When sleuthing a potential or known meteorite impact crater, scientists look for clues. One of these clues is a geological feature known as a “shatter cone.” These objects — also called shocked rocks — bear the patterns of high-energy impacts. They are found only in the bedrock under meteorite impact craters or near underground nuclear explosions. This gives us some idea of the power of a meteorite impact.
This shatter cone comes from the Sierra Madera crater located in southwestern Pecos County, Texas. The crater is 8 miles (13 kilometers) in diameter. Estimates place its age at less than 100 million years. // All photos by Mike Reynolds
Shatter cones show that the rocks near the impact site have been subjected to a significant shock. The pressures associated with such impacts lie between 29,000 pounds per square inch to more than 4 million psi. Atmospheric pressure at sea level measures 14.7 psi, so such impactors create pressures some 270,000 times greater than our bodies normally experience.
Shatter cones are one type of impact evidence beyond the bowl-shaped crater. But the crater might have been erased from sight due to erosion, or it may be covered by growth. Impact “glasses,” breccias, and related material also demonstrate the extremes of an impact. Perhaps one of the most famous sites is the Sudbury Crater in Ontario, Canada, where explorers have found a rich array of impact materials — and, in some cases, are mining them.
If you examine a shatter cone, one of the first things you’ll note is the thin grooves in the rock. These grooves — call striae — form a unique branching, fractured pattern that look like a horse’s tail. Many shatter cones have a characteristic conical shape, with repeating conical patterns in the shatter cone.
The type of shatter cone formed and its specific characteristics depends on the type of rock the meteorite struck. The striae are easier to recognize in fine-grained material, such as limestone, and harder to see in coarse-grained material.
A massive impact 1.85 billion years ago created the Sudbury crater in Ontario, Canada. The blast formed the second-largest impact crater on Earth. Some of the energy from the explosion transferred to the rocks below the crater, forming shatter cones like this one.
Shatter cones can and will be altered over time by Earth’s natural processes. So it is possible a geologist might not easily identify an impact site as such due to erosion or other natural geologic processes.
Shatter cone sizes can vary from microscopic to some 33 feet (10 meters) in length. The largest shatter cone found was at the Slate Islands in Terrace Bay, Ontario.
Meteoriticists are still theorizing how shatter cones form. Theories include the idea that the impact wave compresses the rock. Or perhaps a rebound effect creates tension after the impact pressure subsides in the rock. Shatter cones were first described by Branco and Fraas in 1905 in the Steinheim impact structure in Germany. They initially attributed them to a volcanic eruption.
In my meteorite and crater research arsenal is a selection of shatter cones from various impact sites around the world. The variations, much like meteorites, are indicative of the areas impacted. Sometimes shatter cones are the only evidence that remains from impacts that occurred millennia ago. One does not see as many shatter cones on the market as meteorites and tektites. I try to obtain shatter cone samples — either in person at a site I am researching or in a trade — to give me a better picture of impacts. Plus, shatter cones represent the immense power of such an event, and they are something I can hold in my own two hands.