width = rates of spreading
Height = dependant on age of lithosphere and rate of spreading
50
What are the characteristics of transform boundaries?
Associated with ocean spreading
Represent offsets of the mid ocean ridge - can see the offset of magnetic stripes
Have compression or extension on continents
51
Why do MOR transform faults occur?
Spherical planet therefore plates are shallow bowl shaped
Plates rotate around a pole of rotation (POR)
52
What does the pole of rotation mean for spreading rates?
Further from the pole linear spreading rates must be faster
53
Give an example of a continental transform boundary.
San Andreas fault
Alpine Fault, NZ
54
What is transpression?
Compressional force makes narrow linear mountain belts
Ie southern alps, NZ
55
What is transtension?
Extensional force makes narrow linear basins
Ie Death Valley, CA
56
What happens in ocean-ocean subduction?
Older and colder plate subducts under the other
Subducted plate is recycled
Produces deep sea trench’s, adjacent ranges of folded and faulted rock, chains of island arc volcanoes
Ie Indonesian volcanic arc
57
What happens during ocean-continent subduction?
Dense oceanic plate under-rides buoyant continental plate
Causes magmatism and creation of new continental crust
Development of mountain chains
Closes ocean basins
Ie Himalayas , alps
Eventually leads to continent-comtinent collision which makes larger continents
58
What is the Wilson cycle?
Cycle of creation (ocean spreading) and destruction (subduction)
59
What drives plate tectonics?
Slab pull - sinking of old dense ocean lithosphere into the mantle
Ridge push
Plates with largest % of boundary of subduction zones tend to move fastest
60
what are the main consequences of plate tectonics?
metamorphism
61
how can you use metamorphism to fingerprint tectonic processes?
different plate tectonics settings involve different degrees of burial and heating of rocks
62
what is obduction?
where ocean crust is pushed on top of continental crust instead of under it, forming an ophiolite
63
what is hydrothermal metamorphism?
interaction of hot basalt magma with cold magma
64
what are spilites
hydrated basalt
65
what is metasomatism?
metamorphism by fluid changing the rock composition
66
what does hydrated basaltic crust promote?
promotes oceanic slab melting in subduction zones with the forms volcanoes
67
what are black smokers?
underwater vents that are direct evidence and consequence of hydrothermal metamorphism. gives of Au, Ag, Pb, Zn, Cu
VMS ore deposits
68
what does hydration vary with?
depth
can reach all the way through the crust
69
what is earth's geologic water cycle?
water is brought into the crust at MOR's and via volcanoes
water is returned to mantle at subduction zones
mantle is made of anhydrous minerals, still contain trace of water
more water in mantle than seas
70
what are arcs?
way new continental crust is made
driven by magmatism
71
what do ocean-ocean arcs make?
makes island arcs
72
what do ocean-continent arcs make?
continental volcanic arcs and mountains on the continental margins ie Andes
73
where does subduction metamorphism occur?
in the subduction zone and in the arc and back arc environment
74
what causes high pressure-low temperature metamorphism?
ocean subduction zones
75
give examples of metamorphism in subduction
Glaucophane - blue amphibole mineral found in blueschist metamorphic rocks
Omphacite - green pyroxene found in very high pressure metamorphic rocks
76
why does subduction result in volcanism?
water release from metamorphism of oceanic crust
water lowers the melting temperature
77
when is water added to basalts?
typically when ocean crust is first formed and hydrated at the mid ocean ridges (hydrothermal metamorphism
78
why are arcs not made of basalt?
more granitic or andesitic
fractional crystallisation = more felsic melt
partial melting
79
why is subduction driven by slab pull?
formation of eclogite - negatively bouyant
80
where can we see that ocean crust looks like?
obduction zones ie spreading above a subduction zone, sole formation above old crust, final emplacement onto continent
81
what do mountains represent?
where crust/lithosphere has been thickened, sometimes to more than double its normal
82
what are mountains controlled by?
isostacy (iceberg effect)
ie higher the mountain=thicker the crust
83
what happens to the crust when it forms mountains?
interally deforms during continent-continent collision
84
why does the continental crust deform?
weaker than oceanic crust
85
what are orogens?
mountain ranges caused by accretion/collision
86
how do continents grow?
by having arcs added to its exterior
87
name two orogens in todays earth
cordilleran orogen (west coast of americas)(rocky mountains)
alpine-himalayan orogeny (asia mid east europe
88
what formed the cordilleran orogeny?
accretionary orogeny
continent built up by accreting arcs and sediments
mountains at the edge of the continental plate
89
what is the tasmanides?
an ancient accretionary system ~550Ma
australia, new zealand, antarctica
90
what is zealandia?
a lost continent
accreted continent that is now underwater, sank with australia and NZ separated
91
where do we find mountain belts?
in the middle of new combined continents ie Himalayas
linear belts
92
why does the eurasian plate take up most of the deformation caused by the continent-continent collision with india?
more buoyant and weaker
93
why is mountain building asymmetric?
subduction is one sided
different continental plates have different structure and strength
mountains are typically eroded on one side
94
what happens during slab breakoff?
section of slab breaks off, remaining slab becomes more buoyant and is exhumed and thus rises
ie rocks in the alps that have been down to 100km
95
what does blueschist-eclogite record?
the transition from subduction to collision
expected in high dP/dT
96
what does greenschist-amphibolite-granulite record?
main crustal thickening during ongoing collision ie barrow sequence
97
why is there a clockwise P-T path?
fast burial -> burial slowing down ->retrograde -> fast burial
98
what controls the height of mountain belts?
orogenic collapse
erosion
99
what happens in orogenic collapse?
mountains rapidly thickening but eventually become too big to support themselves.
mountain weight overcomes rock strength = normal faulting
deep hot rocks flow out from beneath the mountain = no support
100
why does earth have an imperfect record?
only have a fragmentary history
further from today = less certainty in predictions
101
what are continents defined by?.
defined by todays plate boundaries but were created by ancient plate boundaries
102
what are continents composed of?
big blobs of archean crust (cratons) glued together by narrow belts of younger Proterozoic rocks (old orogens or mobile belts, forming shields
103
what are narrow younger belts dominated by?
deeply eroded metamorphic rocks (old mountain chains/orogens therefore old accretionary/collisional boundaries)
104
what is a suture?
where two continents/arcs are collided/accreted form a larger continent
105
what are models of continental age based on?
dateable rock units i.e. shales, zircons
106
what percentage of exposed crust is archean?
5%
107
what are complexities in dating the crust?
old crust buried by thick sedimentary packages
making vs reworking of rock
108
how do we know when there was continental collision?
metamorphism = collision
zircons = magmatism and metamorphism
not continuous growth = sampling bias
109
what are supercontinents?
discrepancy
involves all or nearly all of the continental masses
making up a significant part of the continental mass
110
give examples of supercontinents
gondwana
pangea
rodina
111
what do we rely on to reconstruct continents?
rock distributions and geomagnetics
112
when was gondwana?
750-550Ma
113
when was pangea?
450-320Ma
114
what do we expect future continents to look like?
atlantic ocean to keep opening, closing of the mediterranean, suturing of asia, africa and america
115
The Tasmanides is an ancient ________ system that formed on the coast of the ancient supercontinent _________.
Accretionary system
Supercontinent Gondwana
116
The Himalayan mountain chain is a ______ tectonic setting that formed when the ______ plate collided with the ________ plate. The oceanic crust of the _______ plate was subducted under the ________ plate prior to collision.
Colllisonal
Australian-Indian
Eurasian
Australian-Indian
Eurasian
117
What is the main force driving subduction?
Slab pull
118
Give examples of island arcs
Indonesia
Japan
119
What layers of the earth are liquid?
Outer core
120
True or false:
The Eurasian plate is entirely composed of continental crust
False
