Sinkholes: Why Does the Ground Sometimes Just Disappear Right Beneath Us?
By Rebecca J. Rosen in Share 2 Mar 1 2013, 2:15 PM ET 13
And why does it always seem to happen in Florida? A 60-feet-deep. 50-feet-wide sinkhole opened up in Orlando in 2002. The sinkhole swallowed two trees and forced dozens of resident to evacuate. Late last night, in the Tampa suburb of Seffner, the ground gave way beneath a house, and like that, a man was lost and a family bereaved. As biblical as the story sounds, the collapsing Earth was no act of god. Florida’s peninsula is unstable terrain by dint of its particular geology: a bed of limestone is slowly wasting away beneath the soil, taking trees, houses, and lives with it, collapse by collapse.
What feels capricious to those above is the toll of an active planet, one of those improbable collisions of a human timescale and a geological one. On this tailbone of our continent, those collisions become headlines: “Florida Sinkhole Swallows a Home” and “Sinkhole Horror: Family’s Florida House About to Be Swallowed” and “Florida Sinkhole Swallows Toyota Camry.”
Here’s what’s going on underground: The entirety of Florida sits on a bed of limestone, covered in varying degrees by composites of sand, clay, and soil. Limestone is soluble and porous, and over millions of years, acids in water have sculpted out a network of subsurface voids beneath the Floridian ground (think: Swiss cheese). Depending on how strong that top layer of clay and sand is, and how close to the surface any one of those voids is, the land can bear its own weight and that of the infrastructure we build on top of it. But as the holes grow, that surface layer can suddenly give way. According to the USGS, “Many of the numerous lakes and ponds of west-central Florida occupy depressions formed by overburden materials settling into cavities in the underlying limestone.”
Both drought and rain can herald collapses. During long periods of drought, groundwater tables will drop, and caves that were once supported by the pools they had collected become weaker. (Tapping into groundwater for agriculture can have much the same effect.) When rain eventually comes, the additional weight of a soaked-through top layer can become too much for a cave to bear. Florida sees different sinkhole patterns across the state depending on the thickness of that upper strata. In areas where it is thin and sandy, sinkholes are rare and small, as the ground is not strong enough to provide cover for a massive hole below. Where the top layer is thicker and made of clay, the ground can hold its own for millions of years as giant caves form below. Until one day when it can’t. Map of active karst topography in the U.S. (USGS) Geologists call such a landscape karst topography, any place shaped by dissolving bedrock, typically limestone but also other soluble rocks such as dolomite, rock salt, and gypsum.
And it’s not just Florida: According to the U.S. Geological Survey, karst landscapes account for 10 percent of the Earth’s surface. Around the world, some of the most beautiful and strange natural features are the result of this process, the not-so-solid Earth slowly, slowly dissolving away, leaving us with the caves of Slovenia, the hills of Ireland’s western coast, and the pillars of Guilin, China: