Twin Craters in Central Europe Formed in Distinct Events

The Nördlinger Ries and Steinheim Basin in Germany were long considered an impact crater doublet. Left: View across the Ries crater (diameter 15 miles, age 14.8 million years) with St. George’s church in Nördlingen, built from impact rock (suevite), near the center of the image. Upper right: The Steinheim Basin (diameter 2.5 miles) with a prominent central uplift seen from the southern crater rim. Photos by Martin Schmieder. Lower right: Ries-earthquake seismite (bottom, partly in shadow) capped by a layer of Ries ejecta (subhorizontal in the lower half of image, with limestone pebbles) overlain by younger sediments (top), in turn, cross-cut by a prominent, 2 inch-wide clastic dike (vertical, near center) linked with the Steinheim earthquake that followed the Ries impact some hundred thousand years later. Photo by Volker Sach.

For decades, the 15-mile-wide Ries crater in Germany and the 2.5-mile-wide Steinheim Basin, located twenty-five miles to the southwest, have been regarded as an example of an impact-crater doublet. According to this hypothesis, an asteroid pair slammed into the Earth 14.8 million years ago in the mid-Miocene epoch creating the two craters simultaneously. However, a new study by geologists Elmar Buchner (Neu-Ulm University of Applied Sciences), Volker Sach (Sigmaringen, Germany), and former LPI postdoctoral fellow Martin Schmieder offers evidence for an alternative hypothesis.

Investigation of two separate layers of seismite — rock chaotically deformed by a seismic wave — found within 120 miles of the impact craters suggest there were two major earthquakes that rocked the region. The lower seismite unit is capped by pebbles launched from the distant Ries impact site, indicating that the seismite is the direct result of an impact-triggered earthquake. The upper seismite unit consists of dikes that formed when liquefied rock squeezed upward from a water-rich bottom layer into open fractures as the Earth’s crust was still shaking, and reveals a second, distinct disaster. These dikes crosscut the Ries-seismite unit, its ejecta cap, and a few meters of the overlying layers, effectively postdating the Ries impact and its earthquake. These dikes are likely related to the Steinheim impact, which seems to have occurred a few hundred thousand years after the Ries event, thereby contradicting the popular theory of two simultaneous impacts. Independent paleontological evidence supports this hypothesis: the deepest and oldest crater-lake deposits studied inside the Steinheim Basin seem to be several hundred thousand years younger than those at the Ries crater.

The Ries and Steinheim craters may have, finally, been divorced from one another. However, this finding is not entirely surprising. Previous crater-age studies at other impact structures worldwide have shown that most impact craters that were once considered crater doublets or even ‘chains’ of multiple impact craters turned out to have different geologic ages, making them unrelated to one another. READ MORE