Introduction
If you've ever stood on the shore of an ice-covered lake in the early spring you would see that the ice covering the lake doesn't just melt and disappear as one huge mass. It is pulled, jostled, and broken into many smaller pieces. It is at the edges of these pieces, where they touch, that most of the grinding and crunching, the creaking and groaning takes place. Water may even spout out from between the pieces to shoot high into the sky like a geyser. The central portions of these ice pieces, however, remain relatively calm and undisturbed.
So it is with the earth's rigid outer layer—the continents and the ocean floors. They don't slide and creep over the earth as one huge monstrous mass. They are like the ice. They are pulled, jostled, and broken into many pieces. And it is where these pieces meet, where they touch, that most of the earth's action takes place. The central, thickest parts of the pieces are like a small Vermont town; they are relatively quiet and undisturbed.
What kind of action occurs here? If you take a world map and plot the locations of modern volcanoes and earthquakes on it, you will see that these are confined to elongated belts or linear chains (like the "ring of fire" that encircles the Pacific Ocean). By running your finger around these belts you can see that they outline oddly shaped pieces of the earth's surface that look like they belong to a giant jigsaw puzzle. What you have outlined are the places where our rigid earth, the lithosphere, is cracked or broken. The action taking place along these breaks is provided, free of charge, by volcanoes and earthquakes (Weisburd, S., 1985).
The oddly shaped pieces you have outlined are of all different sizes and are called plates by earth scientists..............
