What this lesson teaches
I can describe Earth's layers and explain how convection and slab pull move tectonic plates.
Syllabus K335, Topic 4.1, Key Question 1. What is the plate tectoni theory?
Make a guess
Is a tectonic plate the same thing as a continent?
- No. Plates are found under the oceans, not the land.
- No. One plate can carry ocean and continent.
- Yes, and plates float on liquid magma.
- Yes. Each continent is one plate with its own edges.
Show the answer
No. One plate can carry ocean and continent.
A plate is a moving piece of the lithosphere, the crust plus the rigid top of the mantle. Its edges are often far from any coastline.
A tectonic plate is a moving piece of Earth's outer shell, made of crust and the rigid top of the mantle.
Earth has three main layers. The crust is the thin, rocky outer layer. Below it is the mantle, a thick layer of mostly solid rock. At the centre is the iron-rich core. Its outer part is liquid, and its inner part is solid.
The crust and the rigid top of the mantle together form the lithosphere. It is broken into pieces called tectonic plates. Below them, hot rock is solid but can flow very slowly, over millions of years. Heat from inside Earth drives this flow, which moves the plates.
There are two kinds of crust. Oceanic crust is thinner and denser. Continental crust is thicker and lighter. One plate can carry both. A coastline is where land meets sea. A plate boundary is where two plates meet. They are often in different places.
- Lithosphere
- Crust plus the rigid top of the mantle.
- Oceanic crust
- Thinner and denser than continental crust.
- Mantle
- Mostly solid rock that flows very slowly.
Step by step
Crust
Thin outer rock layer: oceanic or continental.
Mantle
Mostly solid; its hot lower part flows slowly.
Core
Iron-rich; liquid outside, solid inside. Its heat drives mantle flow.
Real examples to use
Crust and plate thickness (USGS)
Under the oceans the crust is only about 5 km thick. Under the continents it averages about 30 km. The lithosphere, which forms the plates, is at least 80 km thick over much of Earth.
Mantle and core (USGS)
The mantle is about 2,900 km thick. Below it, the liquid outer core is about 2,200 km thick and the solid inner core about 1,250 km thick.
Worked example: One plate, land and sea
The South American Plate carries the continent and part of the Atlantic Ocean floor. Its eastern edge is at the Mid-Atlantic Ridge, far from the continent's coast.
Watch out for this
Plates are continents floating on liquid magma.
Plates include crust and the rigid top of the mantle. They can carry oceans and continents, and the mantle below is mostly solid.
Check your understanding
A moving piece of Earth contains oceanic crust, continental crust and rigid upper mantle. What is it?
- The liquid outer core.
- A continent.
- A tectonic plate.
Show the answer
A tectonic plate.
Yes. Plates can carry both kinds of crust.