听力题 The earth is composed of several layers. The top layer of the earth is called the earth’s crust. The earth’s crust is divided into seven sections. These sections are called plates. When two plates push against each other, or when two plates rub against each other, an earthquake will occur. An earthquake causes the ground above the plate to shake. The shaking is caused by the motion of two plates. Earthquakes are likely to happen at the boundaries of the earth’s plates. Each of these plates is about 70 kilometers thick. Let’s take a look at the general boundaries of one of these seven major plates. Take the North American plate for an example. The left boundary of this plate is the west coast of North America and the east boundary of this plate is through the middle of the Atlantic Ocean. That is, this plate extends from the west coast of North America to the middle of the Atlantic Ocean.