In the 1960s，while studying the volcanic history of Yellowstone National Park，Bob Christiansen became puzzled about something that，oddly，had not troubled anyone before：he couldn't find the park's volcano.It had been known for a long time that Yellowstone was volcanic in nature-that's what accounted for all its hot springs and other steamy features.But Christiansen couldn't find the Yellowstone volcano anywhere.
   Most of us，when we talk about volcanoes，think of the classic cone（圓錐體）shapes of a Fuji or Kilimanjaro，which are created when erupting magma（巖漿）piles up.These can form remarkably quickly.In 1943，a Mexican farmer was surprised to see smoke rising from a small part of his land.In one week he was the confused owner of a cone five hundred feet high.Within two years it had topped out at almost fourteen hundred feet and was more than half a mile across.Altogether there are some ten thousand of these volcanoes on Earth，all but a few hundred of them extinct.There is，however，a second less known type of volcano that doesn't involve mountain building.These are volcanoes so explosive that they burst open in a single big crack，leaving behind a vast hole，the caldera.Yellowstone obviously was of this second type，but Christiansen couldn't find the caldera anywhere.
   Just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone.A thoughtful official passed on some of the copies to the park authorities on the assumption that they might make a nice blow-up for one of the visitors' centers.As soon as Christiansen saw the photos，he realized why he had failed to spot the caldera：almost the whole park-2.2 million acres-was caldera.The explosion had left a hole more than forty miles across-much too huge to be seen from anywhere at ground level.At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.

（1）What puzzled Christiansen when he was studying Yellowstone？

D

D

A．Its complicated geographical features.
B．Its ever-lasting influence on tourism.
C．The mysterious history of the park.
D．The exact location of the volcano.
（2）What does the second paragraph mainly talk about？

A

A

A．The shapes of volcanoes.
B．The impacts of volcanoes.
C．The activities of volcanoes.
D．The heights of volcanoes.
（3）What does the underlined word "blow-up" in the last paragraph most probably mean？

C

C

A．Hot-air balloon.
B．Digital camera.
C．Big photograph.
D．Bird's view.