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The Q? Blog

Quarry Geology: Ancient Meteorite Impact in Indiana Benefits Us Today

by Adam Blankenbicker -- Aug 18, 2016
Photo of Adam Blankenbicker, a geology education specialist at the Museum

Adam is a self-proclaimed “volcanologist-turned-educator” who enjoys designing visitor experiences, especially around, you guessed it, geology. He has led...

Geologist Cari Corrigan, left, points out some features in Kentland Quarry in western Indiana. Photo by Don Hurlbert, Smithsonian.
Geologist Cari Corrigan, left, points out some features in Kentland Quarry in western Indiana. Photo by Don Hurlbert, Smithsonian.

I recently had the opportunity to travel with five geologists to unique sites around the country to understand more about the diverse geologic history around us. We visited active quarries which provided unobstructed views of large rock faces that reveal the stories of what has happened in Earth’s past. One of these trips took me to Kentland Quarry in western Indiana, with Smithsonian Geologist Cari Corrigan, to explore a long-ago meteorite strike that now benefits the people who live and work there.

Image iconnhb2013-02874.smaller.pngIt’s easy to get bored looking out the car window at fields of corn and tractors as you travel outside the town of Kentland, but the flat landscape belies the surprising geologic history hidden beneath the surface. Fortunately I was traveling with Cari, who sees the landscape through the lens of her meteorite research. When we entered the quarry, we came upon a giant hole. It was also one of several pits where the mining company, Rogers Group, has been quarrying limestone rock for roads, rail beds, and buildings in this part of the country for more than a century.

I asked Cari, “How do you know there was a meteorite strike here?” She was anticipating the question and answered me with a question of her own. “Why do you think other areas nearby are covered in hundreds of feet of sediment, yet at this site there is bedrock right at the surface?”

As a geologist myself, I knew the presence of limestone meant this was the site of an ocean millions of years ago. By analyzing fossils in the limestone and by comparing these rock layers to others, geologists have estimated this limestone was about 400 million years old. Much later, only some 12,000 years ago, most of the central United States was covered by hundreds of feet of sediments, deposited as the glaciers began retreating at the end of the last ice age. What I remained puzzled about was why the limestone at this site was at the surface, instead of covered with lots of sediment like everywhere else around.

“Think of a drop of water. When it lands in a puddle, it creates ripples that move away from where it hit. But where the drop hits, the water actually rebounds up,” Cari explained. Seeing the doubtful look on my face, she added, “When you have a large impact from something hitting bedrock, the rock can rebound back up like water, and then freeze into place.”

Image iconnhb2013-02977.smaller.pngEureka! I now understood the limestone we were looking at had rebounded up after getting impacted by a meteorite and hardened into the crater with a mound in the middle. Then, much later, as the glaciers receded, sediment filled the crater but left the high parts exposed.

As we walked around, Cari showed me a few other pieces of evidence. Apparently, a large impact is so powerful that it creates some features in the rocks you won’t find anywhere else. She picked up a light-gray, fist-sized rock and handed it to me. “Look closely at the surface and tell me what you see.” I looked at one side and just saw angular edges and faces. But when I turned it over, staring at me was a conical face covered in long, rounded ridges radiating out from a point. “That’s a shatter cone,” she said, “and this place is full of ‘em.” (The image at right shows a cliff face full of shatter cones.) A shatter cone only forms from extremely intense shockwaves.

Image iconshockedquartz.nmnh_.pngIt turns out that another type of impact evidence here is something called shocked quartz, or shocked sandstone (image at left). You can’t see all the details without a microscope, but when you hold a piece you can very easily scratch off grains of quartz. In your hand the rock is friable, or crumbly, but under a microscope you can see two sets of parallel lines where each quartz grain was deformed when it got hit really hard.

All of these clues point to a meteorite strike, but how was that strike important for the people who work in the quarry and use its products? Access! Without the meteorite strike, the limestone bedrock would have been covered by hundreds of feet of sediments deposited by glaciers. Thanks to an event estimated to have occurred many millions of years ago, the hard Indiana limestone is at the surface, already broken up and ready for use.

See more LIMESTONE and SANDSTONE in the Q?rius Collections Zone. Get a printable poster about this quarry story. Get a Journey in Time through Quarries poster that shows five geologists and the geologic histories they explore.

 

 

Categories: Q?rius News, Field Notes