How was Meteor Crater shown to be an impact crater?
After decades of debate, Eugene Shoemaker showed in the early 1960s that Meteor Crater contains coesite and stishovite and shocked rocks, proving it was made by an impact.
Meteor Crater, also called Barringer Crater, is a bowl about 1.2 kilometres across and 170 metres deep in the Arizona desert. Today it is famous as one of the best-preserved impact craters on Earth, but its origin was hotly debated.
Early ideas. Iron meteorite fragments, named Canyon Diablo, were found around the crater in the 1890s. In 1891, the geologist Grove Karl Gilbert of the United States Geological Survey investigated. He concluded, after tests that found no buried iron mass and noted the crater's volume matched its rim, that it was probably a volcanic steam explosion.
Barringer's campaign. In 1903, the mining engineer Daniel Barringer argued that the crater was made by a giant iron meteorite and that its mass lay buried beneath the floor. He spent decades and a fortune drilling, hoping to mine the iron, but never found a large mass. Most of the impactor, we now know, vaporised or melted on impact.
Proof. In the late 1950s and early 1960s, the geologist Eugene Shoemaker studied the crater in detail. He compared it with craters made by nuclear tests and found similar structures. In 1960, with colleagues, he identified coesite, and soon afterwards stishovite, high-pressure forms of silica that form only under shock pressures far beyond volcanic processes. This proved an impact.
Age and impactor. The crater formed about 50,000 years ago when an iron asteroid perhaps 30 to 50 metres across struck at high speed.
Shoemaker's work helped establish impact geology as a science.