How do tectonic plates move?

The lithosphere (the solid bit underneath continents) acts as the major compenant of tectonic plates. The asthenosphere (80-200km deep) is a mostly solid layer, despite being described often as ‘molten rock’. It is described as this because it acts almost like a fluid, by flowing and plastically deforming.

Radioactive decay from the earths core fuels the heating of the mesosphere, beneath the Asthenosphere. Hotspots from the earth’s core cause areas of lower density to form in the asthenosphere. This density difference causes buoyancy (effectively convection) in the asthenosphere, which upwellings being present at spreading centres, specifically at the Mid Oceanic Ridge and volcanic island chains, such as Hawaii. When the asthenosphere comes in contact with the cooler lithosphere it reduces in density, but it cannot fall as quick as a proper fluid like water or air. Instead it ‘flows’ underneath the lithosphere to an area where it is able to sink back down. Eventually it reaches an area known as a subduction zone. This is where the lithosphere from one tectonic plate is being force down beneath another plate. (An example of a subduction zone is the marianas trench).

Ultimately, it is the frictional shear forces that the asthenosphere applies on the underside of the lithosphere which causes the tectonic plates to move. Moving of the plates is evident in the magnetic banding displayed in the mid oceanic ridge but also in zones of subduction. Earthquakes, and thus tsnunamis are created when the shear forces overcome frictional forces between plates, releasing a build-up of potential energy.

(I would assist with this buy drawing a diagram and getting the student to memorise and label key depths and names)