Late in the evening of April 14, 2010, a meteorite streaked across the sky in western Michigan, northern Illinois, southern Wisconsin and eastern Iowa. As the meteor plowed through the earth’s atmosphere, at speeds approaching 70,000 miles/hour, it created a fireball that briefly lit the night sky and caused a sonic boom that rattled windows.
The bright light of the falling meteor was the result of the extreme temperatures created as the rock moved through the atmosphere and was superheated by friction. The bright flash observed in videos of the event suggest that the meteor disintegrated explosively above the earth’s surface and, thus, fragments of the meteor may be strewn over several locations. (Small pieces were reported to have been found in southwest Wisconsin and a piece has been loaned to the Department of Geoscience, University of Wisconsin-Madison, for verification and study.)
The earth receives a near-constant “rain” of meteors entering its atmosphere with most being destroyed before reaching the earth’s surface. The excitement created by the events of April 14th and the small piece of the meteorite found to date makes the 4-mile-wide crater created by the Rock Elm meteorite (see below) that much more spectacular to contemplate.
The Rock Elm meteorite
Sometime around 465 to 475 million years ago, a meteorite streaked across the sky, plowing through the shallow ocean and slamming into the earth at a speed of close to 70,000 miles/hour. It came to rest in what is now west-central Wisconsin.
The moments following impact
The force of the impact created a fireball that was 25 times brighter than the sun and left behind a 4-mile-wide crater in the carbonate rock. The impact blew water, sediment, and rock high into the sky. The center of the crater rebounded from the initial impact, sucking up rock from 1000 feet below the surface. Disrupted rock, sediment, and water slopped back in, depositing broken material around the edges of the new hole.
Within seconds and minutes, the land was transformed in ways that millions of years of deposition, erosion, and even the passage of mountain-leveling glaciers, could not fully erase.
The following images show the sequence of events following a large meteorite strike in Chesapeake Bay (courtesy USGS). Events at Rock Elm would have followed a similar pattern.
A curious fact about meteorites
Meteorites come blazing into our atmosphere as glowing balls of fire. So when they land, you’d expect to have to let them cool off before touching one, right? Not necessarily.
Residents of Colby, Wisconsin, found a large meteorite shortly after it hit and described it as becoming quickly coated with frost, even though it fell on a hot summer day. But when you consider that a meteorite spends eons hurtling through the deep freeze of space and less than a minute entering our atmosphere, it’s not too surprising that the inside may still contain residual cold. So although the outer shell has been turned into molten rock by the fiery entry, the icy interior must gradually warm up to air temperature.
The Rock Elm impact site today
In the millions of years following the impact, as much as 800 feet of sediment was deposited and subsequently eroded. Much of the original evidence from the impact has been stripped away. Still, the jumble of rocks point to a violent moment in their history.
To a geologist’s trained eye, the site in east-central Pierce County forms a striking circle that is 4 miles across. (To the casual observer, the features are much less apparent.)
Rocks immediately outside the impact area consist of an undisturbed, flat layer of dolomite. Known as the Prairie du Chien Group, these rocks are estimated to be around 472 to 488 million years old.
The area inside the circle can best be described as a mess. There are two distinct zones: the upraised center and the basin ring surrounding it.
The upraised center is an oval shape that’s 0.5 miles wide by 1.5 miles long and 180 to 200 feet above the lowest level of the feature’s floor. The center is composed of breccia (rocks containing fragments of other rocks) along with scattered blocks of Mt. Simon Sandstone. The sandstone was sucked up from deep below the surface immediately following the impact and dates back to 500 million years old, making it about 25 million years older than the other material in the area.
Outside the upraised central area lies the basin ring. Along the southeastern wall of the ring sits a curved band of large broken blocks of Prairie du Chien dolomite. These chunks would have broken off and fallen into the crater following the impact. The ring is also filled with a layer of shale and sandstone sediment that’s 150-feet deep and dates back to 461 to 472 million years old (Middle Ordovician period).
How did scientists determine when the meteorite struck?
You can tell a lot by looking at the rocks. The age of the rocks gives geologists clues to when the meteorite strike could have occurred. In this case, because blocks of Prairie du Chien dolomite were scattered along the edge of the basin, we know they were present when the meteorite struck. So the meteorite can’t be any older than 472 to 488 million years old.
The next clue was the age of the sediment filling the bottom of the basin. The oldest undisturbed sediment in the basin marks the youngest possible age of the impact. The oldest shale and sandstone sediment dates back to between 461 and 472 million years old.
The meteorite, then, would likely have struck between 465 and 475 million years ago.
Bill Cordua explains the Rock Elm structure in this video.
Other meteorite impact structures in Wisconsin
To date, we’re aware of only one other possible meteorite impact site in Wisconsin. Glover Bluff, nicknamed Mystery Hill by generations of geology students, is located in northern Marquette County, 4 miles south of Coloma. As with Rock Elm, no meteorite was ever found. Instead, we have evidence of a circular impact area that’s approximately 1 mile across with steeply tilted faulting and complexly jumbled rock layers. Additionally, rare specimens of dolomite displaying shatter cones have been found on the site. Shatter cones are attributed to very high-velocity impacts.
Is the large circular area in western Wisconsin an impact structure?
A large circular area sandwiched between Eau Claire and La Crosse in western Wisconsin shows up on satellite images as apparently being distinctly different from the surrounding terrain.
Was it caused by a meteorite? In a word, no. The area is bounded on three sides by curving river segments: the Chippewa River to the northwest, the Mississippi River to the southwest, and the Black River to the southeast. The northeast “boundary” is marked by relatively low landscapes that likely formed on ancient glacial materials. The bedrock geology both within and surrounding the feature consists mainly of relatively flat-lying sandstone formations that show no signs of folding, faulting, or other catastrophic disturbance. The river segments do not appear to follow any sort of fault structure of the type that might have been caused by a massive meteorite.
While this intriguing topographic feature shows up nicely on satellite images, there’s no evidence of an extraterrestrial origin.