The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States. The plates make up Earth's outer shell, called the lithosphere.

Sinking in one place leads to plates moving apart in other places. The movement of the plates and the activity inside the Earth, is called the theory of plate tectonics.

LIZ BONNIN: Hi, I’m Liz Bonnin. When we think about plate tectonics, we normally picture erupting volcanos, gigantic mountain ranges like the Himalayas, or terrifying tsunamis and earthquakes ...