Sinkholes are natural phenomena which are depressions or actual holes occurring on the ground when water has removed the soil and bedrock beneath. While many sinkholes open up far from humans, financial loss and even personal injury can result when sinkholes occur beneath roads or on private property. Predicting when and where sinkholes will occur is difficult, due to the fact that charting underground water processes is not possible in many inhabited areas.
General Cause of Sinkholes
Sinkholes can occur in a variety of shapes and sizes, from funnel-type holes in the ground to concave depressions. The general cause common to all sinkholes involves the erosion of soluble bedrock, such as limestone, by a source of water, to the point where the surface weight of the ground layer of soil cannot be supported. This phenomenon is not much different than surface erosion which happens near rivers, except that it is occurring underground. Depending on the type and distribution of soil and rock beneath the ground, this process behind sinkholes may vary in a few different major ways.
If the ground consists mainly of limestone, either existing at the surface or covered by only a small layer of sand or soil, solution sinkholes are the most common phenomena. Wherever crevasses exist in the limestone, underground water may move into the limestone. This in turn removes the limestone through these fissures, which gradually increase in size as the limestone is ionized and moved away by the current of the water. Since upper layers of soil generally descend into the limestone as it wears away, large holes and massive cracks in the limestone surface do not usually occur. Clay and sand which flows down into the limestone may even seal the bottom from further runoff, which means that these sinkholes are most commonly seen as slightly concave depressions in the surface.
Cover Collapse Sinkhole
Where limestone or other soluble rock forms the majority of bedrock, and if their aquifer lies below the water table, a more disastrous sinkhole known as a cover collapse sinkhole may develop. Groundwater flow generally wears away the limestone in these cases, and can even form an underground cavern as more bedrock is washed away. Eventually, the rock is unable to support the weight of the soil above it, and a collapse occurs, exposing a large hole in the ground. Since these sinkholes are so unpredictable and involve an actual hole forming abruptly, they are particularly dangerous to both people and property.
Cover Subsidence Sinkhole
The last type of sinkhole usually forms when surface sand or clay occupies a larger layer above the bedrock, and is referred to as a cover subsidence sinkhole. As limestone is removed by groundwater or the water table below, the loose fragments of surface soil move downward to replace bedrock which has been washed away. Since the soil layer is generally incohesive, the weight of surface soil may be dispersed above the removed limestone, so these types of sinkholes may not be detected for years, especially because they often only manifest as small depressions in the surface. Even when cover subsidence sinkholes are noticeable, they may only be a few feet wide and deep, although even sinkholes of this size can wreak havoc when they occur in the wrong place.