1. Intro Flashcards
S waves vs P waves
S waves - shear - solids
P wavees - fluids
Define lithosphere
rigid outer layer (plates)
Define asthenosphere
weaker layer below lithosphere
How has the boundary between the lithosphere and the asthenosphere been defined?
The boundary between them was originally defined thermally (it corresponds to a particular isotherm in the mantle)
Although attempts have been made to locate it using seismic waves, rheology (how the plates move) etc.
In places it lies within the crust, but mostly it lies within the mantle
What two things are evidence for continental drift?
Polar Wander and Seafloor Spreading - magnetic anomalies and frature zones
When was the term ‘‘continental drift’’ first used?
Alfred Wegener in 1912
Explain Polar Wander
- Remnent magnetism
- magnetite+ other mins adopting the direction of earths field at the time the rock cooled below the curie point
- Cores taken and ‘aparent’ location of North Pole worked out
- These are plotted to reveal ‘Apparent Polar Wander Paths’ (APWPs)
- Joining paths together suggests continental drift and varied rates of movement
Explain Seafloor Spreading
- First proposed by Harry Hess in the 1950s (they knew about magnetic field reversals, younger reversals originally known from paleomagnetic measurements on lavas)
- 1958/early 60s - stripy sea floor found by US navy
- In the seafloor spreading process, new oceanic crust is created at mid-ocean ridges by intrusions of dykes and bodies of gabbro, as well as eruption of lavas (Vine, Mathews and Morley hypothesis 1963)
- How far apart the magnetic anomalies are rells us the local spreading rate, and that rate increases the further away from the pole. At the pole in should be 0
- ‘Fracture Zones’ (strike-slip faults that are required to allow spreading in the ridge segments between them) where found running perpendicular to what we now call spreading centres (50s/60s echo sounding from ships, later earthquake evidence)
- Lines drawn perpendicular to these faults join at Euler pole
What initial observations led to the idea of Mantle Hot-Spots and Plumes?
- Pacific islands and seamounts form lines (chains)
- Age increases with distance (+erosion, coral) (no radiometric dating until 70s)
- Volcanoes are active on Hawaii but not others in the chain
What did Wilson (1963) propose about mantle hot-spots and plumes?
Wilson (1963) proposed that island chains were produced by growth of volcanoes over relatively fixed regions of melting in the Earth’s mantle. As the volcanoes grew on plates moving relative to the mantle, they are carried away by that movement, leading to island chains
Define Mantle Plume
ascending bodies of anomalously hot material originating from the deep mantle or core–mantle boundary that are largely independent of the general convective circulation
Define Mantle Hot-Spot
An area on earths surface that exists over a mantle plume
Why are Mantle hot-spots and plumes useful?
We can estimate the movements of the plates relative to earths mantle using dated island and seamount chains. ‘Hotspot reference frame’
How are angle and movement measured usign signal phase?
The method was used very early on to record plate movements using radio telescopes ‘‘very long baseline interferometry’’ or VLBI using quasars radio signals
Short-baseline measurements:
Measure the extra distance ‘‘r’’ travelled by the wave to reach detector B
1. If we know the direction (angle a), we can work out how H is changing (H=r/sin(a))
2. If we know H, we can work out a=arcsin(r/H)
Explain GPS
- Position triangulated from 4 or more satellites
- Distance from satellite calcualted from time taken for signal to reach receiver
- Needs very accurate clocks and positions of satellites
- High accuracies using phase differences between receivers
How have Google and others mapped ocean floors when we have not sent ships everywhere?
Ocean Bathymetry
* Seabed/water interface has a strong density contrast (~1-1.5g/cm3)
* Hence, seabed topography locally affects gravity(both magnitude and direction), distorting ocean surface, which is detected by satellite radar altimetry
* Smith and Sandwell (1997) used the co-variation between free-air gravity (mGal) and depth (m) to work out local variations in bathymetry via Bouger slab formula. Combined with bathymetry data from ships, formed a continuous map of water depth
Explain the Mills Cross principle (multibeam sonar)
- Transmit on along-ship projector transducer array -> beam that is narrow along-track by broad across-track
- Receice on a second array oriented across-ship, producing a beam that is narrow across track but broad along-track. Net effect of ‘‘listening’’ over a strip of the seafloor (ensonified)
- Electronic beam-steering of the received array signals used to create a series of beams at different angles
