Plate Tectonics Flashcards
Adiabatic cooling =
Loss of heat through change in pressure due to expansion
- rises = expands
- = loses energy
The structure of the Earth
Lithosphere - 100km
(Moho = 1300’C isotherm)
Asthenosphere - 350km
Mantle - 2900km
(Gutenberg discontinuity)
Outer core - 5100km
Inner core - 6370km
CORE
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
MANTLE
Mainly peridotite composition
“Rocky part left over from core segregation”
Mostly Mg/Fe/silicates
CONTINENTAL CRUST
Silicates enriched in K/Al/Na
Up to 4 Ga
OCEANIC CRUST
Silicates enriched in Ca/Al
Continuously forming from mantle
<200Ma
More dense due to Fe
What element makes up the bulk composition of the Earth?
Iron, Fe
Direct methods for determining the earth’s structure
DRILLING
XENOLITHS
OPHIOLITES
BASALTS
Direct methods for determining the earth’s structure:
Drilling
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
Direct methods for determining the earth’s structure:
Xenoliths
Xenolith = inclusions in igneous rock during magma emplacement and eruption
Surface info only
Direct methods for determining the earth’s structure:
Ophiolites
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
Direct methods for determining the earth’s structure:
Basalts
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
Indirect methods for determining the earth’s structure
Seismology
Magnetism
Gravity
Heat flow
Comparison with chondritic meteorites
Meteorites and accretion hypothesis
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!
Crust vs bulk earth composition
Much higher e.g. Fe and and Mg in bulk earth composition
Suggested the earth is DIFFERENTIATED (compositionally layers)
Earthquakes
= occur when tension is released from inside the crust
- STRESS
- normal fault, extension - COMPRESSION
- reverse fault - SHEAR
When this occurs, seismic energy travels outwards as waves and releases heat and sound energy
What does the TYPE of earthquake wave depend on?
- Material density
- Elastic/bulk modulus
- how particles relate to one another
- closer = faster (easier to pass energy on)
- dense = slower (harder…)
Types of waves
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
Why do seismic wave velocities vary within the Earth?
Change in composition/crystal structure
Presence of fluid/melt
Presence of open fractures
Seismic reflection/REFRACTION
Seismic reflection =
Waves bounce off layers; interfaces with changes in property
Seismic refraction =
Waves bent as they pass through layers
- determines how velocities change with depth…
- Deeper = faster (more dense)
- 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)
- Sudden decrease in primary wave velocity in outer core due to liquid composition
EVIDENCE FOR LAYERS WITHIN THE EARTH
Seismic shadow zones
S waves
>/= 103’ from the epicentre
P waves
103-142’ from epicentre due to wave refraction at the core/mantle boundary
Using refraction to study the earth; advantages
Fewer source and receiver locations required = less expensive
Little processing needed
Interpretation not complicated
Using refraction to study the earth; disadvantages
Large source-receiver distances required
Velocity must increase with depth
Interpretation made in terms of layers
Only uses 1st arrivals
Using reflection to study the earth; advantages
Small source-receiver distances
Does not require increasing velocity with depth
Interpretation made in terms of complex geology
Uses entire reflected wave field
Using reflection to study the earth; disadvantages
Many source/receiver locations = expensive
Processing is very extensive and requires sophisticated hardware
Interpretation requires more expertise due to this
Uses of gravity
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
Archimedes principal
- Volume of water displaced = volume of submerged part of solid
- If the weight of water displaced < weight of the object = sink
- If that weight of water displaced = weight of the object = float
Applying Archimedes principal to the crust
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
Pratt’s theory of isostasy
FLAT
Density of oceanic crust > density of continental crust
Airy’s theory of isostasy
Heights vary
Continental crust is underlain by a thick low density ROOT
Convection and the rayleigh number
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
Conduction
No material transported, only heat
Material is rigid and does not flow
Occurs in earth’s crust and uppermost mantle
Rheology =
The study of flow of matter
- stress/strain properties of minerals
- soft/strong/weak?
TEMPERATURE IS MAIN CONTROL
- primordial and long term heat sources
Compositional layering of the earth
Continental/oceanic crust
Upper mantle
Lower mantle
Outer core
Inner core
Rheological layering of the earth
Lithosphere
Asthenosphere
Mesosphere
Outer core
Inner core
Lithosphere: rheology
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
Asthenosphere: rheology
Remainder of upper mantle
Weak
Low Velocity Zone (LVZ)
Convection
What is the issue with conduction throughout the earth?
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
Gibbs free energy =
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
Polymorph =
Same composition, different density
Polymorph example: Olivine
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
Phase diagrams; solidus =
Curve below which the mineral assemblage is entirely crystalline
Phase diagrams; liquidus =
Curve above which the mineral assemblage is entirely liquid
Causes of partial melting
- Decompression melting
- upward movement to lower pressure = enables rock to melt - Fluid-flux melting
- lowers melting point - Increase the geothermal gradient
- reaches melting point
Processes leading to magmas of different composition
- Partial melting
- Fractional crystallisation
- Crustal contamination
Equilibrium crystallisation
Final composition of system = initial composition of system
E.g. granite cannot form due to equilibrium crystallisation of a basaltic melt
Magma diversification; partial melting
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
- Forsterite Mg2SiO4
- Fayalite Fe2SiO4
Magma diversification; fractional crystallisation
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
Isostasy =
An equilibrium distribution of mass
Compensation depth =
Imaginary level below the surface where pressures are all the same
Below the compensation depth the density profile is the same
Implications and limitations of Airy/Pratt theories of isostasy
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
How does gravity vary over the Earth’s surface?
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
Geoid =
Surface of equal gravitational potential
DOES NOT EQUAL THE ELLIPSOID
Free air anomaly =
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
Bouger anomaly =
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
Flexural isostasy =
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
Flexural rigidity =
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
Continental drift and evidence
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
Sea floor spreading
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
Plate tectonics theory =
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
Plate motion
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
Consequences of intraplate deformation
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
Wilson Cycle =
Cyclical opening/closing of ocean basins caused by the movement of the Earth’s plates
Describe the Wilson Cycle
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
Whole vs partial mantle convection
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?
Superswells =
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
What are the two plate driving mechanisms?
Mantle drag
Edge force
Mantle drag
Convective motion of the asthenosphere applies drag to the base of the plate
Edge force
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
How do plate velocities vary with:
- plate area
- % plate circumference connected to downgoing slab
- continental area of the plate
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
EDGE FORCE: EFFECT ON PLATE VELOCITY:
- plate area
- % plate circumference connected to downgoing slab
- continental area of the plate
Depends whether mantle drag assists/impedes motion
High % = greatest force = higher velocity
Continental “root” could reduce velocity if mantle drag resists motion
MANTLE DRAG: EFFECT ON PLATE VELOCITY:
- plate area
- % plate circumference connected to downgoing slab
- continental area of the plate
Greater plate area = greater velocity
No correlation
Continental “root” would enhance mantle drag = higher velocity
Why don’t mountain belts flow?
High pressure = want to increase area to reduce pressure BUT rock strength stops the flow
