Plate Tectonics (AQA A Level Geography)
Revision Note
Written by: Rhiannon Molyneux
Reviewed by: Bridgette Barrett
Theory of Plate Tectonics
The tectonic theory of crustal evolution is a scientific theory that revolutionised people’s understanding and study of geological processes and the Earth
It helps to explain geological phenomena such as:
the occurrence of hazards such as earthquakes and volcanic eruptions
the formation of mountain ranges
the movement of continents
the distribution of some mineral resources such as diamonds
Continental Drift Theory
Examiner Tips and Tricks
When you talk about continental drift theory, it is important to remember that this was the very early stages of the development of modern Plate tectonic theory. Wegener’s theory stated that the continents had moved but was unable to suggest how or why, and his ideas did not link to the occurrence of tectonic hazards. It was only decades later that scientists were able to build on his theory to explain how and why tectonic plates move and how their interaction at plate margins creates distinctive processes and landforms.
Tectonic Plate Movement
The tectonic plates move slowly over the asthenosphere
Scientists agree that the plates move but there is still debate over the mechanisms that cause the movement
Convection Currents
In the past the theory of convection currents was used on its own to explain tectonic plate movement
Heat from radioactive decay in the core moves upwards in the mantle
It creates convection currents which rise towards the surface before spreading in the asthenosphere, cooling and sinking
As they reach the asthenosphere they carry the lithospheric plates above with them
Other processes are now recognised as being important in plate movement
Slab pull and gravitational sliding/ridge push
These processes are now thought to be very significant in driving plate movement
A subduction zone is formed when two plates move towards each other
The heavier, denser oceanic plate subducts under the lighter, less dense plate
As the plate sinks, gravity pulls the plate down into the mantle
This is known as slab pull
New oceanic crust is formed at mid-ocean ridges and stands 2-3km above the ocean floor
As the crust moves away from a mid-ocean ridge, it cools and becomes denser and thicker
This causes the lithosphere to slope away from the ridge and gravity pulls the lithosphere down this slope, pushing it forwards
This is known as gravitational sliding (or ridge push)
Ridge push is much less important than slab pull in causing the movement of tectonic plates
Sea floor spreading
Palaeomagnetism provides evidence that the sea floor has gradually moved apart at a mid-ocean ridge
Lava cools and solidifies with the minerals lining up with the magnetic field
The direction of the minerals on either side is a mirror image
Examiner Tips and Tricks
Make sure you can explain the contribution of each of the main processes which drive the movement of tectonic plates. Remember, it is a combination of all of these factors that cause the plates to move, leading to tectonic hazards.
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