判断题
The second-largest volcanic eruption of the 20th century, and by far the largest eruption to affect a densely populated area, occurred at Mount Pinatubo in Philippines on June 15, 1991. The eruption produced high-speed massive falling of hot ash and gas, giant mudflows, and a cloud of volcanic ash hundreds of miles across. The impacts of the eruption continue to this day.
A huge cloud of volcanic ash and gas rose above Mount Pinatubo, Philippines, on June 12,1991. Three days later, the volcano exploded in the second-largest volcanic eruption on Earth in this century. Timely forecasts of this eruption by scientists from the Philippine Institute of Volcanology and Seismology and the US Geological Survey enabled people living near the volcano to evacuate to safer distances, saving at least 5000 lives.
On July 16,1990,a magnitude 7. 8 earthquake (comparable in size to the great 1906 San Francisco, California, earthquake) struck about 60 miles (100 kilometers) northeast of Mount Pinatubo on the island of Luzon in the Philippines, shaking and squeezing the Earth’s crust beneath the volcano. At Mount Pinatubo, this major earthquake caused a landslide, some local earthquakes, and a short-lived increase in steam emissions from a pre-existing geothermal area, but otherwise the volcano seemed to be continuing its 500 year-old sleep undisturbed. In March and April 1991,however,molten rock rising toward the surface from more than 20 miles beneath Pinatubo triggered small earthquakes and caused powerful steam explosions that blasted three outlets on the north flank of the volcano. Thousands of small earthquakes occurred beneath Pinatubo through April, May, and early June, and many thousand tons of noxious sulfur dioxide gas were also emitted by the volcano. Following Mount Pinatubo’s disastrous eruption on June 15,1991, thousands of roofs collapsed under the weight of ash made wet by heavy rains. Ash deposits from the eruption have also been remobilized by monsoon and typhoon rains to form giant mudflows of volcanic materials, which have caused more destruction than the eruption itself.
Nearly 20 million tons of sulfur dioxide were injected into the upper atmosphere in Pinatubo’s 1991 eruptions, and dispersal of this gas cloud around the world caused global temperatures to drop temporarily (1991 through 1993) by about 1 degree. The eruptions have dramatically changed the face of central Luzon, home to about 3 million people. About 20000 indigenous Aeta highlanders, who had lived on the slopes of the volcano, were completely displaced, and most still wait in resettlement camps for the day when they can return home. About 200000 people who evacuated from the lowlands surrounding Pinatubo before and during the eruptions have returned home but face continuing threats from lahars that have already buried numerous towns and villages. Rice paddies and sugar-cane fields that have not been buried by lahars have recovered; those buried by lahars will be out of use for years to come.