Sinkholes

A cave exploration team has discovered an ancient forest at the bottom of a giant karst sinkhole in Leye County in south China’s Guangxi Zhuang Autonomous Region.

Details –

• The sinkhole measures 306 metres in length, is 150 metres in width and 192 metres in depth, with its volume exceeding 5 million cubic meters. Given these dimensions, the sinkhole can be categorised as a large sinkhole.
• Earlier in November 2019, Xinhua Net had reported the discovery of a giant cluster of sinkholes in the same region. Before this, in 2016, scientists had discovered the world’s largest cluster of sinkholes in northwest China’s Shaanxi province.

How are sinkholes formed?

• Sinkholes are depressions formed in the ground when layers of the Earth’s surface start collapsing into caverns. They can occur suddenly and without warning, because the land under the surface of the Earth can stay intact for a period of time until the spaces get too big.
• Sinkholes can be formed due to natural processes or human activity. Typically, sinkholes form in areas of “karst” terrains, where the rock below the surface of the Earth can be easily dissolved by groundwater.
• Essentially, this means that when rainwater seeps into the ground, the rock below the surface of the Earth starts dissolving, leading to the creation of spaces. This process is a slow and gradual one and can sometimes take hundreds or thousands of years.
• As per NASA, karst geology covers about 13 per cent of eastern and southeastern Asia. According to the United States Geological Survey (USGS), karst terrain is created from the dissolution of soluble rocks, mostly limestone and dolomite and is characterised by distinctive landforms such as caves, sinkholes and springs.
• Sinkholes can also be formed due to human activity. According to the British Geological Survey, this can happen due to broken land drains, water mains and sewerage pipes, increased rainfall, storm events, underlying limestone and diverted surface water, among other reasons.

About ‘Karst Topography’ –

• Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite and gypsum. It is characterised by underground drainage systems with sinkholes and caves.
• Karst is most strongly developed in dense carbonate rock, such as limestone, that is thinly bedded and highly fractured.
• Karst is not typically well developed in chalk, because chalk is highly porous rather than dense, so the flow of groundwater is not concentrated along with fractures.
• Karst is also most strongly developed where the water table is relatively low, such as in uplands with entrenched valleys, and where rainfall is moderate to heavy. This contributes to the rapid downward movement of groundwater, which promotes the dissolution of the bedrock, whereas standing groundwater becomes saturated with carbonate minerals and ceases to dissolve the bedrock.
• Karsts are found in widely scattered sections of the world, including the Causses of France; the Kwangsi area of China; the Yucatán Peninsula; and the Middle West, Kentucky, and Florida in the United States.
• 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, Gupt Godavari Cave in Chitrakoot (U.P.), the surrounding coast near Vishakhapatnam(Borra Caves), and Bastar in Chhattisgarh.
  • Borra Caves – Borra Caves, also called Borra Guhalu, are located on the East Coast of India, in the Ananthagiri hills of the Araku Valley.