sinkhole

sinkhole, depression in Earth’s surface formed when underlying limestone bedrock is dissolved by groundwater. It is considered the most-fundamental structure of karst topography. Sinkholes vary greatly in area and depth and may be very large. There are two main varieties, one caused by the collapse of the roof of a cavern (cover-collapse sinkholes), the other by the gradual dissolving of rock under a soil mantle (cover-subsidence sinkholes). Some sinkholes, formed during periods of lowered sea levels in the Pleistocene Epoch, are now half-drowned and are known as cenotes.

Limestone (calcium carbonate) and other soluble rocks dissolve relatively easily in slightly acidic water. Falling rainwater that does not run off quickly into streams and other bodies of water at the surface infiltrates the soil. Water that reaches the soluble bedrock below can create horizontal and vertical cracks through chemical weathering (see also acid-base reaction). This process produces small voids in the rock over time, which may slowly grow into larger underground spaces and caverns as more water comes in contact with the rock (see also cave). If a space becomes large enough and its roof extends close enough to the surface, the roof collapses under the weight of the rock and soil above. In general, such collapsed sinkholes have steep rock sides and may receive streams that then flow underground.

A soil-mantled sinkhole, or cover-subsidence sinkhole, also begins with growing cracks in the limestone bedrock; however, these cracks are filled in with the soil above. As the crack widens, additional soil particles fall into the open space, and depression can appear at the surface over time. A soil-mantled sinkhole tends to be shallower than a collapsed sinkhole and receives local drainage, and water can deliver additional sediment and debris to the depression that helps to insulate the bedrock. In some cases, a thick clay layer at the base of the depression can hold a small pond or lake. In other cases, fast-moving water can liberate the soil forming the plug in the bedrock, which allows the chemical weathering of the rock to resume.

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