As one drives across the desert of northern Arizona, suddenly, and for no apparent reason, there looms ahead a mile-wide, nearly circular hole in the ground called Meteor Crater (Figure 4). In 1891, G. K. Gilbert, chief geologist of the U.S. Geological Survey and one of the most prestigious geologists in the world, attended a lecture in which this feature, known then for its raised rim as Coon Mountain, was described. Scattered around it were curious metallic fragments that were unlike any terrestrial rock and closely resembled iron
meteorites. Gilbert reasoned that if impact created this circular crater, the meteorite must have fallen vertically and could still be buried directly beneath the crater, where its iron magnetism would give it away. Surrounding the crater should be a mixture of ejected rock and meteorite fragments that together would have a greater volume than the now-vacated crater.
Announcing that he was "going to hunt a star," Gilbert and his assistants set out in October 1891 to measure the expected magnetism of the crater floor, but found none. The volume of ejecta turned out (by coincidence, we now know) to just match the volume of the crater. As a responsible scientist who followed where the evidence led, Gilbert had to conclude that impact had not created the crater. Such a conclusion was especially obligatory in this case, since Gilbert had set out his intended investigation of Coon Mountain as a model of the scientific method. He published his findings in 1896, four years after his hotel room experiments.9 Having failed to find the predicted evidence of impact, Gilbert was forced to conclude that something other than impact, most likely a deep-seated gas explosion, had created the crater. Thus developed one of the great ironies in the history of geology: Gilbert correctly concluded that impact created the lunar craters, but incorrectly concluded that it had not created the most visible of all terrestrial craters. For four decades, Gilbert's enormous prestige and apparently meticulous methods put the theory of impact craters to rest.
The crater attracted not only scientific but commercial interest. Geologist and mining entrepreneur D. M. Barringer, "unaware that such ideas were geological heresy," as Marvin puts it," decided that the meteoritic fragments at Coon Mountain meant that a large and valuable mass of meteoritic iron lay buried beneath the crater floor. He began his investigation in 1902 and continued it for 27 years, staking a claim and forming a company to mine the iron ore. Barringer sank exploratory shafts but found only the same fragments of mete-oritic iron that had always turned up at Meteor Crater, as the cavity had come to be called. In 1929 he asked astronomer F. R. Moulton to calculate the amount of iron that should have been left behind. By this time, impact science had advanced enough for Moulton to conclude that the impactor would not have buried itself into the ground, it would have exploded, a fact of which no one in Gilbert's day was aware. Furthermore, Moulton calculated the mass of the meteorite at a mere 300,000 tons, far below Barringer's original estimate. Barringer's role in the search for impact products came to a tragic end only a few months later, just weeks after the stock market crash in 1929, when he died of heart failure. Though only fragments of meteoritic iron ever turned up at Meteor Crater, the work of Barringer and Moulton created an important legacy—the knowledge that at least one terrestrial hole in the ground was formed by an impacting meteorite.
Not long before Barringer died, Eugene Merle Shoemaker was born. He was just slightly too young to serve in World War II." A young man in a hurry, he rushed through Los Angeles's Fairfax High School and the California Institute of Technology, emerging in 1948, at age 20, with bachelor's and master's degrees (and having been a school cheerleader along the way). He immediately joined the U.S. Geological Survey and went to look for uranium ore on the Colorado Plateau. Shoemaker recalled that one day in his first year with the U.S. Geological Survey, on his way to breakfast, it dawned on him that humans were going to "explore space." He thought, "I want to be part of it! The moon is made of rock, so geologists are the logical ones to go there—me, for example."12 Shoemaker was right—humans were going to the moon, but unfortunately, medical problems prevented him from achieving his lifelong goal of being one of them.
By the mid-1950s, Shoemaker, always to be found where the cutting-edge geology was being done, was mapping nuclear bomb craters at the Nevada test site. His work on the Colorado Plateau had drawn to his attention a large cavity there that did not have a nuclear origin: Meteor Crater. Having satisfied the descendants of Daniel Barringer that he was not a disciple of Gilbert, in 1957 Shoemaker began the modern study of Meteor Crater. He used the time-honored methods of the field geologist: Study each rock unit close up and plot its position to produce a geologic map, the universal medium by which geologists communicate.
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