Before a hypothesis is accepted as a scientific theory, it must be challenged, tested and re-tested many times. It was not until 57 years after Barringer and Tilghman presented their first papers that the impact theory was fully accepted by the scientific community.
During the 40’s and 50’s, investigations by Robert S. Dietz and others revealed the existence of structures called “shattercones” at many suspected impact sites, although not at the Barringer Crater. These structures, which can be anywhere from less than an inch to more than six feet tall, can only be created by a sudden intense pressure on existing rock.
Dietz was able to demonstrate that the apexes of the cones at most of these sites pointed upwards, indicating that the force which created them had come from above. This evidence provided strong support for the idea that the sites were created by meteorite impacts. The presence of shattercones is now considered as one of the criteria for identifying an impact structure.
Another feature found only at impact sites is the presence of tiny parallel lines called “planar deformation features” in quartz grains affected by the impact. The intense heat and pressure of the impact causes the crystals to melt along submicroscopic planes, leaving parallel bands of melted and unmelted quartz. No purely terrestrial process can produce enough pressure to create these deformations.
Geologist Eugene Shoemaker published the landmark paper conclusively demonstrating an impact origin for the Barringer Meteorite Crater. Photo: USGS
In 1960, Eugene Shoemaker, Edward Chao and David Milton were responsible for the discovery of a new mineral at the Barringer crater. This mineral, a form of silica called “coesite”, had first been created in a laboratory in 1953 by chemist Loring Coes. Its formation requires extremely high pressures and temperatures, greater than any occurring naturally on earth. Coesite and a similar material called “stishovite” have since been identified at numerous other suspected impact sites, and are now accepted as indicators of impact origin.
Finally, in 1963, Eugene Shoemaker published his landmark paper analyzing the similarities between the Barringer crater and craters created by nuclear test explosions in Nevada. Carefully mapping the sequence of layers of the underlying rock, and the layers of the ejecta blanket, where those rocks were deposited in reverse order, he demonstrated that the nuclear craters and the Barringer crater were structurally similar in nearly all respects. His paper provided the clinching arguments in favor of an impact, finally convincing the last doubters.
Why Should We Care?
Daniel Moreau Barringer’s struggle to prove the impact origin of his crater is an example of the enormous difference that one determined individual can make - even if that individual is a non-scientist working to change the prevailing scientific view. His story tells us about the importance of intuitive leaps, careful data-gathering, stubbornness, and even self-interest in the progress of science. But it also demonstrates the rigorous testing that a scientific hypothesis must endure before it is finally accepted by the scientific community.
“The impact of solid bodies is the most fundamental of all processes that have taken place on the terrestrial planets.”
The progress of science is seldom a simple matter. Gilbert saw the correct answer to the riddle of the crater, but lacked the passion to pursue it further when his first tests failed. Barringer was right when his intuition coincided with his self-interest, but wrong when the truth diverged from it. Both of them ultimately failed to understand the crater because they did not take into account the enormous speed of a meteorite impact, and the enormous release of energy resulting from that speed. A variety of discoveries by different scientists over a period of sixty years would be necessary before the impact theory emerged as the unchallenged winner.
Continuing research at the Barringer Meteorite Crater and other impact sites has taught us not only about our own world, but about the history of the solar system out of which it was born. In the words of Gene Shoemaker, “the impact of solid bodies is the most fundamental of all processes that have taken place on the terrestrial planets… Collision of smaller objects is the process by which the terrestrial planets were born.”