3.1.1 What is a plate?

In order to understand how and why the lithospheric plates move it is first necessary to understand their physical and thermal structure:

• the Earth can be divided into the core, mantle and crust based on its physical and chemical properties

• the lithosphere comprises the Earth's crust and the upper, brittle part of the mantle.

The thickness of the lithosphere is variable, being up to 120 km thick beneath the oceans; it is considerably thicker beneath ancient continental (cratonic) crust. However, the thermal structure of a plate is best illustrated with reference to the ocean basins and how their thermal characteristics change with time.