What Are Plates? (Lectures 5-8) Flashcards
What is the reference surface for gravity observations?
Sea surface
Geoid
What is a geoid?
Equipotential surface
Not horizontal due to non-uniform mass distribution
What is the geoid anomaly?
Difference between height of geoid and theoretical reference surface
Where are geoid anomalies?
Deep in the mantle or core
What does it mean for the geoid anomalies to be very small?
On a planetary scale the Earth resembles a perfect fluid
Continents and oceans are hydrostatically balanced
What were the two theories for isostasy for mountains?
Airy: mountains have roots
Pratt: mountains have lower density than surrounding rocks
Which versions of isostasy apply for continents and oceans?
Airy’s for continents
Pratt’s for oceans
How do oceanic island volcanoes cause gravity anomalies?
Volcano added to cold, strong lithosphere
Lithosphere bends to support the load but not fully compensated
End result of erosion produces a continent at sea level with a crustal thickness of 35km, this means what?
Continental root replaced over time by mantle material, requires mantle flow
Flow takes place by creep as mantle is solid
What is secular variation for the magnetic field?
Non-dipole components of the field migrate on a time scale of decades
Essentially the magnetic pole moves a bit
Magnetism is preserved as natural Remnant Magnetism in which 3 ways?
Thermo-remnant
Chemo-remnant
Depositional remnant
How does thermo remnant magnetism work?
Rock cools from high T through critical Curie point T
Above this, mineral can’t retain a mag field
Below it the field is permanent
How does chemo remnant magnetism work?
Magnetisation acquired by chemical action of phase change during formation of iron oxides at low T
How does depositional remnant magnetism work?
Alignment of magnetised particles in a sediment
What is the evidence the Earth’s mag field is not permanent using remnant magnetism?
Dipole component of mag field reverses roughly every 1 My
Why is secular variation not important over long time frames?
It averages to zero over more than 10000 years
Best fitting average dipole field coincides with what?
What is the equation for this field?
Spin axis
tan(I) = 2 tan(λ)
I is inclination
λ is paleolatitude
What is an apparent polar wander path (APWP)?
How is it obtained?
Change in latitude and orientation with time for a particular continent
Sequence of paleomagnetic poles from rocks of known ages
How can APWPs be interpreted knowing the continents are in relative motion?
(4 points)
Must be similar for continents moving as part of the same mass
If they differ significantly then in relative motion
Can reposition two continents moving relative by superimposing APWPs
APWPs do not give longitudes
What were key discoveries made from bathymetry?
6 points
Most sea floor is flat Mid ocean ridge system Fracture zones offsetting mid ocean ridges Trenches Oldest rocks are 180 Ma Most rocks are basalt
What are observations obtained from magnetometer over the ridge axis?
(3 points)
Symmetrical about ridge axis
Linear anomalies are parallel to ridge axis
Spacing of +ve and -ve anomalies proportional to magnetic reversal timings
Why are magnetic reversals parallel to ridges not equal widths?
Different spreading rates
How is ocean floor created?
Sea floor spreading
What are fracture zones?
Long faults that cut mid ocean ridges
Extend beyond offsets of the ridge axes
What are transform faults?
The active parts of fracture zones
Between offset ridge axes
Earthquakes occur here
If movement on transform faults is horizontal, what are the faults called?
Strike-slip faults
Describe the features of trenches in the ocean
7 points
Long and arcing, on the edge of ocean basins Up to 11 km deep Associated with island arcs Contains andesitic active volcanoes Largest earthquakes, on thrust faults Big gravity anomalies Not in isostatic equilibrium
What is subduction?
Where oceanic lithosphere is returned to the asthenosphere
Occurs at trenches
What are the features of subduction zones?
4 points
Subduct at around 70 mm per year
Cold enough to fail in earthquakes
Deepest earthquakes at 670 km
Andesitic volcanoes usually above where subducting slab is 100 km deep
How do we know that subducting slabs are colder than the surrounding rocks?
P waves travel faster through them
What are the three types of plate boundaries?
Extensional: plates moving apart (spreading ridges)
Convergent: plates move towards each other (subduction zones and mountain building)
Transform: plates move past each other (strike-slip)
Where do earthquakes mostly occur in oceans?
Plate boundaries
‘Horizontal’ motion of the surface of a sphere is equivalent to what?
Where does the axis exit?
Angular rotation about an axis through the sphere
Two Euler poles
The directions of ‘horizontal’ motion about an axis through a sphere follow what?
Small circles
Defining planes that are normal to the rotation axis and don’t have to cut the centre of the sphere
What does it mean for motion on transform faults to be horizontal?
Euler pole describing their motion lies in the direction perpendicular to their strike
If faults bound a rigid plate, what does this mean?
Same Euler pole works for all of the faults
Perpendicular directions to transform faults on same plate boundary converge at the Euler pole
Define rigidity in plate tectonics
Deformation is restricted to plate boundaries and interiors don’t deform at all
What occurs when the plate boundary is drawn in a Mercator projection and using the Euler pole as the projection pole?
Horizontal directions of slip on the plate boundary are parallel to each other and the top and bottom of the map
What observations are made at ridges and for the ocean floor?
(5 points)
No significant gravity anomaly so isostatic equilibrium
Oceanic crust mostly constant thickness so isostasy by density change
Plot of water depth against sea floor age formation, all ridges have the same shape
Depth is proportional to square root of age
Beyond 70 Ma depth flattens out
Which two effects achieve Pratt isostasy for ocean plates?
As new lithosphere moves away from the ridge axis it cools = thermal contraction so it’s denser
Greater water depth away from the ridge
What is the lithosphere-asthenosphere boundary?
An isotherm
Why is the infinite half-space cooling mode for ocean plates half right?
Predicts depth is proportional to square root of age
This is true up to 70 Ma
Older than this is flattens out
What is the plate cooling model for ocean plates?
Sea floor flattens out so lithosphere columns have same density and T
Isotherms are horizontal
What is the asthenosphere temperature?
1300
What is the lithosphere thickness?
125 km
Why would the asthenosphere be at a constant T?
It convects
What is the potential temperature for the asthenosphere?
If the adiabatic gradient were extrapolated to the surface
1300
What is the thermal thickness?
Plate thickness by the change from conduction to convection
Where water depth and isotherms flatten out
What is adiabatic decompression melting?
Material rising follows the adiabatic gradient
Intersects the mantle solidus at 50 km depth and starts to melt
What are the key features of adiabatic decompression melting?
Cools all the way up, only melts by intersecting the solidus
Solidus is onset of melting, only partial melt until liquidus hit
Why does adding water to the mantle i.e. at subduction zones induce melting?
Presence of water reduces the temperature of the solidus
Subducting slab releases water
