A sinkhole measuring 150 feet wide and about 60 feet deep is shown June 12, 2002 a day after it opened near the Woodhill Apartments in Orlando, Fla. A spate of new sinkholes have recently opened up near Tampa. Chris Livingston/
- Persistent cold weather led Florida farmers to spray with groundwater to protect plants from freezing.
- A rapid drop in groundwater levels caused soil to drain into underground cavities, forming new sinkholes.
- Homes and highways can collapse into the sudden sinkholes.
The unusually persistent cold weather that's been so damaging to Florida's citrus crops this winter has also led, strangely enough, to the sudden opening of at least 22 new sinkholes, including one under an interstate highway.
Historically, sinkholes have not been a big problem in agricultural areas. But now there are more homes built in these areas, and that means more chances that a sinkhole will undermine a foundation and render a million-dollar house uninhabitable.
The connection of cold weather to sinkholes is a tale of two colliding phenomena: Florida's naturally cave-riddled ground and fruit growers' financial need to get fruit to market first.
"The real dramatic area (for sinkholes) is the Dover area east of Tampa," said geologist Mark Stewart of the University of South Florida.
That's where lots of strawberries are grown in winter so they can beat the California strawberries to the market in early spring. "It's a very, very big agricultural thing," he explained.
When freezing weather dips into Florida, which it usually does a couple of nights every year, growers protect the strawberry plants by pumping groundwater from wells and spraying it over the plants. When the water turned to ice, it releases heat, which is just enough to protect the plants from any serious frost damage, Stewart said.
The problem is, this year there was a whole week of freezing weather, and that meant a lot of water had to be pumped.
"This year we've seen a 60-foot drop in the aquifer," Stewart told Discovery News. That all-time record drop leads to change underground that can lead to large patches of ground suddenly dropping away -– a.k.a. sinkholes.
Just how the pumping of water triggers the sinkholes to form has everything to do with pressure and where water is headed underground, explained engineer Shiuo-San Kuo, director of the University of Central Florida's Florida Sinkhole Research Institute.
The first thing to understand, explained Kuo, is that the limestone under most of Florida is riddled with millions of cavities created by water over millions of years. Pumping water today does not cause the cavities to form. What the cavities do, however, is provide a void into which soil atop the limestone can suddenly drain into, through holes or cracks.
The reason why sinkholes aren't forming every day is that the sandy, silty material that's draining into holes in the limestone tends to create a funnel of soil crowding downward. That, plus the weight of the soil above, causes a soil plug to form that stops up the hole and forestalls a new sinkhole from forming above, Kuo said.
"Soil will stop in that funnel because of friction," said Kuo, referring to the grain-on-grain friction in the soil that keeps it from flowing like water. To unstop the funnel, something has to counteract that friction.
"When they are pumping water, the water table becomes low and creates downward seepage pressure," said Kuo.
That downward seepage can loosen or wash away soil plugs, which then clears the way for a lot of soil to quickly drain into the cavities, causing large dimples –- or sinkholes –- to form on the Earth's surface.
Ironically, said Kuo, very large buildings like those in downtown Tampa do not tend to get sinkholes forming under them. The reason? The great weight of buildings packs the soil underneath them very tightly, making the soil plugs all the more dense and harder to loosen.
But where smaller structures like homes are intermingled with agricultural lands, sinkholes are quite common and "now we have this conflict," observed Stewart.