Sinkholes are depressions in the ground that form over time because of erosion and gravity. They may reveal themselves gradually or with sudden collapses, although it is the latter kind of sinkhole that often receives the most attention. Sinkholes are usually caused by the water that flows below the topsoil. The water turns acidic from absorbing carbon dioxide and participating in botanical processes, and when it eventually flows down into the bedrock, it begins to whittle away at the structural integrity of the stone formations below. This is especially true if the bedrock is made of a soft mineral, such as limestone or gypsum.
The flowing acidic water can dissolve rock and create underground passages, called conduits, through which more water will eventually flow. Over time, as more water begins to channel through these conduits, underground basins take shape. Called recharge areas, these underground pools of water constantly have water flowing to and from them, which further erodes the bedrock. Eventually, the bedrock will be so eroded that it can no longer support the weight of the topsoil above it. When this happens, the topsoil - - also called overburden - - caves in, creating the sinkhole.
There are three main types of naturally caused sinkholes, distinguished by the types of gaps that form in the bedrock lying below the overburden:
• Cover-collapse sinkhole: This type of sinkhole occurs where the overburden is made of soft material, like clay. As caverns form in the bedrock below, pieces of the overburden tumble into the empty space, making the topsoil level weaker and weaker, until it eventually caves in.
• Cover-subsidence sinkhole: This type of sinkhole is usually characterized by small dimensions, the abundant presence of water and a gradual collapse.
• Dissolution (or solution) sinkhole: These don't exactly sink - - the overburden washes away, exposing the bedrock to erosion.
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