The Great Barringer Meteor Crater
The Great Barringer Meteor Crater
The Great 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 to be an important stepping stone for advances in geology and astronomy. In solving the mystery surrounding the genesis 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 extraterrestrial bodies. In addition, the story behind the early-twentieth-century controversy surrounding the origin of the crater highlights the dangers of prejudice and selective use of evidence in scientific methodology.
The Barringer Meteor Crater (originally named Coon Butte or Coon Mountain) rises 150 ft (46 m) above the floor of the surrounding Arizona desert. The impact crater itself is almost 1 mile (1.6 km) wide and 570 feet (174 m) 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 (1843-1918), then the respected 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 would had to have remained intact through its fiery plunge through the Earth's protective atmosphere. Moreover, Gilbert assumed that most of the meteor must have survived its impact with Earth. Gilbert therefore assumed that if a meteor collision was responsible for the crater, then substantial pieces of the meteor should still exist.
Armed with these assumptions Gilbert began his research. He found no substantial mass inside the crater, so he assumed that the meteor was buried. However, Gilbert was unable to find the elusive meteor, and he 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 (1860-1929), an American entrepreneur and mining engineer, began a study of the Arizona crater. After discovering that small meteors made of iron were 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 phenomena. Acting more like a businessman or miner trying to stake a claim, Barringer seized the opportunity to form a company with the intent of mining the iron from the presumed meteor. Without actually visiting the crater, Barringer formed the Standard Iron Company and sought mining permits.
Over the next decades, Barringer, a self-confident, self-made, and wealthy man, invested his fortunes in proving the meteor impact hypothesis, and in reaping the potential profits from the mining of such a meteor. Barringer collected thousands of investment dollars from people expecting large returns. What had started out as a scientific question now became clouded by profit motive. Evidence was not subjected to scientific scrutiny as much as it was selected to bolster investor "confidence."
For the next 30 years or so, Barringer became the sword and shield of the often rancorous scientific warfare regarding the origin of the crater. In bitter irony, Barringer won the scientific battle—that the crater resulted from a meteor impact—but lost his financial gamble. In the end, the meteor that had caused the impact proved to be much smaller than hypothesized by either Gilbert or Barringer. On the heels of these findings 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 centered on whether 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 with 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 cataclysmic impact. 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 presumably thrown out of the crater at the time of impact). Such a random mixture could only have resulted from a cataclysmic impact.
Barringer's theory gained support from 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 that the heat could not have come from below the surface.
The evidence collected by Barringer also influenced astronomers seeking an explanation for the large, round craters on the Moon. Once again the debate moved between those championing extraterrestrial volcanic activity (gradualism) versus those who favored an impact hypothesis (cataclysm). The outcome of these debates had enormous impact in both geology and astronomy.
One fact that perplexed astronomers was the particular shape of the lunar impact craters: they 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 explain this phenomenon by conducting an experiment: they fired bullets into clumps of rock and mud. Regardless of the firing angle the Barringers demonstrated that the resulting craters were substantially round. More definitive proof was subsequently provided in 1924 by 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 (56,315 kph) at impact proved to be a double-edged 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 were dashed.
In the 1960s American astronomer and geologist Eugene Shoemaker found distinct similarities between the fused rocks found at the Barringer crater and those found at atomic test sites, attesting to the power of the impact. In addition, unique geologic features termed "shattercones" created by immense pressure pointed to a tremendous explosion at or above the impact crater. 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. Having identified more than 150 such impact sites, scientists are researching these sites in hopes of better understanding the Earth's geologic history.
It appears that a catastrophic astronomical collision occurred 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 a vast area of Siberian forest, may have been Earth's most recent significant encounter with an impacting object.
K. LEE LERNER
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