Meteor Crater in Arizona was formed by a volcanic or underground steam explosion.

Meteor Crater is so sharply circular that both volcanic and impact explanations seemed plausible when geologists began studying it seriously. At the time, large terrestrial impact craters were not yet a familiar geological category.

Many geologists favored an explosion from below—volcanism, steam, or some other internal process.

Mining engineer Daniel Barringer argued in the early twentieth century that a meteorite had struck the site. He found meteoritic iron around the crater, but his case was weakened by an incorrect expectation: he believed a huge intact iron mass should remain buried beneath it and spent years trying to locate it.

Modern impact physics explains why that search failed. A high-speed collision releases enough energy to vaporize, melt, fragment, and disperse much of the projectile.

Eugene Shoemaker’s later geological mapping supplied the decisive evidence. Overturned rock layers, ejecta patterns, shocked minerals, and structural relationships matched an explosion directed downward from an impact, not upward from volcanic activity.

Meteor Crater became one of the key terrestrial laboratories for recognizing impact structures elsewhere on Earth and across the Solar System.