A fossilized shockwave
Shatter cones are the only macroscopic geological feature accepted as definitive proof of a meteorite impact. They are conical fractures that form in bedrock subjected to the immense shock pressures of a hypervelocity impact, ranging from 2 to 30 gigapascals (GPa). This is a pressure hundreds of thousands of times greater than Earth's atmospheric pressure at sea level. The intense energy of the shockwave radiates through the rock, creating a unique pattern of stacked, cone-in-cone fractures. These features are often marked with fine grooves, or striations, that radiate from the cone's apex, resembling a horsetail.
These features can form in various fine-grained rocks, including limestone and quartzite. Their size varies dramatically, from microscopic to several meters long. The largest known shatter cone, located on the Slate Islands in Ontario, Canada, measures over 10 meters in length. The orientation of the cones is a important piece of evidence for geologists. The apex of each cone points toward the source of the shockwave—the center of the impact. By mapping the orientation of shatter cones across an area, scientists can pinpoint the location of an ancient impact event, even after billions of years of erosion have erased the original crater.
Reading the crater record
The connection between shatter cones and cosmic impacts was first established by American geologist Robert S. Dietz in 1947. While studying the Kentland structure in Indiana, he recognized these unique fractures and proposed they were diagnostic of an impact origin. This discovery was instrumental in re-evaluating many mysterious geological structures around the world. Previously, circular formations were often attributed to obscure volcanic activity, known as cryptovolcanism. The presence of shatter cones provided a clear, verifiable criterion to distinguish impact craters from terrestrial formations.
One of the most significant sites for shatter cone research is the Vredefort Dome in South Africa, the deeply eroded remnant of the largest verified impact structure on Earth. Formed over 2.023 billion years ago, the original crater is estimated to have been up to 300 kilometers in diameter. The discovery of abundant shatter cones in the Vaal River bed was important to confirming Vredefort's impact origin. Other prominent examples are found in the Sudbury Basin in Canada, a 1.85-billion-year-old impact structure, and the 15-million-year-old Ries Crater in Germany. The study of these formations allows scientists to understand the violent history of Earth and the solar system.