1960

Understanding Impact Craters

Eugene Shoemaker (1928–1997)

There is plenty of evidence around us that the surface of the Earth is constantly being changed by erosional, volcanic, and tectonic forces and processes. What is not so obvious, however, is how much our planet has been influenced by the other major surface geologic process at work in our solar system: impact cratering. One need not look any farther than the Moon and its crater-covered surface to realize that the Earth must also have been similarly bombarded.

It took geologists a very long time to realize the importance of impact cratering as a geologic process on the Earth, as well as on other planets and moons. Leading USGS geologist G. K. Gilbert, for example, was convinced that what was then named Coon Mountain in Arizona was an explosive volcanic crater, even though he acknowledged that many of the circular depressions on the Moon were likely to be impact craters, and even though others like the mining entrepreneur Daniel Barringer were convinced that the feature had been caused by the impact of a small iron-rich asteroid. The definitive proof of the impact origin of what is now called Meteor Crater, and the key pieces of evidence that would eventually help identify nearly 200 other impact structures across the world, would come from another USGS geologist, Eugene (“Gene”) Shoemaker.

Shoemaker was a consummate field geologist and a keen observer and interpreter of the stories the rocks were telling him. In order to better understand the way impact craters could modify a planetary surface, he spent time studying small craters made from nuclear-explosion tests in Nevada. He learned to recognize the telltale signs of such high-energy explosive processes, including the creation of special minerals formed at high shock pressures. In his PhD dissertation in 1960, Shoemaker used his experience to convince the remaining skeptics that Meteor Crater is indeed the result of an impact of a small metallic asteroid some 50,000 years ago. Shoemaker devoted the rest of his career (much of it in collaboration with his wife, the planetary astronomer Carolyn Shoemaker) to searching for and discovering impact structures on the Earth, and to searching for and studying the nature of the large population of near-Earth asteroids that could eventually pose potential future impact hazards to life on our planet.

Geologist Eugene Shoemaker (center, holding hammer) lectures on the geology of impact craters to a group of astronauts at Meteor Crater, Arizona, in 1967.