【 2019· 江苏卷, B 】 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( 圆锥体 )shapes of a Fuji or Kilimanjaro , which are created when erupting magma( 岩浆 )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 les 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. 58. What puzzled Christiansen when he was studying Yellowstone ? 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. 59. What does the second-paragraph mainly talk about ? A. The shapes of volcanoes. B. The impacts of volcanoes. C. The activities of volcanoes. D. The heights of volcanoes. 60. What does the underlined word "blow-up" in the last paragraph most probably mean ? A. Hot-air balloon. B. Digital camera. C. Big photograph. D. Bird's view.