A tectonic plate is a moving piece of lithosphere: the crust and the rigid uppermost mantle.
Earth has three main layers, distinguished by composition. The crust is the thin, rocky outside. Beneath it, the much thicker mantle is mainly solid rock. The iron-rich core is at the centre, with a liquid outer core and solid inner core.
The crust and rigid uppermost mantle form the lithosphere. This is broken into tectonic plates. Hot rock beneath it can slowly deform and flow. Over millions of years, solid rock can change shape without becoming a liquid ocean.
Oceanic crust is generally thinner and denser than continental crust. One plate can contain both. A coastline marks where land meets the sea; a plate boundary marks where two plates meet. They need not follow the same line.
Step by step
Crust
A thin outer rock layer. Oceanic and continental crust differ in thickness, composition and density.
Mantle
Mainly solid rock. Its rigid top is part of the plates; hotter rock below flows slowly.
Core
The iron-rich centre has a liquid outer part and solid inner part. Internal heat supports mantle convection.
Worked example: One plate, land and sea
The South American Plate carries the continent and oceanic lithosphere beneath part of the Atlantic. Its eastern boundary is at the Mid-Atlantic Ridge, far from the continent's Atlantic coastline.
Watch out for this
The plates are continents floating on liquid magma.
Plates contain crust and rigid uppermost mantle. They may carry oceans and continents, and the mantle beneath them is mostly solid.
Check your understanding
A moving piece contains oceanic crust, continental crust and rigid uppermost mantle. What is it?
- A continent
- A tectonic plate
- The liquid outer core