SECTION 4
Good morning. Let's get started! Today we are going to move on to the next chapter-volcano. The first thing we need to know about volcanoes is that they are like a double-edged sword: volcanoes both destroy and create. The catastrophic eruption of Mount St. Helens in 1980 made clear the destructive power of a volcano. However, over a long period of time, even longer than human memory and record, volcanoes have played a key role in forming and modifying the earth. More than 80 percent of our planet's surface is of volcanic origin. Gas emissions from volcanoes formed the earth's earliest oceans and atmosphere. That supplied the essential ingredients to evolve and sustain life.
Ironically, the volcanic soils have attracted people to live on the flanks of volcanoes. So,as more people live in the regions or near the regions of active volcanoes, we should be more aware of the hazards of volcanoes. Probably, you have heard of the story of Mount Vesuvius.The story is set in the year 79 when Vesuvius suddenly exploded and destroyed Pompeii, a Roman city. Within a few hours, hot ash and dust buried the city so thoroughly that its ruins were not uncovered for nearly 1,700 years. The eruption of Mount St. Helens mentioned just now had a huge impact on all the continental states as well as Hawaii and Alaska. So. volcanoes are not that far away from us.
The world volcano comes from a little island called "Vulcano" in the Mediterranean Sea off Sicily.In ancient times,people believed that Vulcano was the chimney of Vulcan,who is the blacksmith of Roman gods. He made iron stuff for Jupiter and Mars and made Vulcan as the outlet of his hot lava fragments and clouds of dust . Today know that volcano eruptions are not super natural but can studied and interpreted by scientists.
Probably you wonder what volcanoes are.Volcanoes are very different from other mountains in that they are not formed by folding and crumpling but by uplift and erosion. A volcano is most likely to be a mountain built around a vent that connects reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and relevant gases are expelled.
During an episode of activity, a volcano commonly displays a distinctive pattern of behavior. Some mild eruptions merely discharge steam and other gases, whereas other eruptions quietly let out a lot of lava. The most spectacular eruptions consist of violent explosions with great clouds of gas debris into the atmosphere. There are about 500 active volcanoes in the world, and those volcanoes have erupted at least once within the recorded history. 50 of those 500 are in the United States-Hawaii, Alaska, Washington, Oregon and California. Most of American volcanoes are hidden under the seas. Many volcanoes are in and around the Mediterranean Sea. Near the island of Vulcano, the volcano Stromboli has been in a state of continuous, mild eruption since early Roman times. At night, sailors in the Mediterranean can see the glow from the fiery molten material that is thrown into the air. Therefore, Stromboli has been called "the lighthouse of the Mediterranean."