Millions of years of geological activity and marine ecology culminated in the the sudden appearance of a 50-foot hole under a Florida suburban household Thursday.
Sinkholes similar to one that swallowed 36-year-old Jeff Bush’s bedroom and, tragically, him along with it in the Tampa suburb of Brandon, are common in Florida, experts say.
Ironically, it’s because the Florida peninsula is one of the world’s most stable areas.
Florida sits atop a plateau that extends more than 100 miles west and 50 miles east of the peninsula.
Surrounded by ocean with little volcanic activity, a thick layer of soft limestone has gone virtually undisturbed below the surface. The remains of millions of years of sea life fused underground, adding another fine layer of material.
The geological condition is called karst, and it means that what lies beneath the surface in Brandon and much of Florida is essentially the same material chalk is made from, said Stephen Kish, a professor with Florida State University’s Department of Earth, Ocean and Atmospheric science.
When that karst layer meets rainwater that has absorbed carbon dioxide – making it slightly acidic – it dissolves.
“It’s like a roof that has a weakness in it,” Kish said. “Eventually, the roof caves in because of weakness and it can’t support its own weight.”
At the surface, the sinkhole Bush fell into was only about 15 feet wide. But engineers determined it is more likely about 50 feet wide underground, and 50 feet deep.
Usually, sinkholes give warning signals by some early sinking, Kish said. A level street may dip 6 inches one day before dropping 6 feet the next.
“Apparently he wasn’t that lucky,” Kish said.
Kentucky and parts of Texas have similar geological conditions, he said. California, on the other hand, sits atop millions of layers of volcanic rock. Any fragile layer of limestone has been overwhelmed by more solid elements.
A Florida database of sinkholes statewide shows nearly a dozen in Brandon between 1979 and 1988, but none recorded in at least the last 30 years. A fire department official for Hillsborough County, which includes Brandon, said sinkholes are rare there
The largest sinkhole in county history was more than 200 feet wide and occurred in Sulphur Springs in 2005, according to the Florida database.
Kish said sinkhole detection is an inexact science, though microwaves sent through the ground can find caverns. The process is too costly to use for single-family homes, he said.