Upheavals in the Earth
What causes earthquakes?
Most of the time, the surface of the earth is firm and steady. But sometimes the ground shakes and trembles, jarring masses of rocks loose from mountainsides, causing fissures (narrow openings) in the surface, and knocking down buildings in cities and towns.

We know that the thin outer crust of the earth is formed of uneven layers of different kinds of rocks. Thesfe rocks are subject to constant pressures, not Pressures within the earth (indicated by arrows), will cause rock layers to break. only from the rock layers that lie above them, but also from forces within the earth itself. These pressures bend the rocks and cause them to change shape.

If these pressures are great enough, the rock layers may suddenly break - in the same way that a stick will give just so far when you bend it in your hands, and then suddenly snap in two.

When this happens, the rocks break apart and snap back just as the two broken ends of the stick do. This sudden snap jars the earth's crust and causes it to shake. And this movement is called an earthquake.
What causes earthquakes?