Structure of the Earth Flashcards
What are P and S waves?
- Body waves (travel through the Earth)
- P waves longitudinal, S waves transverse
- S waves cannot travel through liquids
- P waves generally faster than S
- Obey Snell’s law
What is the structure of the Earth, and evidence for it?
- Not a uniform sphere (lower moment of inertia, avg density greater than surface rock density)
Mantle
* Jump in p-wave velocity at depth (Moho) suggests change in composition
* Continental crust 30km, oceanic crust 8km
Core
* S and P-wave shadow zones: No P waves between 103 and 142deg from source, no S waves lower than 103deg
* Suggests liquid core at 2800km (S waves cannot penetrate, P waves refracted at boundary)
Composition
* Carbonaceous chondrites - meteorites close in composition to Solar System forming nebula
* Suggests mantle mostly silicates, core iron with some nickel
* Iron meteorites - cores of planets destroyed by collision
How is absolute time measured?
- Ratio of stable daughter nuclei to radioactive parents (must be closed system)
- Magnetic polarity reversals (simultaneous everywhere)
- Fossil zones
What are the main mechanical properties of cold rocks?
- Elastic
- Only small deformation possible before fracture
What is cataclastic flow?
When fractures reduce the grain size of rocks until they are able to roll/slide (flow) past each other
What is creep and under what conditions does it occur?
- Slow flow of a crystalline solid under a constant load
- Three main mechanisms:
- Movement of dislocations
- Sliding of crystals at grain boundaries
- Recrystallisation (dissolving and regrowing at grain boundaries)
- Two main types:
- Power-law creep (homologous temp > 0.55): High stress needed, dominated by movement of dislocations
- Diffusion creep (hom. temp > 0.85): Low stress, dominated by migration of atoms across grain boundaries
- Homologous temperature = absolute temp/melting temp
Where is the lithosphere/asthenosphere boundary and what are its characteristics?
- About 125km deep
- Lithosphere is strong and brittle, power-law creep in bottom section
- Asthenosphere is weaker and ductile, diffusion creep
- Mechanical boundary (T dependent) - not a sharp divide. No change in composition (peridotite)
Explain the geoid and its anomalies on Earth.
- Geoid - surface on which g is measured and compared to a reference spheroid
- Anomaly: difference between height of geoid and reference surface
- Geoid anomalies are very small so Earth resembles a perfect fluid - hydrostatic balance (isostasy)
- Airy isostasy (continental mountains) - mountains have roots so excess mass on top does not change g significantly
- Pratt isostasy (oceanic mountains) - mountains less dense than surrounding rock
*Anomalies where there is flexure, e.g. Hawaii, volcano on cold lithosphere
*Erosion - isostatic balance maintained, roots replaced by creep
What generates the Earth’s magnetic field and what are its characteristics?
- Arises from convection currents in the conducting Fe outer core
- Dipole field, angle to axis varies with time (secular variation, averages to zero over 10^5 years)
- Field reverses at random intervals, non-dipole components migrate (not a permanent magnet)
How can palaeomagnetism be used to track the past movements of continents?
- Palaeomagnetism preserved in rocks as remnant magnetism
- Thermoremnant (cooling below Curie point), chemoremnant (phase change during iron oxide formation), depositional remnant (alignment of magnetised particles in sediment)
- Direction of poles recorded for rocks of known ages (apparent polar wander path)
- Same APWP, no relative motion, same continent
- Different APWP = relative motion
- If continents were fixed than moved, APWPs can be traced back to when the pattern was the same
What is the seafloor spreading hypothesis and the evidence for it?
- Ocean floor is made from seafloor spreading at mid-ocean ridges
- Oldest sea floor 200Ma (subduction)
Evidence:
* Seafloor is basaltic - created at mid-ocean ridges
* Profile symmetrical about ridge axis
* Spacing of +ve and -ve anomalies proportional to time between magnetic reversals
* Same reversal timescale works for all ridges
What kind of faults are found at trenches?
Thrust faults
How is the shape of the fault maintained?
Flexure - isostatic imbalance
How can Euler poles describe movement of tectonic plates?
- Plate movement equivalent to angular rotation about an axis through the centre of the earth
- The axis exits the Earth’s surface at two points (Euler poles)
- All points on a continent have the same Euler poles
- Shows plates are rigid and only deformation is at plate boundaries
- GPS confirmed rigidity and rate of plate motion
