To understand continental drift we need to look at the structure of the Earth.
The crust is a thin layer of rock, up to 35km thick, floating on liquid mantle.The crust is thicker on the continents and thinner on the ocean floors. Beneath the crust is the Earth’s mantle. This is comprised of solid rock under extreme heat and pressure. The rock can flow like an extremely thick dough. Further down is the Earth’s core. The core has two regions: the inner core and the outer core. The outer core is molten liquid.The inner core is mainly metals and is solid.
To understand why the continents are drifting we need to realise that they are floating on the mantle. There are huge convection currents of hot rising and cooling mantle.These currents are causing the continents to ‘drift’ on the Earth’s surface.