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## American Museum of Natural History

### Course: American Museum of Natural History>Unit 4

Lesson 1: Plate Tectonics

# Plates on the Move

Look around you. It may seem that the Earth is perfectly still. But the Earth's outer shell or surface is actually moving all the time.
The Earth's thin outer shell is broken into big pieces called tectonic plates. These plates fit together like a puzzle, but they're not stuck in one place. They are floating on the Earth's mantle, a really thick layer of hot flowing rock. The flow of the mantle causes the plates to move in different directions. When the edges of plates meet, four things can happen:
Even though plates move very slowly, their motion, called plate tectonics, has a huge impact on the Earth. Plate tectonics form the oceans, continents, and mountains. It also helps us understand why and where events like earthquakes occur and volcanoes erupt.
Imagine you could travel from one point on Earth straight through the center of the planet and out the other side. Your journey would be nearly 12,870 kilometers (8,000 miles).
Along the way, you’d pass through all of Earth’s layers:
Crust
• The rocky surface of the Earth is a thin outer shell, much thinner than the other layers.
• The land that we see, or continental crust, is about 30 kilometers (19 miles) thick. Under the sea, oceanic crust is much thinner (8 to 10 kilometers, or 5 to 6 miles thick), but it's also much heavier.
• The Earth's crust and the top part of the mantle are broken into ten large plates and many smaller ones.
• Most plates are made of both continental and oceanic crust.
Mantle
• The crust floats on a thick layer of rock, almost 100 times thicker than continental crust.
• This layer of rock isn't like the rock we know. Extreme heat makes it move in circles.
• It flows very, very slowly, but it's enough to cause the plates above it to move over long periods of time.
• The plates move about 8 centimeters (3 inches) per year.
Core
• The core is even thicker than the mantle.
• It's made of a liquid metal outer core that flows around a solid metal inner core.
• The motion in the outer core creates a magnetic field around the Earth. It's the same field that makes a compass work!
• The core gives off incredible heat, which is one of the driving forces that causes the mantle to flow.

## Pangaea

About 200 million years ago, all the continents on the Earth were actually one huge "supercontinent" surrounded by one enormous ocean. This gigantic continent, called Pangaea, slowly broke apart and spread out to form the continents we know today.
Sound amazing? Believe it or not, the continents have come together and spread apart at least three times before. After all, our planet is 4.5 billion years old. On that time scale, 200 million years ago isn't such a long time!
What can make the continents move? Plate Tectonics!
Scientists have found many kinds of evidence that support this idea. Here are just a few:
• The shapes of continents fit together like a puzzle. Just look at the east coast of South America and the west coast of Africa—it's almost a perfect fit!
• Identical rocks have been found on different continents. These rocks formed millions of years ago, before the continents separated. They formed from the same minerals and under the same conditions.
• Fossils of the same kinds of dinosaurs, Mesosaurus, have been found in South America and Africa. These dinosaurs roamed the Earth before the two continents broke apart.
All the Earth’s continents were once combined in one supercontinent, Pangaea. Over millions of years, the continents drifted apart. © AMNH
This article comes from OLogy, the American Museum of Natural History’s website for kids.

## Want to join the conversation?

• How big was the super continent?
• really big, about half the earth was the
supercontinent
• how was the earth formed?
• just like how the continents broke apart can the plates brake apart?
• How are the continents connected to the plates? Are the continents floating on top of them or are the continents part of tectonic plates?
• continents are parts of the plates, they are just the parts elevated high enough to be above the ocean
• what is the epicentre of an earthquake?
(1 vote)
• when earthquakes hit land its very deadly and nobody can predict an earthquake