plate tectonics

The Earth is always changing. Mountains form and erode away. Volcanoes erupt and grow larger. Earthquakes shift the land. Oceans grow and even continents move. What causes all this activity? The Earth's thin, fragile outer shell is broken into large pieces called "plates." These plates slowly move, floating on a layer of hot mantle below. This movement, or tectonic activity, is constantly changing the surface of the Earth.

Definition: a theory that the Earth's crust and upper part of the mantle is made of large pieces that are moving and changing
When the theory was developed: the 1960s
Number of plates: ten large plates and several smaller plates
Types of plate movement: collision (moving toward each other); slip (moving past each other); spread (moving away from each other); subduction (one plate sliding under another)

Image credits: main image, courtesy of AMNH; Ed Mathez: courtesy of AMNH.