Earth’s Interior and Plate Motion
- Earth & Space
- Ages 14–18 · Grades 9–12
What you’ll find out
- Seismic waves reveal Earth’s layers: crust, slowly flowing mantle, liquid outer core and solid inner core.
- Internal heat, partly from radioactive decay, drives mantle convection.
- Plates move by ridge push and, most strongly, slab pull at subduction zones.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Layers of the Earth
No one has ever drilled deeper than about twelve kilometres, so scientists study Earth’s interior indirectly using earthquake waves. These seismic waves travel through the planet. They change speed and direction as they cross boundaries between different materials. One type of wave cannot travel through liquid. Where it disappears tells scientists where the liquid layers are. From such evidence, Earth is known to have four main layers. There is a thin, rocky crust and a thick, hot mantle of solid but slowly flowing rock. Below these are a liquid outer core and a solid inner core, both made mainly of iron and nickel.
What drives the plates
Earth’s rigid outer shell, the lithosphere, is broken into tectonic plates that rest on the hot, slowly flowing mantle beneath. Heat from Earth’s interior drives convection in the mantle. Hotter, less dense rock rises, and cooler, denser rock sinks. This forms vast circulating currents that move over millions of years. Plates are pushed apart at mid-ocean ridges, where new crust forms. At subduction zones, a dense oceanic plate sinks into the mantle. As it sinks, it drags the rest of the plate behind it. Moving a few centimetres a year, plates build mountains, open oceans and shift continents.
Curriculum links: NGSS HS-ESS2-1 · NGSS HS-ESS2-3 · Cambridge IGCSE: Geography · NCERT Geography, Class 11