Just north of Menominee, Mich., a crack the length of a football field zigzags like a stretch mark atop a gentle rise. The uplifted ground and its crack wouldn’t normally garner much attention, except that they suddenly appeared in this southern portion of Michigan’s Upper Peninsula after residents heard a boom.
That was back in 2010. “I had never seen anything like it,” said Wayne Pennington, a geophysicist at Michigan Technological University in Houghton, who first stopped by to see the crack shortly after it formed. Today, he and other scientists have published a paper in Seismological Research Letters about the odd feature and why it suddenly appeared.
When the crack first split open, Pennington’s in-box filled with emails about the boom. Local residents assured him that the crack had just appeared—after all, the trees pointing in odd directions on the ridge were growing straight up only a few days before.
In the video below, a Menominee resident describes the newly formed rise and crack:
Pennington initially suspected that a suddenly collapsing hill was to blame, but when he arrived on the scene and saw the 110-meter-long crack and newly formed ridge, he knew he was wrong. He and his colleagues have now concluded that the crack is a geological phenomenon called a “pop-up.”
The ridge’s tent shape “is the giveaway,” said Norman Sleep, a geophysicist at Stanford University in Stanford, Calif., who first suggested that it could be a pop-up after Pennington circulated pictures to the geophysics community. A tectonic fault couldn’t have caused it, or its shape would have been different—one side higher than the other, for instance. “There is often both horizontal and vertical movement on a tectonic fault,” explained Sleep, who wasn’t involved in the new paper.
Pop-ups occur all over the world, usually recognized as a response to a glacier’s retreat or to large excavations by people—for example, at the base of a quarry after huge amounts of rock were removed, Pennington said.
The Earth’s crust is, in general, being squeezed by the weight of the rock above it, Pennington said. Imagine pushing down on a ball of dough with both fists—the dough under your fists wants to push sideways because of the overlying pressure. “If you remove the weight of the rock above one part of that, then neighboring rocks are still being squeezed” from above and the side. No longer prevented from moving upward, the rocks where the weight was removed can pop up as a new hill or ridge, he continued.
To confirm the feature was indeed a pop-up, the team investigated the underlying limestone using seismic waves, specifically, by slamming a sledgehammer against a metal ball and measuring how fast the waves took to bounce back from the limestone. The team found that the seismic waves traveled slower perpendicular to the crack compared to the waves traveling parallel to the crack. The speed difference suggested that waves moving perpendicular to the crack were traveling across rock fractures, not visible at the surface, consistent with the structure of a pop-up.
But the question still remained—how did the pop-up form? There was no quarry in the area, and the last glaciers retreated over 10,000 years ago, Pennington said. “In our search of the literature, we couldn’t find any example of what appeared to be a naturally occurring pop-up in contemporary times without an obvious cause.”
So the team set out to find any explanation they could, scouring the area for any recent excavations or other clues.
Coincidentally, just a few days before the pop-up appeared, a bolt of lightning struck and damaged a 2-ton white pine tree nearby, Pennington discovered. Residents of Menominee then harvested the tree for firewood, and Pennington thinks that the removal relieved some of the pressure pressing down on the rock.
But the removal of a tree, even a heavy one, is a rather small shift in weight to have triggered a pop-up, Pennington said. When investigating aerial photos, the researchers found another interesting feature: the remnants of a ditch or a pit parallel to the road near the pop-up. Pennington suggests that rainwater falling into the ditch could have soaked into the ground and weakened the surrounding limestone over years and years. This weakened rock might have then been more susceptible to fracturing from even subtle shifts in overlying loads, Pennington said.
Despite these findings, the pop-up’s origins remain a mystery, Pennington said. He hopes that his team’s newly published work will help other scientists who discover pop-ups separate from glaciers or quarries to come up with more definitive answers.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2016), What makes the ground suddenly pop?, Eos, 97, doi:10.1029/2016EO045779. Published on 9 February 2016.