by Peggy Fisherkeller, Curator of Geology
Okay, this is a fictional headline, because there wasn’t anyone around to write it when the impact happened, sometime between 17,000 and 300 million years ago. But with spectacular recent events in Russia, a reminder of Indiana’s very own brush with obliteration is justified.
Kentland, Indiana, is home to one of the larger meteorite craters in the United States, with the area of ground disturbance coming in at more than 7 miles in diameter. You wouldn’t know it though, without the quarry that’s there now. Like most of northern Indiana, the ground surface is covered with glacial till.
The Kentland meteorite crater is part of a great mystery story, because geologists were only convinced that it was a meteorite impact within the past 40 years. Everyone knew that something was up, though, because the rocks there just weren’t right, with layers making a huge irregular bull’s eye pattern where only flat-lying rocks were supposed to be.
What really happened has been roughly sketched out, based on evidence at the site and comparison to more obvious crater sites from around the world. A (big!) meteorite struck with such great force that the ground beneath was compressed, bouncing back hundreds of feet higher than before (though the part that was above the ground has since eroded away). Preserved in these vertical, contorted rock layers were shattercones, pointing toward the direction of impact, micro-sized shocked quartz and brecciated rock (containing angular fragments).
Some mysteries remain. How big was the meteorite? Well, big. When did it strike? We know it hit after the Pennsylvanian Period (~300 million years ago), because the crater crosses through rocks of that age. We know that it happened before the end of the last glaciation, because glacial till is deposited on top. With chemical and physical techniques, researchers have done a little better, putting the impact maybe between 97 million and 300 million years.