[originaltext] (20)Scientists who study the earth tell us that the continen

(20)Scientists who study the earth tell us that the continents and ocean floors are always moving. Sometimes, this movement is violent and might result in great destruction. Today, we examine the process that causes earthquakes. Scientists now know that the surface of the earth is not as permanent as it had been thought. Scientists explain that the surface of our planet is always in motion. Continents move about the earth like huge ships at sea. They float on pieces of the earth’s outer skin, or crust. Only since the 1960s have scientists begun to understand that the earth is a great, living structure.(21)Some experts say this new understanding is one of the most important revolutions in scientific thought. The revolution is based on the work of scientists who study the movement of the continents—a process called plate tectonics(构造学)Earthquakes are a result of that process. Plate tectonics is the area of science that explains why the surface of the earth changes and how those changes cause earthquakes. Tectonic plates can cause earthquakes as they move. Modern instruments show that about 90% of all earthquakes take place along a few lines in several places around the earth.(22)These lines follow underwater mountains, where hot liquid rock flows up from deep inside the planet. Other earthquakes take place at the edges of continents. Pressure increases as two plates move against each other. When this happens, one plate moves past the other, suddenly causing the earth’s surface to split. Earthquakes are among the most frightening events that nature can produce. At times like these, we remember that the ground is not as solid and unchanging as people might like to think.
20. What may lead to earthquakes according to the passage?
21. What do the experts say about this new understanding?
22. Where does the earthquake likely happen?