What Causes Sinkholes To Open Up?
MINNEAPOLIS (WCCO) -- On Sunday morning, part of Kellogg Boulevard in St. Paul crumbled. It happened about 40 minutes after a water main broke below ground.
Sinkholes can be caused by a number of different things. So, what causes the ground to open up?
"They essentially happen because of the same cause," says Anna Lindquist, a visiting professor of geology at Macalester College. "In all cases, it's material being removed from underground and then the surface collapsing in."
Some sinkholes happen naturally, like after a heavy rainstorm. Others occur because of something humans did.
For example, there's 4-6 inches of pavement on Kellogg Boulevard, according to Steve Schneider with St. Paul Regional Water Services. Underneath the pavement is 4-6 inches of gravel and underneath that is seven feet of native soil. The water main lies below that soil, about eight feet underground.
When the 1890 water main broke, it sent water into the soil, displacing it and creating a hole underneath the gravel and pavement. Eventually, gravity set in and the pavement collapsed.
Naturally-occurring sinkholes often happen in places where limestone exists underground. The rain or groundwater slowly eats away at the limestone, dissolving it, and creating a hole. This can happen over months or centuries.
"The really dramatic ones will take hundreds of thousands of years," says Lindquist.
In Minnesota, there is limestone, but Lindquist says Minnesota is less likely to have naturally-occurring sinkholes because of the giant glaciers that existed 12,000 years ago. Those glaciers were heavy enough to have collapsed any large holes that might have formed.
"Southern Minnesota has some small sinkholes," she says. "But, nothing to worry about it you're building a house."