Dynamic Earth Flashcards
Airy’s model
High mountains are balanced by deep crustal roots. The pressure at a compensation depth below any level of any lateral density variations has to be equal.
E.g. Iceberg
Pratt’s model
A model for the lithosphere that accounts for isostatic anomalies by assuming there is a level of compensation that lies at a constant depth everywhere. Above this, density decreases as topographic elevation increases.
(Mountains underpaid by material of lower average density). Similar thickness, lower density.
E.g. A Ship
Lithospheric plates
Outer cool skin of the earth. They move apart at divergent plate boundaries and together at convergent boundaries.
Plate model
Outer stiff layer floating on soft substrate.
Evidence for soft substrate layer.
POSTGLACIAL UPLIFT. Explained by viscosity.
Moho discontinuity
The seismic discontinuity that marks the boundary between the Earths crust and mantle. The mantle has p wave velocities about 8.1 km/s higher than that of the lower density crystal rocks. ~20km deep.
How do we know that plates move?
Geology of ocean floors:
- magnetic anomaly patterns
- transform faults
Past plate motions
- continental fit
- hotspots
- palaeonagnetism
Active plate tectonics
- earthquakes
- direct measurement
Very long baseline interferometry
Direct measurement using satellites from distant quasars.
GPS
Global positioning satellites
Instant fix - combine satellites providing reference frame
Opening of the Atlantic
Starts with rifting of the continent
Then formation of new basaltic lower SiO(2) ocean crust
Continental margins are where we find most oil and gas
What is going on beneath a mid ocean ridge
Upwelling of not mantle (low in SiO2) with partial melting
Rise of melt towards seafloor
Solidification of melt to form a layered crust ~ 6km
Mantle potential temperature controls
CRUSTAL THICKNESS
Mantle potential temperature
Is the temp of a piece of mantle if we could get it to zero pressure
Hydrothermal circulation
Cools the lithosphere at ridge crests
Removes mg from ocean water, adds Ca
Supplies heat and nutrients to life without photosynthesis
Produces massive sulphide ore deposits in modern and ancient rocks
Hydrates the crust
Importance of a liquid, conducting, rapidly convecting outer core?
produces a magnetic field which is a self exciting dynamo which without we would be exposed to radiation.
