Sinkholes; how do sinkholes form?

Imagine the ground beneath your feet or your house suddenly collapsing and forming a large hole in the ground. It's a terrifying thought. Sinkholes are uncommon, but when they do develop, they can be fatal. Sinkholes form when the ground underneath the land surface is unable to support it. Sinkholes can occur for a variety of causes; continue reading to learn more about them.

Sinkholes are widespread in areas where the geology beneath the land surface contains limestone, carbonate rock, salt beds, or rocks that can be dissolved naturally by groundwater passing through them. Underground caves and caverns form as the rock degrades. Sinkholes are spectacular because the land normally remains intact for a long time until the subsurface areas get too large. A abrupt collapse of the ground surface can occur if there is insufficient support for the land above the voids.

Formation of sinkhole; Image credits - cartleton.edu


What is a "sinkhole"?

When it rains, water stays inside the sinkhole and sinks into the subsurface. Sinkholes can range in size from a few feet to hundreds of acres, and can be anywhere from a few feet to more than 100 feet deep. Some have vertical walls and are formed like shallow bowls or saucers; others store water and form natural ponds. Sinkholes usually emerge slowly enough that no change is evident, although they can appear rapidly when a structure collapses. If it occurs in a city, such a collapse can have a dramatic impact.


Types of sinkholes

Dissolution sinkholes: The most intense dissolution of the limestone or dolomite occurs where the water first comes into contact with the rock surface. Where flow is centered 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 touch with the atmosphere, aggressive dissolution occurs.

Formation of dissolution sinkhole, Source; Land Subsidence in the United States, USGS

Subsidence sinkholes: Where the covering sediments are permeable and contain sand, cover-subsidence sinkholes tend to form gradually. Cover-subsidence sinkholes are more prevalent, smaller, and can go undiscovered for lengthy periods of time in regions where the cover material is thicker or the sediments contain more clay.

Formation of subsidence sinkhole, Source; Land Subsidence in the United States, USGS

Collapse sinkholes: Cover-collapse sinkholes can form quickly (in a matter of hours) and inflict significant damage. They form when the overlaying sediments contain a large amount of clay. Surface drainage, erosion, and deposition have turned the sinkhole into a shallower bowl-shaped depression over time. Surface drainage, erosion, and sediment deposition change the steep-walled sinkhole into a shallower bowl-shaped depression over time.

Formation of collapse sinkhole, Source; Land Subsidence in the United States, USGS

Sinkholes can be human-induced: Land-use patterns, particularly groundwater pumping and construction and development practices, have been linked to new sinkholes. Sinkholes can also emerge when natural water-drainage patterns are disrupted or new water-diversion systems are created. When the land surface is altered, such as when industrial and runoff-storage ponds are built, sinkholes can arise. The additional material's heavy weight may cause a subsurface collapse of supporting material, resulting in a sinkhole.


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