What are plate tectonics?
Framework for understanding the Earth’s large scale behaviour.
What are some ideas about plate tectonics?
- Earth lithosphere divided into nearly rigid plates
- Oceanic crust can be part of continental crust, but is much denser, so more easily created or destroyed
- Surface area constant: sea floor spreading balanced by consumption of plate material elsewhere
What are 6 pieces of evidence for plate tectonics?
- Jigsaw fit
- Geological evidence (similar rock types across continents
- Palaeontology (similar species)
- Palaeomagnetism (record of magnetic field reversals)
- Seismic activity (earthquake activity shows pattern of stress in crust)
- Volcanism (hot spots indicate mantle plumes)
What are the 3 types of plate boundary?
- Constructive
- Destructive
- Conservative
Describe a constructive plate boundary.
- Plates moving away from eachother (parallel or at angle)
- Creates new lithosphere, usually at mid-ocean ridges
- Magma upwells and cools, creating basaltic crust
What are the two types of spreading for constructive plate boundaries and what is the equation for their relative velocities?
Normal spreading is parallel, whereas oblique spreading is at an angle.
v0 = |B V(A)| = |A V(B)|
Describe a destructive plates boundary.
- Volcanoes and earthquakes occur
- One plate being subducted under another
- Plate material forced into mantle
- Usually dense oceanic crust under less dense continental crust
Describe a conservative plate boundary.
- Plates slide past each other
- No lithosphere created or destroyed
- Many small earthquakes to release stress build up
What do real plate boundaries normally look like?
Segments of constructive boundaries connected to segments of conservative boundaries, as boundaries are not straight lines.
What are plates described as? What can we approximate?
Plates are ‘spherical cap segments’. We can approximate a flat earth ~100km.
What is a triple function?
Where three plate boundaries meet. many different types depending on the 3 types of boundary.
What can we do to calculate the unknown velocity of one of the plate boundaries?
Can use vector addition.
c) VB = (c) VA + (a) VB (use pythagoras to determine third velocity
What can we say plate motion is similar to?
A rotation of the plate about an axis passing through the centre of Earth.
What do we need to fully describe the relative motion between 2 plates?
The latitude and longitude of one pole of the rotation axis, plus it’s rotation vector.
What is the notation for each pole on the rotation axis?
P = positive rotation pole = clockwise, N = negative pole = anticlockwise
What is the equation for the velocity of a specific point?
v = ωR(earth)sin(Δ), where Δ is the angular distance from the pole to the point (centred at earths centre) and ω is the angular velocity of the rotation pole.
What is the standard convention for describing plate velocities?
Half the velocity as is 2 plates moving relatively, so each plate has v/2
How do we find the position of the rotation pole?
Found by constructing perpendicular lines from conservative boundaries.
What are 3 other ways to measure plate velocities?
- Displacement survey (how much a river moves after earthquake)
- GPS (measure change in relative position of GPS ground stations)
- Very long baseline interferometry (measure separation of radio telescopes using astronomical objects)
