What Is a Sinkhole? — In Plain Words

General information only.

The short answer

A sinkhole is an area of ground that has no natural external surface drainage. When it rains, the water stays inside the sinkhole and typically drains into the subsurface — the ground below the surface.

Where sinkholes are common

Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or other rocks that can naturally be dissolved by groundwater circulating through them.

How a sinkhole forms

As the rock dissolves, spaces and caverns develop underground.

Sinkholes are dramatic because the land usually stays intact for a while, until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur.

These collapses can be small, or they can be huge, and they can occur where a house or road is on top.

Typically, sinkholes form so slowly that little change is noticeable, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic effect if it occurs in an urban setting.

How big sinkholes can be

Sinkholes can vary from a few feet to hundreds of acres across.

They can be from less than 1 foot to more than 100 feet deep.

Some are shaped like shallow bowls or saucers, whereas others have vertical walls. Some hold water and form natural ponds.

Rock types where collapse sinkholes can happen

USGS describes areas of the United States where certain rock types that are susceptible to dissolution in water occur. In these areas, underground cavities can form, and catastrophic sinkholes can happen.

These rock types are:

• Evaporites — salt, gypsum, and anhydrite.
• Carbonates — limestone and dolomite.

Evaporite rocks underlie about 35 to 40 percent of the United States, though in many areas they are buried at great depths.

Where the most damage tends to occur

USGS says the most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.

The three types of sinkholes

USGS uses Florida to discuss the different types of sinkholes, because Florida is prone to sinkholes. USGS says the process of dissolution — where surface rocks that are soluble to weak acids are dissolved, and where cavities form below the land surface — is responsible for virtually all sinkholes in Florida.

Dissolution sinkholes

Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focused in preexisting openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation, where groundwater is in contact with the atmosphere.

Rainfall and surface water percolate through joints in the limestone. Dissolved carbonate rock is carried away from the surface, and a small depression gradually forms.

On exposed carbonate surfaces, a depression may focus surface drainage, accelerating the dissolution process. Debris carried into the developing sinkhole may plug the outflow, ponding water and creating wetlands.

Gently rolling hills and shallow depressions caused by solution sinkholes are common topographic features throughout much of Florida.

Cover-subsidence sinkholes

Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand.

In areas where cover material is thicker, or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.

Cover-collapse sinkholes

Cover-collapse sinkholes may develop abruptly, over a period of hours, and cause catastrophic damage. They occur where the covering sediments contain a significant amount of clay.

Over time, surface drainage, erosion, and deposition of sediment transform the steep-walled sinkhole into a shallower bowl-shaped depression.

The Florida freeze event of January 2010

More than 110 sinkholes formed in the Dover area of Florida during a freeze event in January 2010.

Groundwater levels dropped to record-setting lows as farmers pumped water to irrigate their plants for protection from the cold temperatures.

The sinkholes destroyed homes, roads, and sections of cultivated areas.

Sinkholes can be human-induced

New sinkholes have been correlated to land-use practices, especially from groundwater pumping and from construction and development practices.

Sinkholes can also form when natural water-drainage patterns are changed and new water-diversion systems are developed.

Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created. The substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole.

The overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by groundwater fluid pressure. The water below ground is actually helping to keep the surface soil in place.

Groundwater pumping for urban water supply and for irrigation can produce new sinkholes in sinkhole-prone areas. If pumping results in a lowering of groundwater levels, then underground structural failure, and thus sinkholes, can occur.

General information only.