wind，water，fire and ice—these powerful natural forces have shaped the land of america in the past. they are still changing it today.
the colorado river slowly cut down through stone to make the grand canyon (大峡谷)．
long ago，ice sheets cut yosemite valley and the great lakes，and glaciers (冰川) are still on the move in alaska.
wind and rain storm hit the land from time to time. fires sometimes burn down forests and destroy the homes of wild animals: this happened in
dead volcanic (火山的) mountains such as haleakala on maui and carter lakein oregon are beautiful to look at. but there are several active volcanoes in the us，especially along the pacific coast. there are also many earthquakes in this area. scientists can do nothing to control earthquakes or volcanoes. this was shown clearly on may 18,1980.at 8: 32 that morning，mount st helens in washington state broke out. the top of the mountain was blown off.
over 60 people—campers，scientists，journalists，forest workers—were killed. hundreds of square miles of forests were knocked down. in towns over 100 miles away，day suddenly became night. an ash cloud hid the sun for many hours. towns and fields in washington，oregon，and idaho were covered with dirty gray ash.
scientists knew that mt st helens might erupt (爆发) soon. but no one could say when. and no one expected the terrible strength of the eruption. many people were surprised and unready. harry r．truman,84，owned a small hotel in mt st helens. scientists had asked people to move away because the volcano was becoming dangerous.
but harry r．truman would not leave his home. now it is covered with many feet o资源难易程度:★★★★★★★★★★★★★★★
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