The action of surface water and groundwater in the chemical weathering or chemical erosion of soluble carbonate rocks such as magnesium carbonates (dolomites) and calcium carbonates (limestone) produces landforms that are called karst topography.
Geological features such as caves, sinkholes, underground rivers, barren and rocky ground and lack of surface water bodies are all results of the chemical processes of karst topography. The ontology of the term derives from its original reference to the Karst region in Slovenia, a limestone region near the Gulf of Trieste (Encyclopaedia Britannica).
Karst topography characteristically occurs when a layer of carbonate rocks occur just below surface soil, whose dissolution sometimes causes surface soil to funnel in, sometimes suddenly without warning.
The trigger for a collapse, Dr. Anthony Cooper, a geologist with the British Geological Survey claims, most commonly occurs due to changes in groundwater levels, or a sudden increase in levels of surface water.
In dry spells, when the water table is reduced, the withdrawal of groundwater levels leaves cavities once filled by water and silt might lead to a weakening of soil foundations. Also additions to the weights placed on surface soil or heavy downpours coupled with soil subsidence might also trigger a sinkhole.
As about 10 per cent of the Earth’s surface is comprised of karst topographies, it is not uncommon to find sinkholes. In fact the entire state of Florida in the US is classified as a karst landscape, such that insurance is a legal obligation for insurers to home owners.
The famous eroded cliffs at the western Irish coast, the caves in Slovenia, the 662 meter deep Xiaozhai tiankeng sinkhole in China, the 612 meter deep sinkhole Dashiwei tiankeng in China, the 530 meter deep Red Lake in Croatia, the 510 meter deep Minye sinkhole in Papua New Guinea and the Sotano Del Barro (410 meters) and the Sotano de las Golondrinas (372 meters) sinkholes in Mexico are some of the most significant examples of karst topographies (Jon Henley, 2013).
In India karst topography is present in the Vindhya region (mainly southwestern Bihar), the Himalayas (parts of Jammu & Kashmir, Robert Cave, Sahasradhara, the eastern Himalayas, areas near Dehradun), Pachmarhi in Madhya Pradesh, the surrounding coast near Vishakhapatnam, and Bastar in Chhattisgarh. It is reported that the 2004 tsunami originating near the Andaman and Nicobar islands might have caused sinkholes to appear all over certain islands of the Nicobar island group. Three Nicobar islands are particularly affected – Campbell Bay, Car Nicobar and Kamorta.
Mitigation measures for sinkholes basically bear the precautionary goals of filling sinkholes, maintenance of groundwater levels and checking contamination of groundwater, preventing surface water drainage into risk zones, and maintaining safety standards at risk zones . There are various approaches depending on the nature of the risk zone and the surrounding topography, land use and the climate.
The highest priority is provided to drainage in areas having karst topography and the approach is based on geological and engineering assistance that is site specific. Due attention is provided both to surface and ground water. It is necessary that inspections of risk areas take place on a regular basis, especially after major rain events. The fostering of vegetative buffer zones also works to prevent excessive run off of surface water, although it is advisable to plant native plant breeds and avoid the propagation of noxious plants and weeds.
Soil can also be investigated for assessing culpability of bearing karst features. Traditional methods such as boring soil and rock, and also percussion methods necessitates penetrating the ground in order to extract information on the underground geological properties of soil.
The invasiveness of these methods when dealing with the problem of sinkholes has led to a turn towards technological non-invasive methods for determining the properties of suspect soil. Some of these non-invasive methods include ground penetrating radar, resistivity imaging, seismic surveying and microgravity surveying. These on-site assessments are the necessary pre-requisite for engineering and development processes in suspect karst topographies.
In 2016 scientists are reported to have measured the world’s deepest sinkhole in the South China Sea, toppling Dean’s Blue Hole in the Bahamas, previously thought to be the deepest sea sinkhole in the world.
The sinkhole in the South China Sea has been named the Dragon Hole and is 987 feet or 300 meters deep, located at a coral reef nearby the Paracel islands. A blue hole is an undersea sink hole that is visible from the surface due to the difference in the hues of the waters.
Such sinkholes are likely to provide undersea passages to cave systems and are usually steep-walled depressions. Due to the poor water circulation they are usually anoxic, meaning that oxygen percolation is very poor and diminishes after a certain depth, lending them averse to most life at great depths other than bacteria.