20
Feb

PR 17 February 2012

The Mines and Geosciences Bureau (MGB)-7 will send a geologist to conduct a site assessment on the reported and recently formed sinkhole in Sitio Plaza, Barangay Camboang, which is roughly three kilometers from the municipal hall in Dumanjug, southwest of Cebu.

This developed after MGB-7 received the information from the Regional Disaster Risk Reduction and Management Council (RDRRMC)-7 that a sinkhole was newly formed within a private property of a certain Joy Distelkamps, which allegedly measures 20 meters by 15 meters in diameter and a depth of 7 meters.

Meanwhile, the rapid geohazard assessment on rain-induced landslides and flooding in 2009, a team of geologists from the MGB-7 observed the presence of sinkholes in 14 barangays in Cebu province. Eleven barangays in Tuburan, namely: Bulwang, Lusong, Montealegre, Santo Niño, Siotes, Kabkaban, Libo, Taminjiao, Kalangahan, Bagasawe, and Sandayong.

On the other hand, barangays Linut-od in Argao, Kang-actol in Dumanjug, and Vive in Ronda, Cebu were observed to have sinkholes.

A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote is a natural depression or hole in the surface topography caused by karst processes – the chemical dissolution of carbonate rocks.

MGB-7 Regional Director Loreto B. Alburo said that Senior Science Research Specialist Abraham R. Lucero, Jr. will check the area.

Based on the initial account, the collapse or cave-in of the cave roof which resulted to the formation of a sinkhole may have been trigged by the 6.9 magnitude earthquake and many aftershocks, and intense and prolonged precipitation brought about by an active low pressure area.  The earthquake may have caused ground or tension cracks.  These factors may have weakened the cave roof.

According to the Visual Dictionary of the Earth, caves commonly form in areas of limestone. Limestone is made of calcite (calcium carbonate), which dissolves in the carbonic acid naturally present in rainwater, and in humic acids from the delay of vegetation. The acidic water trickles down through cracks and joints in the limestone and between rock layers, breaking up the surface terrain into clints (blocks of rock), separated by grikes (deep cracks), and punctuated by sinkholes into which surface streams may disappear.

As precautionary measures, Alburo added MGB-7 initially recommended the appropriate installation of fence or barrier of the affected area, proper signage or warning, and continuous monitoring of the subsidence by the local government units.

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