Menominee Crack: Michigan Researchers Finally Identify Mysterious Pop-Up Feature

In 2010, a massive split in the ground mysteriously opened just north of Menominee, Michigan. It appeared right after a loud boom sound that people in the surrounding area heard. Six years later seismologists from the Michigan Technological University have finally identified what the feature is: A geological pop-up.

The split, which measures almost 360 feet long and 30 feet wide at its largest point, formed in Oct. 2010 following a magnitude-1 earthquake. Aside from uprooting some trees and causing others to tilt, the crack poses no threat, according to a news release.

Pop-ups generally occur in places where the earth rebounds upward after being relieved of an overbearing weight from a removed quarry or a melted glacier. However, the Michigan pop-up appears to be "one-of-a-kind."

"We wanted to look into the crack because we could not find information in the literature on pop-up structures forming outside specific areas," Wayne Pennington, study leader from Michigan Tech, said in the university's release. "As far as we can tell, this is a one-of-a-kind event; but in case it is not, we wanted the information about it to be available for other investigators."

The first clue suggesting that the split was in fact a pop-up was that it occurred at the top of a ridge. Pennington and his team investigated the split by running seismic refraction tests. This involved measuring the speed of sound as it travels through layers of the earth and the wave's changes in direction through different rock types, fractures and other unseen variables by striking a large metal ball with a sledgehammer and capturing the sound waves. Ultimately, this revealed that the brittle limestone underlying the crack was highly fractured and bowed upwards, causing the ground above to separate or split. 

While researchers have identified what the Menominee Crack is, the cause of the crack remains a mystery -- and an especially puzzling one in a region not known for having earthquakes. Although pop-ups generally occur when a great deal of pressure is relieved, they can happen spontaneously. Researchers also suspect the removal of a large tree from the area might have something to do with it. Nonetheless, the feature is surely unique. 

Their findings were recently published in the journal Seismological Research Letters.

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