The Earth's crust is made up of a number of plates that sit on top of the mantle, which is made up of molten rock. The movement of these plates is called plate tectonics. Plate movement can, over time, cause mountains to form, volcanoes to erupt and earthquakes.
One of the most active movers is the Pacific Plate, which stretches from the Pacific Coast of North America to Asia and nearly as far south as Antarctica. It pushes against the Antarctic Plate, the Nazca Plate off South America and the Eurasian Plate. Sometimes known as the "ring of fire," the places where these plates meet are prone to earthquakes and volcanic activity.
One of the best examples of plate tectonics are the Hawaiian Islands. The Pacific Plate has steadily been moving northwest for millions of years. As it passes over the Hawaiian hot spot, a thinner part of the Earth's crust that spews lava, the resulting volcano forms an island. As the plate moves, another island forms.
This Pacific Plate conveyor belt created all the Hawaiian Islands, and it's still moving. Currently the hot spot is spewing lava on the Big Island, adding land to the southeast coast. The plate has moved enough to cause another undersea eruption, and another island. Named Loihi, it will be thousands of years before this island breaks the surface.