The Glasford Structure

Picture this:  It’s the year 455,000,000 BC. It’s a warm day in Illinois. We know it was warm because Illinois was just below the equator then. Sun sparkles on the water’s surface. (Or perhaps the moon was shining. Who knows?) Water? What water? Illinois was totally covered with a shallow inland sea then. A bright light appears in the sky, and it’s getting brighter and brighter. Was it a star? Maybe a nova? There was no one there to wonder. It’s a meteorite, burning up as it enters our atmosphere. It slams into the Earth, just south of present-day Peoria, Illinois. And you thought nothing exciting ever happens in Peoria! 

The meteorite vaporizes on impact, but its energy creates an impact crater about 2-1/2 miles across. The force on the earth is so great that the interior of the crater rebounds like a trampoline, creating a dome. For one frozen moment, the water sprays from the crater, and then, with a roar, comes rushing back. Boiling, steaming, roiling water fills the crater. Then, all is quiet, and the next day, the sun will shine on the water again.

By meteor impact standards, this was small, not an extinction event. Perhaps many primitive sea creatures died. Perhaps the broken bodies of giant trilobites floated in the water, but then . . . time passed. Sands and sediments of the sea slowly filled in the crater. More time passed, and after some coming and going, the seas retreated from Illinois. Sand and mud left by the retreating seas turned to stone. Soil blew in from the eroding Rockies. Any sign of a crater was gone.

But it left behind an anomaly in the earth called the Glasford Structure. Deep well drilling in this spot revealed intensely disturbed, chaotic, and contorted rocks, and further investigation revealed the buried crater. 

This meteor impact may be part of something called The Ordovician Meteor Event. Somewhere around 468 million years ago, an asteroid in the asteroid belt broke apart and scattered fragments into Earth-crossing orbits. The result was at least six meteor impacts on the Earth, including, possibly, one in present-day Des Plaines, Illinois (near O’Hare Airport), where a Des Plaines Structure has been detected.

Thinking about meteor impacts on Earth can be a bit unnerving, and scientists suggest that at least 17 meteorites a day hit our planet. But it’s the way the Universe rolls and I guess we should get used to it! Meanwhile, as you can see by the photo below, all is peaceful in Peoria County . . . for now.

Photo: Charles O’Dale. Taken from “just inside the location of the South rim looking north toward the centre of the Glasford Structure.

Meteor, Meteoroid or Meteorite?

In the course of learning about the Glasford Structure, I learned that when a small body is moving through the solar system, and has the potential to hit a planet, it’s called a meteoroid. When a meteoroid enters the Earth’s atmosphere and burns up (a “falling star”) it’s a meteor. If the meteoroid survives its trip through the atmosphere and hits the ground, it’s a meteorite. Phew. I’m so glad I have that cleared up!

To learn more about craters, please check out Mr. O’Dale’s blog, the Crater Explorer.

3 comments

  1. Very cool stuff.

    Sometimes, in August, during the Perseid Meteor shower, we hike late at night up to a remote, elevated view point to sit back and watch the meteor shower. In total dark, at 2800 feet elevation, just staring up at the sky, you can see a meteor or two every minute. It is an awesome show.

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      • The first time we did this I saw orders of magnitude more shooting stars in two hours than I had in my whole life. There were three of us, looking up at the night sky, going ‘ooh!’ and ‘ah!’ every couple of minutes. Parenthetically, one of the guys is afraid of heights and this is a very steep trail where you pass within about three feet of cliff edges. Because we did it at night he was able to manage but during the day, he said, he would have been frozen. Magical!

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