Primitive forest at the bottom of a giant sinkhole in China

Primitive forest at the bottom of a giant sinkhole in China

Subject : Geography

Section : Geomorphology

Context:

• 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.

Giant sinkhole in China

• 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. In Mandarin, giant sinkholes are called Tiankeng or “heavenly pit”.
• The sinkhole has three big caves in the walls and its bottom has a well-preserved primitive forest with trees nearly 40 metres high.
• 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.

What is a primitive forest?

• The primitive forest biome is characterized by being hot and wet year-round, though not to as extreme a degree as the tropical rainforests. Though the forest’s range has decreased considerably since its original formation, many environments with a similar climate and biota still persist in many areas.

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 south eastern 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.

Sinkholes in China and around the world

• In China, the mining of coal, zinc, lead and iron ore deposits in karst areas have been associated with the formation of sinkholes due to human activity, according to a 1997 paper published in the journal Environmental Geology.
• Sinkholes are not uncommon in other parts of the world.

About 20 percent of the US is made up of karst landscapes. In Florida, in an area that is classified as a karst landscape, insurance agencies must provide homeowners coverage against damage that can accrue from ground cover collapse.

• The largest sinkhole in the US is called the “Golly Hole”, which collapsed suddenly in 1972 and is over 325 ft long, 300 ft wide and 120 ft deep.
• Other sinkhole-prone areas around the world include Mexico, parts of Italy and Russia.