In the past 5 months villagers from Lianyuan in southern China‘s Hunan Province have been left alarmed after seeing over 20 holes suddenly open up in the ground (Main Photo). From last September the cave-ins, which range in size, have seen houses collapse and rivers dry and left villagers terrified about where the next will be. Initial investigation showed reckless coal mining has damaged the underground water system and caused cave-ins in farm fields and riverbanks in recent months.
WHAT IS A SINKHOLE?
A sinkholes is a depression or hole in the land surface, or may be hidden from view below the surface. It can be shallow or deep, small or large. Some are shaped like shallow bowls or saucers whereas others have vertical walls, some hold water and form natural ponds. Hydrologic conditions, including lack of rainfall, lowered water levels, or, conversely, excessive rainfall in a short period of time can all contribute to sinkhole development. The essential factor of all sinkhole development is the dissolving of the underlying limestone by slightly acidic water. As rain falls through the atmosphere, it absorbs carbon dioxide and forms a weak Carbonic acid. As this water moves through the soil zone, it reacts with living and decaying plant matter and becomes more acidic. The acidic water slowly dissolves limestone, especially along the fractures and weak layers. This chemical erosion eventually causes voids or cavities into which overlying sediments may collapse or subside. The result of chemical erosion of limestone,
followed by physical collapse or subsidence, is a sinkhole.
SINKHOLES CAN BE HUMAN INDUCED:
Increased numbers of sinkholes can generally be attributed to
changing or loading of the earth’s surface with development
such as retention ponds, buildings, changes in drainage patterns,
heavy trafﬁc, drilling vibrations or a sudden or gradual decline in
groundwater levels. In urban areas, all these impacts may occur
at the same time, accelerating any sinkhole tendencies. The water below ground is actually helping to keep the surface soil in place. Ground-water pumping for urban water supply and for irrigation can produce new sinkholes. If pumping results in a lowering of ground-water levels, then underground structural failure, and thus, sinkholes can occur.
by:Let’s Explore & Learn