SINKHOLES......

Why there are Sinkholes?

See a video below is the link, copy & paste in the URL.
http://msnvideo.msn.com/?videoid=96771e79-d3be-47f1-a1e4-8c6adff24320&src=v5


It’s a natural cavity, or a sunken space, as a hole worn by water. Sinkhole, also known as a swallow-hole, swallet or doline, is a natural depression or hole in the ground caused by some form of collapse of the surface layer (wikipedia). Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry places in certain locations.

The formation of sinkholes involves natural processes of erosion or gradual removal of slightly soluble bedrock (such as limestone) by percolating water, the collapse of a cave roof, or a lowering of the water table. Sinkholes often form through the process of suffusion. As the rock dissolves, spaces and caverns develop underground. These sinkholes can be dramatic, because the surface land usually stays intact until there is not enough support. Then, a sudden collapse of the land surface can occur.

Occasionally a sinkhole may exhibit a visible opening into a cave below or a cavern disturbs or hit by mining drilling. In the case of exceptionally large sinkholes, an underground stream or river may be visible across its bottom flowing from one side to the other.But what causes that void? It’s both natural and man-made triggers. It is often a man-made structure that causes the change in groundwater flow and leads to a collapse. It is also formed as a result of the collapse of old mine workings close to the surface, a man-made hole. So that in other instances due to obsolescence of old mining materials used in the chambers and roof support of the shafts, the above surface of the earth will cave in. Such as the case of the rare but still occasional collapse of abandoned mines and salt cavern storage in salt domes in places like Louisiana, Mississippi and Texas.

Sinkhole is formed by rainwater leaking through pavement and carrying soil into a ruptured sewer pipe or it is formed by moving water, from above eroding the substrata and causing collapse of the top soil. They can also occur from the over pumping and extraction of groundwater and subsurface fluids.

Cebu, Philippines

Sinkholes can also form when natural water-drainage patterns are changed and new water-diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created; the substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole. Let us view some of the sinkholes:

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Latest information made by Mines and Geosciences Bureau (MGB) in the Philippines in the aftermath of a 7.2 magnitude earthquake that hit Bohol, Cebu and other provinces in Central Visayas, Philippines on October 15, 2013 was their discovery of the "ruptured sinkhole" in Lapu-Lapu City and was looking into reports of six others in Calape town, and a "concealed sinkholes" are reported to exist in Bohol and Cebu. Bohol is 80 percent limestone, while 50 percent of Cebu is limestone, and that sinkholes usually occurred in limestone. They give priority to locate these "concealed sinkholes" as they pose a greater hazard with the continuing aftershocks jolting the Central Visayas. A classification made by MGB: Ruptured Sinkhole is apparent and seen as a hole in the ground caused by the collapse of the top soil. A Concealed Sinkhole has the top soil intact but is hollow underneath.

As we recall the 7.2 magnitude quake that struck Bohol and Cebu Provinces that killed at least 190 people and destroyed thousands of homes and a dozen or more centuries-old churches considered already a "heritage property" in the Philippines.