c. 50,000 BCE

Arizona Impact

Grove Karl Gilbert (1843–1918), Daniel Barringer (1860–1929), Eugene M. Shoemaker (1928–1997)

We know that the Earth has been slammed by asteroid and comet impacts throughout its history—we need only look at the record of the Late Heavy Bombardment preserved in the ancient, airless surface of our celestial neighbor the Moon. Given the erasure of impact craters on our planet via the constant erosion of wind and water and the continuous renewal of the seafloor through plate tectonics and volcanism, it’s no wonder that it took terrestrial geologists a long time to realize the importance of impact cratering as a geologic process on Earth and other terrestrial planets, moons, and asteroids.

One of the best natural laboratories for that realization is the Meteor Crater (also known as Barringer Crater or Canyon Diablo Crater), just east of Flagstaff, Arizona. The crater is about 1,200 meters (3,900 feet) wide and 170 meters (557 feet) deep. Up until the 1960s, geologists heatedly debated the origin of this feature. In the 1890s, G. K. Gilbert, one of the first advocates for the impact origin of the circular craters on the Moon, argued that the lack of significant debris from the impactor itself meant that the crater was formed by an explosive volcano. In the early twentieth century, mining engineer Daniel Barringer purchased the crater and spent years drilling in vain for what he believed was a giant buried iron meteorite from an impact event. Finally, geologist Eugene M. Shoemaker, who had studied craters created by the US government’s nuclear test program in Nevada, confirmed the crater’s origin as an impact feature, based primarily on the discovery of certain forms of quartz minerals that could only form in the high-pressure, high-temperature environment of an impact rather than a volcano.

Since then, more than two hundred other impact craters have been recognized on Earth—most larger but much less preserved than Meteor Crater. Lab and computer studies of impact physics now reveal that impactors—in this case, an approximately 164-foot (50-meter) iron-rich asteroid traveling at more than 6 miles (10 kilometers) per second—are almost always completely vaporized on impact, resolving the question about the lack of debris in the area around the crater.

View from the rim of Meteor Crater, a hole in the Arizona desert about ¾ mile (1.2 kilometers) wide, created about 50,000 years ago by the impact of a small iron-rich asteroid traveling more than 6 miles (10 kilometers) per second.