Why in News?
Recently, an enormous sinkhole swallowed a bus and pedestrians in the Xining of China. This incident leads to sparking an explosion, killing six people and more than 10 people are missing. Sinkholes are not unknown in China, where they are often blamed on construction works and the country’s rapid pace of development. Even in 2016, atleast three people fell into the huge sinkhole, which swallowed a section of road and pedestrians.
What is Sinkholes?
Sinkholes are cavities in the ground that form when water erodes an underlying rock layer. Sinkholes have both natural and human causes. The formation of sinkholes involves natural processes of erosion or gradual removal of slightly soluble bedrock (such as limestone rocks, carbonate rocks) by percolating water, the collapse of a cave roof, or a lowering of the water table. Sinkholes often form through the process of suffosion.
Manmade sinkholes are created when city development compromises the structural integrity of underlying rock. Roads, buildings, and other types of construction may cause water to collect in certain areas and wash away the supporting rock layer (especially at low sea levels and after a heavy rainfall).
Types of Sinkholes?
There are three types of sinkholes exist.
Dissolution Sinkhole: Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Rainfall and surface water percolate through joints in the limestone. Dissolved carbonate rock is carried away from the surface and a small depression gradually forms.
- Cover Subsidence Sinkhole: Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker, or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.
- Cover Collapse Sinkhole: Cover-collapse sinkholes may develop abruptly (over a period of hours) and cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression. Over time, surface drainage, erosion, and deposition of sediment transform the steep-walled sinkhole into a shallower bowl-shaped depression.