Barringer Meteor Crater (World of Earth Science)
The Barringer Meteor Crater in Arizona was the first recognized terrestrial impact crater. The confirmation of a meteor impact (subsequently identified as the Canyon Diablo meteorite) at the site proved an important stepping-stone for advances in geology and astronomy. In solving the mystery surrounding the origin of the Barringer crater, geologists and astronomers made substantial progress in understanding the dynamic interplay of gradual and cataclysmic geologic processes both on Earth and on extra-terrestrial bodies.
The Barringer Meteorite Crater (originally named Coon Butte or Coon Mountain) rises 150 feet above the floor of the surrounding Arizona desert. The impact crater itself is almost a mile wide and 570 feet deep. Among geologists, two competing theories were most often asserted to explain the geologic phenomena. Before the nature of hot spots or plate tectonic theory would have convinced them otherwise, many geologists hypothesized that the crater resulted from volcanic activity. A minority of geologists asserted that the crater must have resulted from a meteor impact.
In the last decade of the nineteenth century, American geologist Grove Karl Gilbert, then the head of the U.S. Geological Survey, set out to determine the origin of the crater. Gilbert assumed that for a meteor to have created such a large crater, it was necessary for it to remain intact through its fiery plunge through the earth's protective atmosphere. Moreover, Gilbert assumed that most of the meteor survived its impact with Earth. Gilbert, therefore, assumed that if a meteor collision was responsible for the crater, substantial pieces of the meteor should still exist and there should be ample and direct physical evidence of the size of the meteor. When upon observation it became apparent that there was no substantial mass inside the crater, Gilbert assumed that the meteor might have been covered with the passage of time. Assuming the meteor to be like other known meteorites and similar in percentage of iron composition to the smaller meteors found around the crater, Gilbert looked for magnetic evidence in a effort to find the elusive meteor. Gilbert's repeated tests found no evidence for such a buried mass. After carefully examining the crater, Gilbert concluded that, in the absence of the evidence he assumed would be associated with a meteor impact, the crater had resulted from subterranean activity.
In 1902, Daniel Moreau Barringer, an American entrepreneur and mining engineer, began a study of the Arizona crater and took up the opposing view. After discovering that small meteors made of iron had been found at or near the rim of the crater, Barringer was convinced that only a large iron meteor could be the cause of such a geologic phenomenon. Acting more like a businessman or miner trying to stake a claim, and before doing any studies on the potential masses and energies that would have to be involved in such an impact, Barringer seized the opportunity to form company with the intent of mining the iron from the presumed meteor for commercial profit. Without actually visiting the crater, Barringer formed the Standard Iron Company and sought mining permits.
For nearly the next thirty years, Barringer became the sword and shield of often-rancorous scientific warfare regarding the origin of the crater. In bitter irony, Barringer won the scientific battle, the proof eventually accumulated that the crater resulted from a meteor impact, but lost his financial gamble. In the end, the meteor that caused the impact proved much smaller than hypothesized by either Gilbert or Barringer, and the nature of the impact obliterative. On the heels of these finding in 1929, Barringer died of a heart attack. His lasting legacy was in the attachment of his name to the impact crater.
The debate over the origin of the Great Barringer Meteor Crater came at a time when geology itself was reassessing its methodologies. Within the geologic community there was often vigorous debate over how to interpret geologic data. In particular, debates ranged regarding the scope and extent of uniformitarianism. In its simplest form, uniformitarianism asserted only that the laws of physics and chemistry remained unchanged during the geologic history of the Earth. Debate centered on whether the predominantly dominant gradualism (similar to evolutionary gradualism) of geologic processes was significantly affected by catastrophic events.
Barringer confidently asserted that the Coon Butte crater supported evidence of catastrophic process. Although he argued selective evidence, Barringer turned out to be correct when he asserted that the finely pulverized silica surrounding the crater could have only been created in a process that created instantaneously great pressures. Beyond the absence of volcanic rocks, Barringer argued that there were too many of the iron fragments around the crater to have come from gradually accumulated separate meteor impacts. Moreover, Barringer noticed that instead of defined strata (layers) there was a randomized mixture of the fragments and ejecta (native rock presumable thrown out of the crater at the time of impact). Such a random mixture could only have resulted from a cataclysmic impact.
Barringer's cause gained support of mainstream geologists when American geologist George P. Merrill tested rocks taken from the rim and floor of the crater. Merrill concluded that the quartz-like glass found in abundance in the presumed eject could only have been created by subjecting the native sands to intense heat. More importantly, Merrill concluded that the absence of sub-surface fusions proved the heat could not have come from below the surface.
The evidence collected by Barringer also influenced astronomers seeking, at that time to explain large, round craters on the moon. Once again, the debate moved between those championing extra-terrestrial volcanic activity (gradualism) versus those who favored an impact hypothesis (cataclysm). This outcome of these debates had enormous impact in both geology and astronomy.
One fact that perplexed astronomers was that it appeared that all of the lunar impact craters were generally round. If meteors struck the Moon at varying angles, it was argued, then the craters should have assumed a variety of oblique shapes. Barringer and his 12-year-old son set out to experiment with the formation of such craters by firing bullets into clumps of rock and mud. Regardless of the firing angle, the Barringers demonstrated (and published their results in both popular and scientific magazines) that the resulting craters were substantially round. More definitive proof was subsequently provided in 1924 by calculations of astronomers who determined that forces of impact at astronomical speeds likely resulted in the explosive destruction of the impacting body. Importantly, regardless of the angle of impact, the result of such explosions would leave rounded craters.
The confirmation that a meteor weighing about 300,000 tons (less than a tenth of what Barringer had estimated) and traveling in excess of 35,000 mph at impact could have produced the energy and catastrophic phenomena observed proved a double edge sword for Barringer. In one stroke, his hypothesis that the crater was caused by a meteor impact gained widespread support while, at the same time Barringer's hopes of profitably mining the meteor vaporized like much of the exploded meteor itself.
The scientific debate on the origin of Barringer crater was essentially closed when it was dramatically demonstrated that meteor impacts could impart such large energies far above even the tremendous power of nuclear weapons. In the 1960s, American astronomer and geologist Eugene Shoemaker found distinct similarities between the fused rocks found at Barringer crater and those found at atomic test sites. In addition, unique geologic features termed "shattercones" created by the instantaneous application of tremendous pressure pointed to a tremendous explosion at or above the impact crater. Determinations made by later, more sophisticated dating techniques placed the age of the crater at roughly 50,000 years.
Scientists subsequently understood that massive cataclysmic collisions result in what is now termed shock metamorphism. These shock metamorphic effects have been shown to be exclusively associated with meteorite impact craters. No other natural process on earth can account for the observed results.
A great deal of such evidence methodologies derived from the Barringer crater controversy now points to a catastrophic astronomical collision at the end of the Cretaceous Period 66 million years ago. The effects of this collision are thought to have precipitated the widespread extinction of large species, including the dinosaurs. The enigmatic Tunguska explosion of 1908, which devastated an vast area of Siberian forest, may have been Earth's most recent significant encounter with an impacting object vaporized so as to leave little physical remains beyond the manifest effects of a tremendous explosion.
Methods used to confirm Barringer crater as a meteor impact crater have been used to identify many other impact sites around the world. Once scientists became aware of the tremendous energies involved in astronomical impacts, large terrestrial impacts, often hidden by erosive effects, became a focus of study. With more than 150 such impact sites identified, impacts have taken on an important role in understanding the Earth's geologic history. The accumulated evidence led to a synthesis of gradualism and catastrophic theory. In accord with uniformitarianism, the gradual and inexorable shaping processes taking place over geologic time were understood to be punctuated with catastrophic events.