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.
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.
either move towards each other, move away from each other, or slide past each
Mouse over the arrows located on the plate boundaries to learn what happens
at each type of boundary.