A sinkhole is a depression or hole in the ground that forms when the surface layer collapses into a void beneath it. This geological phenomenon is a natural process that can occur gradually over centuries or happen with sudden, catastrophic failure. Sinkholes are common features in landscapes where the subsurface geology is susceptible to natural erosion of underground rock.
The Necessary Geological Setting
Sinkholes occur most frequently in regions characterized by Karst topography. This landscape develops where the ground is underlain by soluble bedrock, meaning rock that can be dissolved by water. The most common types of soluble rock are carbonate rocks like limestone and dolomite, but gypsum and subsurface salt beds are also susceptible. Karst regions are defined by an internal drainage system where water travels underground through channels instead of flowing across the surface.
The Natural Process of Underground Cavity Formation
The mechanism behind sinkhole formation is the chemical process of dissolution. As rainwater percolates through the atmosphere and soil, it absorbs carbon dioxide, creating a weak solution of carbonic acid. This naturally acidic water seeps down through cracks and fissures to reach the soluble bedrock below. Over vast periods, this acidic groundwater chemically reacts with and slowly dissolves the rock. This continuous dissolution widens cracks and joints, creating an intricate network of subterranean voids and caverns beneath the surface.
Three Mechanisms of Surface Collapse
The collapse of the land surface into the underlying void occurs through three distinct mechanisms, depending on the soil layer above the bedrock. Dissolution sinkholes form where the soluble bedrock is exposed or covered by only a very thin layer of soil. These develop gradually as water dissolves the rock, forming a bowl-shaped depression that slowly deepens over time.
Cover-Subsidence Sinkholes
Cover-subsidence sinkholes occur when the surface layer is composed of permeable, sandy, or granular sediments. As the underlying rock dissolves, the loose soil particles above slowly filter and wash down into the expanding void. This gradual downward migration of soil results in a gentle, saucer-shaped depression on the surface.
Cover-Collapse Sinkholes
The third and most dangerous type is the cover-collapse sinkhole, which forms where a thick, cohesive layer of clay or other strong material covers the bedrock. This solid layer acts as a temporary ceiling, holding its shape even as a large cavity forms beneath it. The surface remains intact until the cavity grows so large that the cover can no longer support its own weight, leading to a sudden, catastrophic collapse.
How Human Activity Accelerates Sinkhole Development
While sinkholes are a natural phenomenon, human activities can significantly accelerate their development in susceptible Karst regions. Heavy pumping of groundwater for irrigation or municipal supply lowers the water table, reducing the buoyant support for the roofs of underground cavities. The weight of new construction, such as large buildings or retention ponds, can also place excessive stress on unstable ground, triggering the failure of a fragile subterranean ceiling.
Altering surface water drainage is another common human trigger. A concentrated flow of water, often from leaking water mains or poorly designed stormwater runoff systems, can rapidly erode and wash away the supporting soil. This high-volume flow accelerates the dissolution of the bedrock and the removal of the covering soil, causing a sinkhole to form much faster than under natural conditions.