|from Sinkholes, West-Central Florida USGS|
Sinkholes can vary from small shallow depressions in the earth to holes that are hundreds of feet deep and cover hundreds of acres. Some sinkholes even hold water and form natural ponds and lakes. Typically, sinkholes form so slowly that little change is seen in one's life- time, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic and devastating effect if it occurs in an urban or suburban setting as recently happened in Hillsborough County near Tampa, Florida when a sinkhole opened beneath a house swallowing a man and his bedroom. The body was never recovered and the house was demolished. Western central Florida has a long history of sinkholes and because of geology and groundwater pumping is particularly susceptible to sinkholes. In the water well fields for St. Petersburg located in Hillsborough County and surrounding counties sinkholes have occurred in conjunction with development of each of the well fields as well a throughout the region. Sinkhole formation is highest during dry months of the year and during drought, but overall appear to be increasing in frequency according to the U.S. Geological Survey, USGS, though there might be some reporting bias to the data.
A landscape that forms sinkholes, sinking streams, caves, and springs is called a karst landscape. A karst landscape most commonly develops on limestone, but can develop on several other types of rocks, such as dolostone (magnesium carbonate or the mineral dolomite), gypsum, and salt, which are types of evaporates rocks. Rain is naturally mildly acidic, and slowly over time the weakly acids rainwater dissolves these deposits creating fissures. The deposits are highly permeable, and surface water passes through them quickly to underlying aquifers, eating away at the limestone and evaporates bedrock. Overtime this creates the voids that become sinkholes. There are three general types of sinkholes: dissolution sinkholes—depressions in the limestone surface caused by the erosion of limestone by rain; cover-subsidence sinkholes—formed as overburden materials gradually fill below surface fissures formed by the infiltration of rain; and cover collapse sinkholes—which occur in limestone terrain with a thick overburden or mantle after the fissures forms large cavities and the cover materials collapse into the subsurface voids. This third type of sinkhole is what occurred last week near Tampa.
Hundreds of collapse sinkholes of various sizes occur throughout the country each year and start unnoticed when infiltrating water or groundwater flowing in the subsurface creates a void where soil is washed away. Eventually the void or hole grows large enough that the soil above it can no longer bridge it. The soil bridge then suddenly collapses into the void below and a sinkhole forms. Often this happens when the water level that has been exerting an upward pressure- helping to hold up the soil bridge falls. This process usually takes many years to occur in nature, but it can be aggravated by human activities. Any activity that increases the amount of water flowing into the subsurface can speed up this process. Parking lots, streets, altered drainage from construction, irrigation, leaking swimming pools and roof guttering are some things that can increase runoff; even severe weather can cause sinkholes.
The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania thought large areas of the United States are underlain by evaporates rocks (salt, gypsum, and anhydrite) and carbonates (limestone and dolomite), the rock types that are most susceptible to being dissolved away by water. Even when evaporite rocks are buried at great depths, in so called Mantled Karst terrain sinkholes can form. These sinkholes are the most sudden when the mantle give way. The western central portion of Florida is an area of Mantled Karst terrain, but most of Florida is prone to sinkhole formation because it is underlain by thick carbonate deposits and is so rich in groundwater. Development and overuse of the groundwater resources for municipal, industrial and agricultural water supplies has resulted in falling groundwater levels that play a role in sinkhole formation as well as development.
According to Ann B. Tihansky of the U.S.G.S. in Tampa and author of Sinkholes, West-Central Florida,“Induced sinkholes are generally cover-collapse type sinkholes and tend to occur abruptly. They have been forming at increasing rates during the past several decades and pose potential hazards in developed and developing areas of west-central Florida. The increasing incidence of induced sinkholes is expected to continue as our demand for groundwater and land resources increases. Regional declines of ground-water levels increase sinkhole occurrence in sinkhole-prone regions.” The sinkhole prone regions of the country can be seen below.All of Florida is karst terraine, but as can be seen below karst terrain also covers much of the Valley and Ridge Province of Virginia in the western third of the state. Small karst areas occur in the Cumberland Plateau, Piedmont and even the Coastal Plain provinces.
If you have questions or worries about sinkholes, settling or earth movement in your yard, Florida has an excellent question and answer web site.