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
Explain how GPS is used for tectonics
- For tectonics, need resolution of < 1cm
- This is done by recording over long periods of time (uncertainties reduce ~square root of no. of observations)
- Campaign mode (same place diff. time)
- Continuous recording stations
Explain how InSAR works and how it is used
InSAR: Interferometric Synthetic Aperture Radar
* Satellites on repeating orbits compare phase difference on successive passes on the same location, shows uplift/subsidence
* Ambiguity is resolved by counting phase from areas of no deformation
* closely spaces = greater changes
* Where fringes are absent, this usually implies trees or bushes which move between the two radar scenes
* InSAR interpretatio usually requires numerical modelling
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
What are multibeam sonars?
Multibeam sonars - produce a series of geo-referenced beams which are each narrow fore-aft and across-track
Detect range and bearing to the seabed echo, and echo strength (travel time to the seabed and angle relative to the ship)
Transducers can be mounted permanently on the hull or temporarilly on a pole
GPS & motion sensor must also be on vessel
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
What corrections are need for multibeam sonar?
Corrections needed for real world coordinates:
- angular (roll, pitch, heading)
- position (heave, sway)
* Internal navigation sytems INS (using accelerometers) provides orientation and position data, while GPS provides heading and position
* Dual GPS- heading from phase differences
* GPS and accelerometre data are combined because GPS is very noisy over small timescales and very accurate over long whereas accelerometres are the opposite
* Roll, pitch, yaw and heave data used to adjust soundings for motion
* Variations in sound velocity affect range calculation and cause refraction. In software, correct using a sound-speed profile generated either from a conductivity-temperature-depth (CTD) or sound velocity probe (Snell’s Law)
* Processed data is projected onto the map coordinates, gridded, contoures and used to make 3D images
What are examples of practical applications of multibeam sonars?
Very small systems can be used to image harbour walls or even smaller objects
Systems can be used to check water depths in channels used by shipping
Submarine volcanic ridge around Hawaii showing volcanic cones (academic example)
Much of the fine detail in the oceans on Google Earth
