The 2010 meteorite
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 meteorite 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 this 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 suggests 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 was 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 14, 2010, and the small piece of the meteorite that was found makes the 4-mile-wide crater created by the Rock Elm meteorite that much more spectacular to contemplate.
News report on the 2010 meteorite.
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 diagrams 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 points 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 Rock Elm 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.
Scientific papers about the Rock Elm impact structure
—Rock Elm structure, Pierce County, Wisconsin: A possible cryptoexplosion structure, William S. Cordua, 1985, Geology 13:372–374
—The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz, Bevan M. French, William S. Cordua, and J.B. Plescia, GSA Bulletin, January 2004, 116:200–218
—Geology of the Rock Elm Complex, Pierce County, Wisconsin, William S. Cordua and Thomas J. Evans, 2007, WGNHS Open-File Report 2007-02.
Bill Cordua explains the Rock Elm structure in this video.
Other meteorites in Wisconsin
Other impact structures
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.
Other meteorites in Wisconsin
Wisconsin has been hit by at least 13 meteorites since the 1860s. The meteorites ranged in size from less than 1 pound to around 530 pounds.
Meteorites are called “falls” when the meteorite was observed falling to the ground; those recovered later are called “finds.”
Click below to see the list of where meteorites have fallen in Wisconsin. (Adapted from the Minerals of Wisconsin database)
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Locations of documented meteorite falls and finds in Wisconsin
Algoma: A meteorite massing nearly 9 pounds was found during plowing about 4 miles west of Algoma (Kewaunee County). It contained 88.6% iron and 10.6% nickel. This meteorite is now on display at the Geology Museum at UW–Madison.
Angelica: Approximately 3 miles north of Angelica (Shawano County), a 33-pound iron meteorite was found in 1916 during plowing.
Belmont: A stony meteorite weighing 58.28 pounds was found in 1958 near Belmont (Lafayette County). It contained mostly bronzite and olivine, but also about 23% iron-nickel alloys. The Belmont meteorite is currently in the Geology Museum at UW–Madison.
Colby: Two stony meteorites with a combined weight of over 200 pounds fell at 6:15 P.M. on July 4, 1917 near Colby (Clark County). Samples are in the Geology Museum at UW–Madison.
Hammond: An iron meteorite massing almost 53 pounds was recovered from a plowed field near Hammond (St. Croix County) in 1884. Troilite was present in nodules and as fracture fillings. Chemical analysis gave 89.78% iron and 7.6% nickel with trace amounts of cobalt, phosphorous, silicon, carbon, copper and tin.
Kilbourn: A 772-gram stony meteorite fell through a barn roof near Kilbourn (Columbia County) at 5:00 P.M. June 16, 1911. Samples and a replica are in the Geology Museum at UW–Madison.
Mifflin: Named after Mifflin Township in southwest Wisconsin, this stony meteorite fell on April 14, 2010. Several pieces, the largest of which weighs 142 grams, are on display in the Geology Museum at UW–Madison.
Oshkosh: A 4-ounce fragment of stony meteorite was found on gravel approximately 2 miles NW of Oshkosh (Winnebago County).
Trenton: A number of large pieces from an iron meteorite have been found in Trenton township, east of West Bend (Washington County). Fragments were first found in 1858, with more turning up in 1873. A detailed search of the area with a metal detector revealed more fragments in 1952 and 1964. At least 13 fragments had been found so far. The largest chunks weighed 527 pounds and 413 pounds. Sawn and treated fragments show troilite nodules and lenses. Specimens from this meteorite are on display at the Geology Museum at UW–Madison.
Vernon County: The Claywater Meteorite was observed to fall at 9 A.M. on March 25, 1865. It came in as a rotating fireball and exploded near ground level. Two fragments with a combined mass of 3.3 pounds were recovered. The meteorite was mostly stony, containing olivine and enstatite. It also contained about 17% iron-nickel alloys.
Waushara County: An iron meteorite with stony inclusions, nicknamed the Pine River Meteorite, was found in 1894 near Saxeville. Its mass is nearly 8 pounds. This proved to be an unusual meteorite for the octahedrite with many silicate inclusions and an “anomalous member of chemical group IA.” The silicate inclusions consist of granular crystalline intergrowths of orthpyroxene, olivine, plagioclase feldspar, troilite and iron nickel metal with accessory chromite, diopside and schreibersite.
Waushara County: A stony meteorite massing 1.5 pounds was found near Mt. Morris. The Mt. Morris Stone is a coarsely crystalline sulfide rich stone with forsterite, enstatite, kamacite, schreibersite, troilite, graphite, chromite, daubreelite and chalcopyrite. Later detailed geochemical analysis suggests that the Mt. Morris meteorite is a fragment from the Pine River Meteorite.
Zenda: The Zenda Meteorite was found in 1955, about a half-mile west of Zenda (Walworth County). It massed 8.16 pounds and was deeply oxidized. It was an iron meteorite containing kamacite, schreibersite patches, troilite, taenite, and lawrencite. It also contained minor graphite, olivine and pyroxene.
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.
I think I found a meteorite!
Please note that WGNHS does not offer meteorite testing and is unable to identify meteorites.
Genuine meteorites are extremely rare finds, so the odds that a rock you found is a meteorite are incredibly slim. If you think you’ve found a meteorite, visit the resources we’ve listed below. The first one on the list is particularly helpful because it offers a plethora of information, photos, and guidelines to help you learn more about whether your rock could truly be a meteorite, but all of these resources are worth perusing.
Some Meteorite Information (https://sites.wustl.edu/meteoritesite/)
From Washington University in St. Louis. Don’t be fooled by the modest title — this site offers a comprehensive collection of information, photos, and a checklist to help you learn more about whether your rock is a meteorite.
Meteorites (and Meteowrongs) (https://www.ualberta.ca/science/meteorites/)
This page from the University of Alberta includes a short list of questions and a video on identifying potential meteorites
Have you found a space rock? (https://geology.com/meteorites/meteorite-identification.shtml)
This introductory guide to meteorite identification was written by Geoffrey Notkin of Aerolite Meteorites.
Other meteorite resources
More about meteorites
Geology Museum, UW-Madison (https://museum.geoscience.wisc.edu/)
The museum houses a collection of many of the meteorites found in Wisconsin.
Center for Meteorite Studies, Arizona State University (https://meteorites.asu.edu/)
Their website has educational resources about meteorites as well as photos highlighting some of their collection of more than 40,000 specimens.
Rocks from Space: Meteorites and Meteorite Hunters (second edition), by O. Richard Norton, 1998, Mountain Press Publishing Company, 447 pages
An exhaustive but approachable discussion of meteorite science. Covers meteorite discoveries and profiles the individuals involved.
Meteorites lesson plan, WGNHS (https://home.wgnhs.wisc.edu/meteorites-lesson-plan/)
This free lesson plan uses meteorites to help students learn about the scientific method.
Meteorite impact craters
Terrestrial impact craters (www.solarviews.com/eng/tercrate.htm)
Includes a photo gallery profiling over a dozen of the world’s largest impact craters.
Earth Impact Database, Planetary and Space Science Center, University of New Brunswick
A database of all confirmed meteorite impacts in the world with reference lists for each impact site.
Barringer Meteor Crater, Arizona (barringercrater.com)
At 50,000 years old, the Barringer crater is still quite fresh, geologically speaking; it shows none of the weathering seen at the Rock Elm impact site in Wisconsin.