Plate Tectonics & Mantle Convection Flashcards
Evidence for continental drift?
- jigsaw fit, with similar rocks and fossils (e.g. Mesosaurus) on either side
- Palaeomagnetism with mirrored stripes on either side of MAR
- Movement of plates apart from each other by using magnetic inclinations.
What is the Geocentric Axial Dipole Hypothesis?
The hypothesis that while the magnetic field of Earth is always changing due to imperfections in the core, if averaged out over 3000 years or more, it will average at the geographical pole.
Inclinations equations?
tan(inclination) = 2tan(latitude)
3 ways to test tectonic theory?
Slip vectors - measurements at several places around the plate allows the deduction of rotational movement
Transform faults - examples at spreading ridges are moving opposite to their offset, showing spreading.
Marine magnetic anomalies - pole reversals are identical either side of ridges
How can lithospheric thickness be defined?
- Thickness above the low-velocity zone. Oceans are up to 80km, and continents from 100-200km
- The thickness of Earth that transfers heat by conduction. Ocean are ~125km, 100-200km for continents
How does topography over slow and fast spreading ridges differ?
Slow spreading ridges tend to have very rough topography, with the ridge being the low point as fissures moving it down (axial rift). Fast spreading ridges have smooth flanks and the ridge is the high point (axial horst).
What is the Benioff zone?
The Benioff zone is the dipping plane of earthquakes, representing the deformation of the descending plate. The plate remains cool and stiff quite a distance into the upper mantle (up to 680km)
What is the ellipsoid and geoid?
Ellipsoid - a flat surface of theoretical gravity based solely off of latitude.
Geoid - a surface of constant gravitational potential that approximates the mean sea level taking into account the ‘lumpiness’ and density differences of Earth.
How are gravity measurements corrected?
- Latitude comparison - the anomaly is the observed - expected.
- Free Air correction - takes into account the altitude of the measurement as it decreases with height. This is 0.3mGal/m in height. This is added to the latitude anomaly
- Bouguer correction - takes into account the mass of rocks beneath the measurement down to the geoid. For a slab, this is ∆g=2πGρl and this is subtracted from the FAA
What is isostasy and what are the types?
The theory of isostasy holds that variations in surface height are supported by variations in density beneath the surface such that the pressure at some ‘compensation depth’ in the mantle is the same everywhere.
Pratt isostasy assumes the mass of all columns plus the mass of the mantles beneath are equal.
Airy isostasy assumes with different densities, the masses of all columns are equal. Both can be correct in different instances.
What is heat flux?
the flow per unit area per unit time of heat. In 1D, this is q=−kA dT/dz where T is temp, k is thermal conductivity and A is x-sec area. Its unit is Wm-2
What is Rayleigh number?
Rayleigh number =
buoyancy/viscous drag * momentum diffusivity/thermal diffusivity
Ra = gaΔTL^3 / ηk
g - gravitational attraction
a - coefficient of thermal expansion
k - thermal diffusivity
Gentle convection is 1000, with the mantle being 1,000,000.
What is the Preliminary Earth Model?
Shows how seismic waves can change speed.
- Generally, velocity increases since density increases.
- The initial increase is not linear, as phase changes in minerals like olivine occur, as elements like silica change coordination number.
○ Olivine’s silica begins as a 4-CN, then changes to a 6-CN spinel structure. Eventually, it goes to a perovskite structure.
- The s-waves cannot propagate through the liquid outer core.
Equation for plate motion?
v=ωRsinθ
v = absolute velocity
ω = angular velocity
R = Earth radius - 6000km
What is θ in the plate motion equation?
θ = cos-1[sin(λx) sin(λp) + cos(λp) cos(φp-φx)]
λ - latitudes
φ - longitudes
x - for a plate
p - for a pole
