Unusual rock formation leads to investigation in Michigan.
When you have geologists involved, every rock, hole in the ground, or fissure has a story to tell and a recent rock formation in Michigan is under scrutiny for its unusual appearance. Back in 2010, according to a story on eurekalert.org a crack formed in the surface of the Earth north of Menominee, Michigan, located in the state’s Upper Peninsula area.
For certain, Michigan is not known for seismic activity, and the formation of the crack registered less than a magnitude 1, but was the first recorded earthquake in the state’s history. Of course, that piqued the interest of scientists in the area, and a research team from Michigan Technological University went to investigate.
The team surveyed the area and took seismic refraction tests and verified the formation was a pop-up structure. Pop-ups occur when a large weight is removed from the surface, allowing the earth and rocks to rebound underneath the missing weight. Pop-ups have almost always been found where giant rocks have been removed through quarrying, or where glaciers have receded while melting.
Calling the Menominee Crack a one-of-a-kind feature, team leader Wayne Pennington, dean of the College of Engineering at Michigan Tech, said the group has confirmed the crack was caused by highly fractured limestone that had bowed upwards, fitting the definition of a pop-up.
Pennington, who said the team wanted to investigate because they could not find any literature about pop-ups forming outside specific areas, noted the crack was not as important as the accompanying ridge, formed during the upheaval. He explained the ridge was an indication of a deeper structure.
But what caused the release of downward pressure that formed the crack in the first place? There were no quarries in the immediate area around the formation, and the last of the glaciers vacated the area over 11,000 years ago.
Pennington says there was a large pine that had to be removed after toppling about the same time as the pop-up formed, and that certainly provided an interesting coincidence. But still, he admits they can not be certain the tree removal contributed to the pop-up formation, and they find the appearance of the crack quite puzzling, especially in an seismic zone.
Pennington adds, “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.”