Plate Boundaries

The surface of the Earth is broken into rigid plates. These plates are 100 to 120 kilometers thick and include the crust and a small part of the upper mantle. Many plates contain both continental and oceanic crust. Scientists often refer to this crust/upper mantle layer as the lithosphere. The plates sit on top of a softer, more plastic layer of the mantle called the asthenosphere (from the Greek word "weak").

There are 12 major plates plus a number of minor ones. The plates are named after the regions where they are located. The North American plate, the Pacific plate, and the Caribbean plate are examples.

All of the plates are in constant motion, some moving faster than others. They move in different directions at about the same rate as your fingernails grow, approximately five centimeters each year on average. That might seem slow, but over millions of years, the plates and the continents riding on them move a long way.

Scientists don't fully understand why the plates move. The most accepted theory is that convection currents in the asthenosphere drag them along. Convection currents occur when one part of a fluid is warmer than another part. Think of a pot of water being heated by a single flame at its center. The water at the bottom of the pot in the center heats up and rises to the top. The cooler water on top moves towards the sides of the pot and sinks to the bottom, creating a convection current.

Similarly, columns of hot mantle material deep down in the Earth rise towards the surface. When the mantle material reaches the Earth's thin lithosphere, it moves along the surface away from the column of hot material. It then cools and sinks back down into the deep mantle.

The red is earthquake and hot zones. Notice on the map how most of the
earthquakes and volcanoes occur along narrow bands. These bands correspond
to the boundaries of plates.