Investigating a Mystery

Although meteorite falls had been observed for hundreds of years, until the twentieth century no one had ever identified a crater created by a meteorite.

Geologist Grove Karl Gilbert
Geologist Grove Karl Gilbert. Image: USGS

Most scientists rejected the possibility of such a crater, believing that all natural landforms had been created slowly, over thousands or even millions of years, rather than in a single catastrophic moment.

Enter Grove Karl Gilbert

In 1891, however, one scientist became interested in the giant crater in the Arizona desert. Grove Karl Gilbert, the chief geologist for the U.S. Geological Survey, was the most renowned geologist of his generation. He was tolerant, generous, and fair-minded, with an intense dislike of controversy of any kind; he has been described as “perhaps the closest equivalent to a saint that American science has yet produced.”

Gilbert’s prestige was so great that none of his colleagues or successors was willing to publicly question his conclusions, even when it became apparent that some of those conclusions had been wrong.

Testing Two Hypotheses

In keeping with his careful, methodical approach to science, Gilbert visited the crater to test two hypotheses about its formation. The first was that it had been formed by the impact of a giant meteorite. The second was that it was the result of an explosion of superheated steam, caused by volcanic activity far below the surface.

Gilbert assumed that if the crater had been formed by a meteorite, that meteorite must have been nearly as big as the crater itself. He also assumed that it was still there, taking up space underneath the crater floor. He therefore decided to test the impact hypothesis in two ways:

  • By comparing the volume of the hollow of the crater to the volume of the ejected material.
  • By experimenting with magnets to test for a large mass of buried iron. 

Gilbert concluded that the crater was created by a steam explosion, and that the meteorite fragments were simply a coincidence.

If the meteorite were still under the crater floor, he reasoned, the space inside the crater would be smaller than the volume of material thrown out by the impact. And a large mass of iron would attract a magnet, altering the direction of a compass needle.

The results of both tests were negative. Gilbert concluded that the crater was created by a steam explosion, and that the thousands of meteorite fragments lying around it were simply a coincidence. He later used his investigation of the crater, and his own abandonment of the impact hypothesis, in a series of lectures illustrating the application of the dispassionate scientific approach.

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