unit 10 Frightening nature

    although there are thousands of volcanoes in the world，most of them are dead. on1y about 500 sometimes start to throw out lava again. a famous volcano which is now dead is mount fujiyama in japan. it is covered with snow in winter.
    vesuvius is the name of a very famous volcano in italy. it first came to life many，many years ago. it was quiet for hundreds of years. then in the year 79 it suddenly burst. a great cloud of smoke shot up into the sky with great burning rocks.which fell all around. hot lava poured down its sides. about 3 000 people were killed.
    this has happened again many times since that year. sometimes no damage was caused，or only little damage. but there was serious damage in the years 472，1631，1794，1861，1872 and 1906.you can see that a volcano can stay alive for many years. in 1906 part of the top of the mountain fell off. there was also serious damage in 1914 but there has not been any since that year.
2 .what is a tornado?     
what exactly is a tornado? the general picture is familiar enough. the phenomenon usually occurs on a hot sticky day with south winds and an ominous sky. from the base of a thunderhead，a funnel-shaped cloud extends a violently twisting spout toward the earth. as it sucks in matter in its path，the twister may turn black, brown, or occasionally (over snow) even white. the moving cloud shows an almost continuous display of sheet lightning. it lurches along in a meandering path,    usually northeastward, at 25 to 40 miles per hour. sometimes it picks up its finger from the earth for a short distance and then plants it down again. the funnel is very slender: its wake of violence averages about 400 yards wide. as the tornado 'approaches, it is heralded by a roar as of hundreds of jet planes or thousands of railroad cars. its path is a path of total destruction. buildings literally explode as they are sucked in by the tornado's low-pressure vortex(where the pressure drop is as much as 10 percent) and by its powerful whirling winds (estimated at up to 500 miles per hour). the tornado's lifetime is as brief as it is violent. within a few tens of miles (average: about 16 miles), it spends its force and suddenly disappears.   
3.earthquakes   
earthquake is a sudden, violent shaking of the earth's surface. it's a kind of crustal movement form of expression.   
earthquake is regarded as one of the most damaging forces known to man: since records began to be written down, it has been estimated that earthquake-related fatalities have been numbered in the millions, and that earthquake-related destruction has been beyond count.