A strange crack that suddenly appeared in rural Michigan five years ago may have an explanation, although even the geologists proposing it admit a lot of uncertainty remains.
It sounds like something out of "Doctor Who," but in October 2010, a large crack appeared north of Menominee, Michigan. The formation is 110 meters (360 feet) long, 0.7 meters (2 feet) across in places and up to 1.7 meters (5.6 feet) deep. For parts of its length, the ground rose 2 meters (6.6 feet) to form a ridge. The crack's arrival was accompanied by a loud boom and the first (albeit tiny) earthquake ever recorded on the Upper Peninsula, which shook nearby houses.
Since then, the mystery has deepened with similar booms heard in nearby parts of Wisconsin.
The crack has done no harm, besides tilting some trees at dangerous angles, and has even become a small tourist attraction. However, if it had occurred in a built-up area, it might have done considerable damage.
A section of the crack. Wayne Pennington/Michigan Technological University
Dr. Wayne Pennington of Michigan Technical University has identified the crack as a “pop-up,” a consequence of rock layers rebounding after being weighed down by ice. The curious aspect is that glaciers left the Upper Peninsula 11,000 years ago.
"One of our reasons for publishing this was that in our search of the literature we could find no other mention of modern pop-ups that didn't occur at something like the base of a quarry, where people had removed massive amounts of rock earlier," Pennington explained in a statement. However, discussion with locals and the wooded nature of the area makes clear nothing like that happened at Menominee, leading Pennington to say, "As far as we can tell, this is a one-of-a-kind event."
The ridge is a layer of limestone that previously lay deeper within Earth, typical of other pop-ups, and has other familiar features like the bending of seismic waves indicative of brittle limestone. Pennington said the question of what caused it to push through the dirt above has “became the puzzle.”
In Seismological Research Letters, Pennington presents a theory that he thinks has a 60 percent chance of being correct. He proposes that the melting of long-gone glaciers primed the limestone almost to breaking point, where it waited for a final push. The final straw was provided by the toppling two weeks earlier of a double-trunked pine estimated to weigh 2,000 kilograms (4,400 pounds).
A close-up of the crack..Wayne Pennington/Michigan Technological University
“Since we haven't seen this kind of thing elsewhere, and the tree is such a small effect, we wonder if there might be something else,” Pennington said. One alternative theory is that an unexplained rectangular feature seen in old satellite images channeled rainwater beneath the soil, weakening the rock during winter as it froze and thawed. Both the tree and the rectangular feature were near the crack's northwestern end.
"It may be a one-of-a kind phenomenon," Pennington said. "But if it happens again, we'll be all over it, trying to figure it out."
