What Makes a Sinkhole a Sinkhole?

A subsidence incident or sinkhole on First Street in San Francisco, blocking traffic to the Bay Bridge. By telstar on Flickr.

The Florida Geological Survey probably deals with more sinkholes than most agencies. The most frequent geologic cause of sinkholes is dissolution of limestone and other soluble rocks and the collapse of the soil cover over them. However, there are other events that are often called “sinkholes” by the media and frequently civil engineers even if they are not a geological sinkhole. The FGS uses the term “subsidence incident” to denote all types of sinkhole phenomena and only after investigations to they formally call them sinkholes. Here’s how they define sinkhole on their website: (Photo of a “subsidence incident” on First Street in San Francisco by telstar on Flickr)

Sinkholes are closed depressions in areas underlain by soluble rock such as limestone, dolostone, gypsum, or salt.  Sinkholes form when surface sediments subside into underground voids created by the dissolving action of groundwater in the underlying bedrock.

Other subterranean events can cause holes, depressions or subsidence of the land surface that may mimic sinkhole activity. These include subsurface expansive clay or organic layers which compress as water is removed, collapsed or broken sewer and drain pipes or broken septic tanks, improperly compacted soil after excavation work, and even buried trash, logs and other debris. Commonly, a reported depression is not verified by a licensed professional geologist to be a true sinkhole, and the cause of subsidence is not known. Such an event is called a subsidence incident.  The Florida Geological Survey maintains and provides a downloadable database of reported subsidence incidents statewide.  While this data may include some true sinkholes, the majority of the incidents have not been field-checked and the cause of subsidence is not verified.