(Original post by Strawberrry)
I need help understanding 'plate tectonic theory of crustal evolution':
- tectonic plate
- plate movement
- gravitational sliding
- ridge push
- slab pull
- convection currents
- sea floor spreading
Please could someone explain this all to me? Thanks
Hi Strawberrry, I've covered a lot of these in my degree so far. This is how I would describe/explain them:
The outer layer of the Earth (the lithosphere) is divided into several large slabs called tectonic plates, or lithosphere plates. Lithosphere plates comprise the crust (both oceanic and continental), plus the upper part (the top 100km or so) of the mantle, which is brittle, in contrast to the asthenosphere below, which is 'weak' due to a small degree (approximately 1%) of partial melting. Tectonic plates sit on top of the asthenosphere.
Plate movement refers to the movement of tectonic plates. Tectonic plates move relative to each other, driven by a series of different forces - slab pull, ridge push and trench suction are believed to be the most important forces driving plate movement.
At mid-ocean ridges (large fractures in the oceanic crust through which magma is being released to form new ocean floor), hot buoyant magma rising underneath the ridge pushes the ocean floor upwards, producing a topographic high. Gravitational instability means that the crust naturally wants to sink back down, and as it moves away from the ridge, cools and becomes more dense, it begins to subside towards the mantle - this process is known as gravitational sliding.
Ridge push is one of the forces which is thought to drive plate movement. It works in two ways: 1) plates are pushed apart by the injection of magma at mid-ocean ridges, and 2) gravitational instability, due to the topographic high at the mid-ocean ridge (caused by the hot buoyant material pushing the ridge up), making the plates wanting to move downwards away from the ridge.
Slab pull is another force which drives plate movement. The idea is that the subducting plate is more dense than the surrounding mantle, so it subducts more quickly under its own weight due to gravity.
Convection currents occur in both the mantle and the outer core (due to the outer core of the Earth being liquid), but I'll just describe mantle convection here.
Material at the base of the mantle gets heated by the heat radiating from the core. This hotter material expands and becomes less dense, rising up through the cooler mantle rocks towards the surface. When the hot material reaches the base of the crust, it spreads out laterally, allowing time for the heat to be transferred into the crustal rocks via conduction. As the mantle material cools, it contracts and becomes more dense, then sinks back down towards the outer core before getting heated up again. These convection loops are thought to drive plate tectonics.
Sea floor spreading is where new material is being created at mid-ocean ridges, and the plates are moving apart, away from the mid-ocean ridges.
I hope this helps