50,000 years ago, a large crater was formed in the northern Arizona desert of the United States. To get an idea of the size, imagine twenty American football games being played simultaneously on its floor, while more than 2 million spectators watch from the sloping sides. What created this crater? Was it volcanic in nature? And how can we tell? Though these days we know Meteor Crater to be an impact crater formed from an iron-nickel meteorite about 150 feet across, back in the 19th to early 20th century, there was a long dispute over the origin. Given the San Francisco volcanic field lies only about 40 miles to the west, the thought was that the crater was formed due to a volcanic steam explosion. The problem with this? No volcanic rocks were found in the region. Fortunately, the relatively young age of the crater, paired with the dry Arizona climate has prevented much erosion, allowing for more detailed study.
In 1903, mining engineer and businessman, Daniel M. Barringer, proposed the hypothesis that the impact was due to a large iron-metallic meteorite – he spent his life searching and mining for the large metallic body, not knowing at the time that a meteorite could disintegrate during impact through vaporization, leaving being only fragments – not a single large mass underground at the center of the impact. Soon, the surrounding plains were found to be covered with about 30 tons of oxidized iron meteorite fragments. A key discovery by Eugene Shoemaker many years later was the presence of the minerals coesite and stishovite, rare forms of silica found only where quartz-bearing rocks have been severely shocked by an instantaneous overpressure. This cannot be created by volcanic activity – only an impact or nuclear explosion can produce it. The impact at Meteor Crater created what’s called “inverted stratigraphy” as well, so the layers immediately exterior to the crater rim were found to be stacked in the reverse order in which they were originally laid. Normally, the oldest layers are on the bottom and the youngest on top. In a case of inverted stratigraphy, the oldest is on top!
Meteor Crater was the first crater to be confirmed as created by an extraterrestrial impactor. During the 1960s and 1970s, NASA astronauts trained in the crater to prepare for the Apollo missions to the Moon. Given the cratering process ejects material that actually originates below the surface of the crater, when astronauts went to the Moon, they knew they should be able to collect material on the surrounding ejecta blankets that originated beneath the cratered region – a valuable sampling technique learned at Meteor Crater.