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.