Sinkhole
A sinkhole is a depression or hole in the ground that results from subsurface instability, primarily caused by erosion or water drainage patterns affecting the surface layer. There are two main types of sinkholes: cover-subsidence, which form gradually, and cover-collapse, which appear suddenly and can be particularly dangerous. A third type, dissolution sinkholes, arises when water dissolves underlying bedrock in areas with little soil cover. Sinkholes are most prevalent in regions with soluble bedrock, such as limestone and gypsum, which are common in parts of the United States like Florida, Alabama, and Kentucky.
While some sinkholes occur naturally, others can be triggered by human activities such as poor drainage management, mining, or construction practices. Notable incidents, including deadly sinkholes in Guatemala City and Florida, highlight the potential risks associated with sinkhole formations, particularly in urban areas. Interestingly, some sinkholes develop unique ecosystems that can host diverse plant and animal species, which may be found nowhere else on Earth. Despite anecdotal claims that sinkholes are increasing due to human impact, definitive conclusions on this trend are yet to be established.
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Sinkhole
A sinkhole is a hole or a large depression in the ground caused by some form of subsurface instability. The most common causes of sinkholes are erosion and water drainage patterns that destabilize the ground's surface layer, but they can be caused by human activity. Scientists recognize two main types of sinkholes: cover-subsidence sinkholes, which develop gradually over an extended period, and cover-collapse sinkholes, which develop quickly and manifest suddenly. Some models recognize a third type, known as dissolution or solution sinkholes, which occur when water dissolves the underlying bedrock in a geographic area with little soil cover or surface vegetation.
Sinkholes can be deadly and very destructive if they form in places with dense human populations, such as urban areas. One such incident took place in Guatemala City, Guatemala, in 2010, when a 65-foot-wide, 100-foot-deep sinkhole suddenly opened below a factory, causing the deaths of fifteen people. Another prominent case took place in Florida in 2013, when a sinkhole formed below a house, causing the death of a resident who was asleep inside. A sinkhole formed near a police department in Hinton, West Virginia, in 2022. In 2024, numerous sinkholes opened throughout Florida following Hurricane Milton.
Background
While some sinkholes occur quickly and suddenly, others are preceded by detectable warning signs. The slope of the affected ground may show signs of change and appear to slump in a particular direction. Shallow ponds or pools of water may collect in areas where they did not previously exist, and shifts in the surface soil layer can expose tree roots, building foundations, and other elements that were previously underground. Grass and other types of surface vegetation may die, leaving barren patches of ground that usually take a circular shape. If a sinkhole is forming below an occupied building, residents may find that windows may not open and close properly, as slight shifts in the ground skew the horizontal plane of the building.
According to the United States Geological Survey (USGS), sinkholes are most likely to occur in the states of Alabama, Kentucky, Missouri, Pennsylvania, Tennessee, Texas, and particularly Florida, as these areas have geographic characteristics that make them more prone to subsurface rock erosion. Geologists working in Florida are currently attempting to develop a mapping tool to identify areas where sinkholes are most likely to occur. Other sources have claimed that as much as 20 percent of the landmass of the United States has a heightened sinkhole risk, though these claims have not yet been verified.
Other parts of the world where noteworthy sinkholes have formed include Qattara, located to the west of Cairo, Egypt, which is home to what is believed to be the largest naturally occurring sinkhole on Earth. The Qattara Depression measures 50 miles (80 kilometers) in length and 75 miles (120 kilometers) in width. Berezniki, Russia, is the site of a sinkhole that has been continually getting deeper since its original formation in 1986; it currently reaches a depth of about 650 feet (200 meters). The remote Cerro Sarisariñama region of Venezuela has a collection of perfectly circular sinkholes of uniform size, which continue to confound scientists and researchers who have been unable to explain their symmetrical formation.
Overview
In most cases, sinkholes affect landmasses that contain subsurface layers of soluble bedrock, which is bedrock that can be dissolved through consistent exposure to large quantities of water. Limestone and gypsum are both noted as soluble bedrock types that are particularly prone to sinkhole collapses.
Cover-subsidence sinkholes cause the gradual erosion of this bedrock over time, and in most cases, the underlying rock will become exposed and ponds or shallow pools of water will form on the surface, above the point where the erosion activity is taking place. In the case of cover-collapse sinkholes, erosion takes place at a deeper level, widening natural cracks and hollow areas in the underlying bedrock. The sudden appearance of a cover-collapse sinkhole is due to the bedrock no longer being able to bear the weight of surface elements.
While most sinkholes occur as the result of natural processes, others are caused by human activity. Some construction sites are configured to redirect rainwater to specific drainage areas, where it can seep into the ground and destabilize the subsurface bedrock, leading to the eventual appearance of a cover-collapse sinkhole. In urban areas, sinkholes often form as the result of poorly designed sewage and drainage infrastructure or as the result of pipe or water main breakages. Mining activities and salt storage have been known to cause the sudden appearance of cover-collapse sinkholes. Sinkholes can also be triggered by the overly aggressive removal of water and other fluids from subsurface areas.
Cover-collapse sinkholes are considered particularly dangerous, as they are very difficult or impossible to detect without the use of specially designed geological equipment. Sinkholes can form underwater, in which case they are usually called "blue holes." One of the deepest such sinkholes that has been discovered is known as Dean's Blue Hole, which is located off the coast of the Bahamas. A massive sinkhole in Chile is also among the deepest. Longer than a basketball court, it is about 656 feet (200 meters) deep.
In most cases, sinkholes will naturally fill in with soil if they are left untreated and become a drain mechanism for incoming rainwater, which will then collect in the region's groundwater reserves. Alternately, sinkholes may settle into a formation geologists call an "open throat," in which the rock at the bottom of the sinkhole remains visible and the sinkhole connects to adjacent subsurface caves.
Some sinkholes go on to develop their own unique ecosystems, which thrive at the bottom of the sinkhole and are powered by incoming sunlight and rainwater. One prominent example is located in the Hubei province of central China, where a large sinkhole contains scores of plant and animal species, which have developed in the oxygen-rich and highly humid environment. The sinkhole's unique ecosystem is largely untouched by humans, and only a small number of researchers have ever studied it. In Venezuela, the Sarisariñama sinkholes are home to ecosystems that contain some species of plants and animals that are found nowhere else on Earth.
While some researchers have claimed that sinkholes are becoming more common, likely as the result of destructive human activity, it is currently not possible to draw such a conclusion with any certainty. Unlike extreme weather and natural events such as hurricanes, tornadoes, tropical storms, blizzards, and earthquakes, sinkholes are not individually tracked.
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