Finding the Evidence
How do we know today that the Barringer Meteorite Crater was caused by a meteorite? Features of the rocks in and around the crater provide several crucial pieces of evidence for an impact origin.
Geologist Eugene Shoemaker helped to prove the impact origin of the crater. Photo: USGS
Overturned Rock Layers
In normal stratification of sedimentary rock, materials are deposited horizontally over time, with the youngest layer on top. In the ejecta blanket of an impact crater, this stratigraphy is inverted – the youngest layers appear at the bottom, and the oldest layers appear at the top. Daniel Moreau Barringer and others described these overturned rock layers at Barringer Meteorite Crater.
In 1963 Eugene Shoemaker demonstrated that the layers of rock inside the crater and in the ejecta blanket were structurally similar to craters caused by nuclear blasts, and produced a geologic description comparing the features of the crater to the features of the Teapot Ess nuclear crater.
Shoemaker and Ed Chao had also discovered the shock metamorphism of quartz to coesite and stishovite. The collective work of these men provided the evidence for the impact origin of the crater.
Rocks in Shock
Extreme temperatures and pressures from a meteorite impact cause extensive heating, melting, and deformation. Deformation occurs under the high pressures experienced as a shockwave passes through the target rocks during an impact (known as shock metamorphism). Because they can only be formed under these special conditions, these target rocks are considered indicators of shock.
Scientists often look for shock indicators to prove that a given terrestrial structure is impact-related. Several shock features that scientists look for include planar deformation features and planar fractures (these two are usually identified using thin sections and microscopes); high-pressure polymorphs (coesite and stishovite); and shatter cones. They can range greatly in size, from microscopic to >10 meters in size.
Scientists have found evidence of shock metamorphism at Barringer Crater in breccia deposits (rock made up of fractured rocks held together by a melted matrix), in the pulverized rock of Coconino sandstone near the crater rim, and in shocked sandstone rocks found on the edges of the ejecta blanket.