Barringer Crater, often called Meteor Crater, is a large and striking geological structure in the northern Arizona
desert. The crater
, which is roughly circular, is about 1.2 kilometres across and 200 metres deep, with a rocky, substantial rim
rising around 45m above ground level. Lying in one of the driest parts of the country, the structure is the best preserved meteoritic crater on Earth and was the first terrestrial crater to be ascribed to meteorite
For over 100 years the crater has been among the most discussed and debated geological structures ever found. The area around the crater is strewn with lumps of iron, and in the 19th century it was proposed that there might be enough iron beneath the surface to justify mining. It was then that the debate about the crater's origin began. It was soon recognised that the crystal structure of the iron identified it as meteoritic in origin, and the obvious implication was that the crater had been formed by a meteorite; but no one had previously ascribed to a crater this sort of origin.
The eminent geologist Grove Karl Gilbert investigated the issue and concluded that the crater was probably not meteoritic but rather the result of a steam explosion caused by underground volcanic activity: he could not find any evidence of a large, subterranean mass of iron corresponding to the meteorite itself, and, moreover, the crater lay in an area of volcanic activity.
During the early 20th century, evidence accumulated to challenge this view and establish Barringer Crater as the result of meteoritic impact. Most prominent in this process was Daniel Barringer himself, a mining engineer who decided almost immediately on hearing of the crater that it was indeed meteoritic. Over the course of several years he made many crucial discoveries to support this hypothesis, among them that the area contained a vast quantity of material formed by intense heat and pressure, including quartz glass and pulverized silica; that rocks and boulders from the upper crater had been thrown distances of a mile or more, which did not seem to be likely from a steam explosion; that the underlying rock layers had been turned upside-down during crater formation (stratigraphic inversion); that much of the iron was of a magnetic oxide form (magnetite) not resembling any terrestrial substance; and that there was no evidence in the area either of hot spring activity or of volcanic rocks.
Like Gilbert, Barringer believed that if the crater was meteoritic, there should still be a large iron meteorite somewhere beneath the surface. However, years of ever-deeper exploration, much of it at Barringer's personal expense, consistently failed to find such a object. Rather than being solved in a single step, the problem gradually faded as the process of impact cratering became better understood in the 1920s and '30s. It had been assumed that the crater's circular shape was the result of a more-or-less vertical impact, and that its great size meant the meteorite itself must have been many hundreds of metres across. Newer discoveries showed that both assumptions were false: circular craters are caused by all but the most oblique collisions, and the meteorite itself was probably much smaller than the crater. That, and the possibility that the meteorite had vaporized on impact, led to the search being abandoned.
On the strength of Barringer's evidence the meteorite hypothesis came to dominate by the 1930s and is now universally accepted. According to the most contemporary evidence, the meteorite, which struck about 50,000 years ago, was 40 to 50 metres across, weighed in the order of 100,000 tons, struck at around 20 kilometres per second and released energy equivalent to around 20 million tons of TNT. All life within a radius of around 20 kilometres was destroyed by the impact.
For pictures and further information, visit the crater's official website at http://www.barringercrater.com.
Beatty, J. K., Petersen, C.C. and Chaikin, A. The New Solar System (fourth edition). 1999. Cambridge University Press, Cambridge, UK.
Mark, K. Meteorite Craters. 1987. The University of Arizona Press, Tucson.
Melosh, H. J. Impact Cratering: A Geologic Process. 1989. Oxford University Press, New York.