In the 1960s, while studying the volcanic history of Yellowstone National Park, 20世纪60年代，美国地质勘测局的鲍勃·克里斯琴森在研究黄石国家公园火山史的时候，

　　Bob Christiansen of the United States Geological Survey became puzzled about something that, oddly, had not troubled anyone before: 对一件事感到迷惑不解：而且，说来也怪，以前竟然没有人为这件事费过神：

　　he couldn't find the park's volcano. 他找不到这个公园的火山。

　　It had been known for a long time that Yellowstone was volcanic in nature—that's what accounted for all its geysers and other steamy features 很久以来，大家已经知道，黄石公园是火山形成的——因此有那么多喷泉和其他散发蒸汽的地貌。

　　and the one thing about volcanoes is that they are generally pretty conspicuous. 火山有个特点，即：一般来说比较明显。

　　But Christiansen couldn't find the Yellowstone volcano anywhere. 但是，克里斯琴森哪里也找不到黄石公园的火山，

　　In particular what he couldn't find was a structure known as a caldera. 他尤其找不到一种名叫破火山口的结构。

　　Most of us, when we think of volcanoes, think of the classic cone shapes of a Fuji or Kilimanjaro, 一提到火山，大多数人马上会联想到富士山或乞力马扎罗山那样典型的火山锥。

　　which are created when erupting magma accumulates in a symmetrical mound. 那是由喷出的岩浆堆积而成的一个对称的小山冈。

　　These can form remarkably quickly. 火山锥形成的速度可能相当快。

　　In 1943, at Paricutin in Mexico, a farmer was startled to see smoke rising from a patch on his land. 1943年，墨西哥帕里库廷的一个农夫吃惊地看到自己的一块地里冒起了烟。

　　In one week he was the bemused owner of a cone five hundred feet high. 一个星期以后，他做梦似的已经成了一个152米高的火山锥的主人。

　　Within two years it had topped out at almost fourteen hundred feet and was more than half a mile across. 不到两年，它的高度达到了430米，直径800多米。

　　Altogether there are some ten thousand of these intrusively visible volcanoes on Earth, all but a few hundred of them extinct. 地球上总共有大约1万座这种一目了然的火山，其中只有几百座是活火山。

　　But there is a second, less celebrated type of volcano that doesn't involve mountain building. 但是，还有一种不大有名、并不造山的火山。