Un enorme cratere (sinkhole) si e’ formato in seguito al collasso di ingenti quantita’ di terra di fonte ad una scuola elementare della provincia di Ningxiang in China. Il “buco” si era iniziato a formare lo scorso gennaio e la situazione si e’ aggravata in seguito alle abbondanti pioggie primaverili che ne hanno accelerato il collasso.
Per lo meno altri otto “buchi” si sono formati in diverse regioni cinesi durante le scorse settimane, disseminando panico fra la popolazione che vede in questi fenomeni i segnali premonitori di una imminente attivita’ sismica a livello nazionale.
Cosa e’ e come si forma un sinkhole?
Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by ground water circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. These collapses can be small, as this picture shows, or they can be huge and can occur where a house or road is on top.
The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. The picture to the left shows a sinkhole that quickly opened up in Florida, apparently eating a swimming pool, some roadway, and buildings.
Areas prone to collapse sinkholes
The map below shows areas of the United States where certain rock types that are susceptible to dissolution in water occur. In these areas the formation of underground cavities can form and catastrophic sinkholes can happen. These rock types are evaporites (salt, gypsum, and anhydrite) and carbonates (limestone and dolomite). Evaporite rocks underlie about 35 to 40 percent of the United States, though in many areas they are buried at great depths.
Sinkholes can be human-induced
New sinkholes have been correlated to land-use practices, especially from ground-water pumping and from construction and development practices. 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.
The overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by ground-water fluid pressure. The water below ground is actually helping to keep the surface soil in place. Groundwater pumping for urban water supply and for irrigation can produce new sinkholes In sinkhole-prone areas. If pumping results in a lowering of ground-water levels, then underground structural failure, and thus, sinkholes, can occur.