source: GeologicalSociety 2017年7月5日
Soluble (karstic) rocks including limestone, gypsum and salt all dissolve underground resulting in cavities and cave systems that may collapse causing sinkholes to form at the surface. Limestone dissolves at a slow rate and is well-known for hosting extensive cave systems.
Gypsum and salt dissolve much faster than limestone and also produce cave systems, but they can evolve on a human timescale rather than a geological one and are seldom seen. As a consequence they pose a geological hazard that can cause sinkholes to develop and rapidly spread. Natural gypsum dissolution has resulted in a severe sinkhole problem affecting the city of Ripon in North Yorkshire, and sinkholes further north such as those at Croft, near Darlington, in County Durham.
These are both places frequented by Lewis Carroll and sinkholes are thought to have inspired the story of Alice in Wonderland. Gypsum dissolution-prone areas can be identified both by the occurrence of sinkholes and the presence of sulphate-rich springs. Salt is highly soluble dissolving in the subsurface to feed saline springs which were called wiches in Medieval times. Consequently, the towns of Droitwich, Northwich and Middlewich are all situated on salt springs.
In these towns salt production utilised the natural springs, then increased brine extraction caused catastrophic subsidence in the surrounding areas. The natural dissolution of soluble rocks continues, but by understanding the controlling mechanisms the worst sinkhole areas can be avoided. In addition, some measures can be put in place to help development in the less sinkhole-prone areas.
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