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Understanding Earth Sciences > Plate Tectonics > Flashcards

Plate Tectonics Flashcards

1
Q

Adiabatic cooling =

A

Loss of heat through change in pressure due to expansion

  • rises = expands
  • = loses energy
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2
Q

The structure of the Earth

A

Lithosphere - 100km

(Moho = 1300’C isotherm)

Asthenosphere - 350km

Mantle - 2900km

(Gutenberg discontinuity)

Outer core - 5100km

Inner core - 6370km

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3
Q

CORE

A

Iron-nickel

Inner = solid

Outer = liquid

  • important because allows magnetism to take place as liquid needed for convection
  • protects earth from radiation

Formed early due to:
1. Segregation
2. Sinking of metal phase
= transferred GPE to thermal energy

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4
Q

MANTLE

A

Mainly peridotite composition

“Rocky part left over from core segregation”

Mostly Mg/Fe/silicates

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5
Q

CONTINENTAL CRUST

A

Silicates enriched in K/Al/Na

Up to 4 Ga

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6
Q

OCEANIC CRUST

A

Silicates enriched in Ca/Al

Continuously forming from mantle

<200Ma

More dense due to Fe

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7
Q

What element makes up the bulk composition of the Earth?

A

Iron, Fe

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8
Q

Direct methods for determining the earth’s structure

A

DRILLING

XENOLITHS

OPHIOLITES

BASALTS

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9
Q

Direct methods for determining the earth’s structure:

Drilling

A

Reaches max. 15km

Need stable crust

  • earthquakes
  • high pressure water stores

This also happens to be the thickest (-ve)

Oceanic crust is expensive and hazardous

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10
Q

Direct methods for determining the earth’s structure:

Xenoliths

A

Xenolith = inclusions in igneous rock during magma emplacement and eruption

Surface info only

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11
Q

Direct methods for determining the earth’s structure:

Ophiolites

A

Ophiolite = pieces of oceanic plate which have been obducted onto the edge of continental plates

e.g. Can find pieces of peridotite at the surface on the bottom of an ophiolite

Example: Cyprus

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12
Q

Direct methods for determining the earth’s structure:

Basalts

A

Gives an idea of what the mantle is like as they have a similar composition

Only a similar composition to the mantle at the surface

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13
Q

Indirect methods for determining the earth’s structure

A

Seismology

Magnetism

Gravity

Heat flow

Comparison with chondritic meteorites

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14
Q

Meteorites and accretion hypothesis

A

Meteorites originate in asteroid belts
Planets that did not form or are disaggregated
Mostly 4.5 billion years old
- N.B. haven’t been through the rock cycle in the same way as Earth so can accurately date
Fall to Earth

Accretion hypothesis states that the earth should have the same bulk composition as meteorites
Can compare solar/meteorite/earth element abundances:
- fewer with high atomic numbers as they require more fusion/processes
- sun has more H/He bc our gravity is not strong enough to hold them!

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15
Q

Crust vs bulk earth composition

A

Much higher e.g. Fe and and Mg in bulk earth composition

Suggested the earth is DIFFERENTIATED (compositionally layers)

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16
Q

Earthquakes

A

= occur when tension is released from inside the crust

  1. STRESS
    - normal fault, extension
  2. COMPRESSION
    - reverse fault
  3. SHEAR

When this occurs, seismic energy travels outwards as waves and releases heat and sound energy

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17
Q

What does the TYPE of earthquake wave depend on?

A
  1. Material density
  2. Elastic/bulk modulus
    - how particles relate to one another
    - closer = faster (easier to pass energy on)
    - dense = slower (harder…)
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18
Q

Types of waves

A

BODY WAVES
Primary/longitudinal/compressional

Secondary/shear/transverse

  • particles oscillate at right angle to the direction of propagation
  • DO NOT TRAVEL THROUGH LIQUID

PRIMARY WAVES ARE FASTER THAN SECONDARY WAVES

SURFACE WAVES
Rayleigh waves

Love waves

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19
Q

Why do seismic wave velocities vary within the Earth?

A

Change in composition/crystal structure

Presence of fluid/melt

Presence of open fractures

Seismic reflection/REFRACTION

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20
Q

Seismic reflection =

A

Waves bounce off layers; interfaces with changes in property

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21
Q

Seismic refraction =

A

Waves bent as they pass through layers
- determines how velocities change with depth…

  1. Deeper = faster (more dense)
  2. S waves don’t travel through outer core, only the inner (P wave hits and some of the energy splits and becomes a shear wave)
  3. Sudden decrease in primary wave velocity in outer core due to liquid composition

EVIDENCE FOR LAYERS WITHIN THE EARTH

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22
Q

Seismic shadow zones

A

S waves
>/= 103’ from the epicentre

P waves
103-142’ from epicentre due to wave refraction at the core/mantle boundary

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23
Q

Using refraction to study the earth; advantages

A

Fewer source and receiver locations required = less expensive

Little processing needed

Interpretation not complicated

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24
Q

Using refraction to study the earth; disadvantages

A

Large source-receiver distances required

Velocity must increase with depth

Interpretation made in terms of layers

Only uses 1st arrivals

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25
Q

Using reflection to study the earth; advantages

A

Small source-receiver distances

Does not require increasing velocity with depth

Interpretation made in terms of complex geology

Uses entire reflected wave field

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26
Q

Using reflection to study the earth; disadvantages

A

Many source/receiver locations = expensive

Processing is very extensive and requires sophisticated hardware

Interpretation requires more expertise due to this

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27
Q

Uses of gravity

A

GRAVITY - any two objects in the universe exert a gravitational attraction on each other

Detect density changes within the subsurface and infer variations

  • gravity is a force in a direction therefore can be DEFLECTED
  • higher gravity where there are dense bodies of mass

e.g.
Hydrothermal fluid flow
Mineral extraction

Problems…

  • density is not diagnostic
  • gravity also varies with elevation (r) GREATLY therefore need to control height before interpretations can be made
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28
Q

Archimedes principal

A
  1. Volume of water displaced = volume of submerged part of solid
  2. If the weight of water displaced < weight of the object = sink
  3. If that weight of water displaced = weight of the object = float
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29
Q

Applying Archimedes principal to the crust

A

We have a bimodal distribution of elevations

Continental crust is ~1-2km above sea level

Oceanic crust is ~3-4 km below sea lebel

  • oceanic is more dense therefore has to displace more water to float
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30
Q

Pratt’s theory of isostasy

A

FLAT

Density of oceanic crust > density of continental crust

31
Q

Airy’s theory of isostasy

A

Heights vary

Continental crust is underlain by a thick low density ROOT

32
Q

Convection and the rayleigh number

A

Buoyancy forces are able to overcome viscous resistance

Takes place in parts of earth’s mantle

Want a Ra number greater than 10^3:

HIGH THERMAL EXPANSION COEFFICIENT
- expands more = density lowered more = rises

HIGH DENSITY
- will sink

HIGH HEIGHT OF FLUID IN CONVECTING REGION
- rises more for a given expansion coefficient

HIGH CHANGE IN TEMPERATURE
- stronger gradient = more vigorous convection

LOW FLUID VISCOSITY
- viscous fluids resist convection

LOW THERMAL DIFFUSIVITY
- = lowers efficiency of conduction which would inhibit

33
Q

Conduction

A

No material transported, only heat
Material is rigid and does not flow
Occurs in earth’s crust and uppermost mantle

34
Q

Rheology =

A

The study of flow of matter

  • stress/strain properties of minerals
  • soft/strong/weak?

TEMPERATURE IS MAIN CONTROL
- primordial and long term heat sources

35
Q

Compositional layering of the earth

A

Continental/oceanic crust

Upper mantle

Lower mantle

Outer core

Inner core

36
Q

Rheological layering of the earth

A

Lithosphere

Asthenosphere

Mesosphere

Outer core

Inner core

37
Q

Lithosphere: rheology

A

Crust and uppermost mantle

Strong, forms tectonic plates

70km thick beneath oceans

125-250km thick beneath continents

Thickness controlled by 1300’C isotherm
- older = colder = lower isotherm

Conduction

38
Q

Asthenosphere: rheology

A

Remainder of upper mantle

Weak

Low Velocity Zone (LVZ)

Convection

39
Q

What is the issue with conduction throughout the earth?

A

Conduction within the Earth follows a geothermal gradient of approximately 25’C/km

If this occurred throughout the Earth temperatures would be far too high

= CONVECTION

  • adiabatic gradient of 0.3’C/km
  • hot/cold mantle circulated = more stable overall temperature
40
Q

Gibbs free energy =

A

Measure of the chemical energy of the system

  • melting/crystallisation take place to minimise

Low Gibb’s free energy = more stable

Varies with entropy and volume which in turn vary with temperature and pressure

41
Q

Polymorph =

A

Same composition, different density

42
Q

Polymorph example: Olivine

A

At 410km there is a mantle transition from olivine to wadsleyite
(Also at 660km from wadsleyite to ringwoodite)

At high pressures and low temperatures, wadsleyite is more stable

At low pressures and high temperatures, olivine is more stable

= wadsleyite found in subduction zones
= olivine found due to partial melting

43
Q

Phase diagrams; solidus =

A

Curve below which the mineral assemblage is entirely crystalline

44
Q

Phase diagrams; liquidus =

A

Curve above which the mineral assemblage is entirely liquid

45
Q

Causes of partial melting

A
  1. Decompression melting
    - upward movement to lower pressure = enables rock to melt
  2. Fluid-flux melting
    - lowers melting point
  3. Increase the geothermal gradient
    - reaches melting point
46
Q

Processes leading to magmas of different composition

A
  1. Partial melting
  2. Fractional crystallisation
  3. Crustal contamination
47
Q

Equilibrium crystallisation

A

Final composition of system = initial composition of system

E.g. granite cannot form due to equilibrium crystallisation of a basaltic melt

48
Q

Magma diversification; partial melting

A

Upper mantle, in comparison to oceanic crust

  • enriched in Fe
  • depleted in Mg

Olivine (Mg, Fe)2SiO4
= solid solution and continuous variation between two end members

  1. Forsterite Mg2SiO4
  2. Fayalite Fe2SiO4
49
Q

Magma diversification; fractional crystallisation

A

Occurs if crystals are removed from contact with cooling magma e.g. crystal settling within a magma chamber
= layers

A granite CAN form due to fractional crystallisation of a basaltic melt

50
Q

Isostasy =

A

An equilibrium distribution of mass

51
Q

Compensation depth =

A

Imaginary level below the surface where pressures are all the same
Below the compensation depth the density profile is the same

52
Q

Implications and limitations of Airy/Pratt theories of isostasy

A

Both mechanisms of LOCAL isostatic compensation

= imply lithospheric blocks bounded by vertical faults
- faults are actually at angles e.g. normal/reverse/strike-slip

Implies lithosphere has a uniform strength and is weak
- unlikely due to earthquakes; must have some strength to hold stress up to this point

53
Q

How does gravity vary over the Earth’s surface?

A

LATITUDE

  • not a perfect sphere = greater at poles
  • centrifugal force greater at the equator = lower gravity at the equator

ELEVATION
- greater elevation = lower gravity

= weigh less at equator

54
Q

Geoid =

A

Surface of equal gravitational potential

DOES NOT EQUAL THE ELLIPSOID

55
Q

Free air anomaly =

A

Measured gravity anomaly after the Free Air Correction (amount added to get it to reference level) applied to correct for ELEVATION

  • matches topography
  • small = in isostatic equilibrium
56
Q

Bouger anomaly =

A

Corrects for additional mass above/below Mean Sea Level
- positive for high density
N.B. Mountains have low density roots

E.g. negative anomalies in the Scottish Highlands and Cornish Peninsula in Great Britain associated with granite - relatively low density compared with other igneous rocks/highly indurated sedimentary/metamorphic rocks

57
Q

Flexural isostasy =

A

Lithosphere flexes rather than sinks in response to loads

Suggests that lithosphere does have a strength and can, up to a limit, support its own weight

Affected by flexural rigidity

58
Q

Flexural rigidity =

A

Strength of the lithosphere as it flexes

LARGE LITHOSPHERE THICKNESS
- isotherm deeper beneath old, cold cratonic crust

LARGE YOUNG’S MODULUS
- “how stiff”

SMALL POISSONS RATIO
- “if you compress it in one direction, how much does it increase in the other (i.e. play doh analogy)

= large zone of deformation
= decreased depth deformed

59
Q

Continental drift and evidence

A

Wegner 1912 = continental drift

CONTINENTAL FIT
- more convincing 2000m below sea level

FOSSILS
- mesosaurus S America and S Africa

MOUNTAINS

COAL
- in UK/Arctic

POLARITY/PALAEOMAGNETISM

GEOLOGICAL EVIDENCE
- violet quartzite in S Africa and Brazil

60
Q

Sea floor spreading

A

Magnetic “stripes” of alternating polarity every 450,000 years

Parallel and symmetric about ridges

Magnetite = iron-rich

  • contains magnetic domains within that are free to move and line up with current magnetic field when hot
  • cools = remains in place
61
Q

Plate tectonics theory =

A

The Earth’s solid outer crust (lithosphere) is separated into plates that move over the asthenosphere, the molten upper portion of the mantle. The oceanic and continental plates come together, spread apart and interact at boundaries all over the planet

62
Q

Plate motion

A

Plate tectonics theory assumes plates act as rigid stress guides

“The motion of a rigid body on a sphere can be represented as rotation about a Euler pole”

Can use Euler poles to describe plate motion along with speed of rotation

N.B. Means vectors on a flat map may not appear consistent

63
Q

Consequences of intraplate deformation

A

CONTINENTAL lithosphere deforms in a NON-RIGID manner

Continental lithosphere has a variable crust thickness which causes crustal buoyancy forces to come into play

Imagine two columns

A:
- thick continental crust to the compensation depth

B:
- thinner continental crust; air above and then denser mantle below to the compensation depth

Pressure currently greater in A>B
= A wants to spread sideways
- increase area = decrease pressure

= extensional faults e.g. Tibet

64
Q

Wilson Cycle =

A

Cyclical opening/closing of ocean basins caused by the movement of the Earth’s plates

65
Q

Describe the Wilson Cycle

A

Thick continental lithosphere with a deep root = high T and P
= instability

Rifts = ocean basin

Denser oceanic plate subducts = subduction zone

Plates move towards each other over time

Compression

66
Q

Whole vs partial mantle convection

A

Seismic tomography suggests whole mantle convection as subducting slabs penetrate the transition zone BUT

  • low resolution
  • big sample sections
  • not very clear ‘slabs’

The Mantle Transition Zone (MTZ) could be a barrier with the olivine/wadsleyite phase transition BUT

  • Wadsleyite is more stable at high pressure; why would it rise/convect?
  • Olivine is more stable at high temps; why would it fall/convect?
67
Q

Superswells =

A

Regions greater than 1000km in diameter of elevated topography and bathymetry e.g. in Africa and the Pacific

Studies show they are not underlain by warm, low density material in the upper mantle - RULES OUT PRATT THEORY OF ISOSTASY

Instead another dynamic theory: dynamic uplift due to hot upwelling in the lower mantle

68
Q

What are the two plate driving mechanisms?

A

Mantle drag

Edge force

69
Q

Mantle drag

A

Convective motion of the asthenosphere applies drag to the base of the plate

70
Q

Edge force

A

Cold, relatively dense slab sinks into hot asthenosphere, dragging the rest of the plate towards the subduction zone

If a segment of slab breaks loose and sinks = ceases to drag parent plate downwards but produces other forces that ‘suck’ the parent plate downwards

71
Q

How do plate velocities vary with:

  • plate area
  • % plate circumference connected to downgoing slab
  • continental area of the plate
A

No correlation between plate velocity and area

High % plate circumference = high velocities

Greater continental area of plate = lower velocity

THEREFORE EDGE FORCE MECHANISM MORE LIKELY

72
Q

EDGE FORCE: EFFECT ON PLATE VELOCITY:

  • plate area
  • % plate circumference connected to downgoing slab
  • continental area of the plate
A

Depends whether mantle drag assists/impedes motion

High % = greatest force = higher velocity

Continental “root” could reduce velocity if mantle drag resists motion

73
Q

MANTLE DRAG: EFFECT ON PLATE VELOCITY:

  • plate area
  • % plate circumference connected to downgoing slab
  • continental area of the plate
A

Greater plate area = greater velocity

No correlation

Continental “root” would enhance mantle drag = higher velocity

74
Q

Why don’t mountain belts flow?

A

High pressure = want to increase area to reduce pressure BUT rock strength stops the flow

Understanding Earth Sciences (8 decks)

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