The volcano is one of the most surprising frightening forces of nature. Maybe you have seen pictures of these “fireworks” of nature. Sometimes when a volcano erupts, a very large wall of melted rock moves down the side of a mountain. It looks like a “river of tire. ”Sometimes volcanoes explode, throwing the melted rock and ashes(灰)high into the air. But where does this melted rock come from? The earth is made up of many layers( 层). The top layer that we see is called the crust. Under the crust are many layers of hard rock. But far, far beneath the crust whose rock is so hot, that it is soft. In some places it even melts. The melted rock is called magma. Sometimes the magma breaks out to the surface through cracks(爆裂声)in the crust. These cracks are volcanoes. Most people think of mountains when they think of volcanoes. But not every mountain is a volcano. A volcano is simply the opening in the earth from which the magma escapes. The hot magma or lava as it is called, cols and builds up on the surface of the earth. Over thousands of years, this pile of cooled lava can grow to be very, very big. For example, the highest mountain in Africa, Kilimanjaro, is a volcano. It towers more than 16,000 feet above the ground around it.