Volcanoes







Scientists say the explosions of volcanoes in Japan and the Philippines have similar causes. They do not think the activity is linked. Yet they fear it may be the start of a very destructive geological period.

The volcano in Japan is Mount Unzen. The volcano in the Philippines is Mount Pinatubo. Both are part of a group of volcanoes around the Pacific Ocean known as the “ring of fire.”Up to ninety percent of the world’s active volcanoes are in this group.

Scientists say volcanos are driven by powerful underground forces, These forces are caused by the movement of huge pieces, or plates, of the Earth’s outer skin, the crust. At one edge of the ring of fire, the Pacific Ocean plate is moving under the North American continental plate.

When this happens, the ocean plate drops into the mantel under the crust. The mantel is extremely hot. It also is under a great deal of pressure.

In the mantel, the ocean plate melts. The result is gas and liquid rock. The pressure in the mantel pushes these materials to the Earth’s surface. They come out through an opening, usually a volcanic mountain.

Scientists have become skillful at knowing when a volcano will explode. But they are not yet able to say how long the activity will continue.

Volcano expert Lindsay McClelland works at the Smithsonian Institution in Washington. He says scientists observe volcanoes very closely. To learn if volcanic activity is easing, they look for fewer and smaller explosions.

There is a problem with this method, however, Mister McClelland says. Fewer and smaller explosions may mean that volcanic activity is easing. It also may mean that the volcano is preparing for even greater explsions. Mister McClelland says scientists have several ways to help tell the difference. One way is to measure any earthquake activity linked to the volcanic activity. This shows if liquid rock is still rising to the surface.

Another way is to study changes in the gasses that rise to the surface. A third way is to note any changes in the size of the volcano itself. It liquid rock is moving under it, the volcano will expand.

Even with these methods, experts say, it is still very difficult to know exactly when a volcano will stop exploding and remain quiet.

• 6小时掌握学英语的秘诀！——点击看答案
• 专治男女记不住单词的秘密