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TITLE: Juan de Fuca Ridge

[Photo: Plate Tectonics 101]

Plate tectonics is the concept that Earth's outermost layer is made up of moving, rigid plates that interact dynamically.

Here is our planet drained of water and stripped of vegetation.

  • Its surface is a mosaic of huge plates that move imperceptibly — creating continents, island chains, mountains, spreading ridges, and deep-sea trenches.
  • At plate boundaries, the Earth quakes, volcanoes erupt, and mountains rise.

Over time, plates are born ... and they die.

[Photo: Knife slicing through an apple]

How is the Earth Like an Apple?

They’re both thin-skinned. And their insides look very different from their outsides. But unlike an apple, the Earth is in constant motion and its interior is scorching hot.


What is a Plate?

Earth's plates are huge slabs of rigid rock that:

  • vary from 100-200 km (62-125 mi) thick;
  • are made of the crust and uppermost mantle, which together are called the lithosphere;
  • ride on a layer of hot flowing mantle rock called the asthenosphere.

In this cross section of Earth, you can see how the plates ride on hotter rock below. The plates make up only 2-3 percent of Earth’s radius.


Why Do Plates Move?

Heat escaping from our planet's hot interior to cold outer space causes mantle rocks to flow, contributing to the motions of the overlying plates.


Why Do Plates Move? (Part Two)

Gravity helps move the plates away from high spreading ridges, where they form, to deep-sea trenches, where they are subducted.


How Many Plates Are There?

Right now, there are eight large plates and nine smaller ones.

  • Notice that the major plates include both continents and oceans.
  • The number of plates and their sizes, shapes, and motions have changed throughout Earth's history ... and continue to do so today.
  • Different plates move at different speeds. How fast is the plate you live on moving? In which direction?


What Happens at Plate Boundaries?

redPlates move apart at spreading ridges.

yellowPlates come together at convergent margins.

greenPlates pass by each other at transform faults.


Why Do the Continents Rise Above the Ocean Floor?

Earth's rock factory makes two principal products:

  • basalt, a dense rock that makes up most of the oceanic crust; and
  • granite, a less dense rock that makes up most of the continental crust.

Because continental crust is less dense and thicker than oceanic crust, it "floats" higher on the underlying mantle rock. Notice how high the continental crust rises above the oceanic crust in this cross section of the North American Plate. Oceanic crust is made of dense basalt; continental crust consists of less dense granite.

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits