What to know about unpredictable sinkholes, how they form and where they can happen
Rescuers were searching for a Pennsylvania grandmother who is believed to have fallen into a sinkhole while searching for a lost cat Monday evening.
Elizabeth Pollard, 64, apparently fell into a sinkhole near the Union Restaurant in Unity Township, about 40 miles southeast of Pittsburgh, Paste BN reported.
More than 100 people were involved in the search. Crews have closed several roads in the area to set up equipment and establish "a safe area for crews to operate," Unity Township fire officials said.
The search has revived national concern about the unpredictability of sinkholes. Here's what you should know about sinkholes, how they're created and where they can happen.
How do sinkholes form?
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A sinkhole is a hole or deep depression in the ground that opens up when layers of certain types of rock – usually limestone, carbonate rock and salt beds – dissolve beneath the surface and collapse.
Sinkholes usually form over time by a phenomenon called karst processes in which groundwater breaks down minerals in the underlying soluble rock. The dissolved minerals create spaces inside the rock, weakening it until it collapses.
Sinkholes can range in diameter from a few feet to hundreds of acres. Their depths can be less than 1 foot to more than 100 feet. Some have vertical walls, others look like saucers or shallow basins, and some naturally generate ponds by retaining water, according to the U.S. Geological Survey.
Where are sinkholes usually found in the US?
Sinkholes are common in Florida because of its limestone bedrock. Alabama, Kentucky, Missouri, Tennessee, Texas and Pennsylvania are prone to sinkholes as well, also because of their bedrock.
Nearly every state has limestone, gypsum and other soft rocks that are susceptible to a collapse. "In 99% of all cases, sinkholes pop up in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite)," the science news website ZME Science says.
Over the past 15 years, sinkhole damage costs an average of $300 million a year in the U.S.
There are several ways to identify a sinkhole in progress. Slumping or tilting trees or fence posts and small ponds of rainwater forming where water has not collected before can be warning signs, according to the USGS. In buildings, doors and windows that suddenly don't close properly can be another indication.
CONTRIBUTING James Powel and Alan Gomez
SOURCE Paste BN Network reporting and research; U.S. Geological Survey; CBS affiliate KDKA; American Geosciences Institute; ZME Science