Landslides and Mass-Wasting II Sinkholes, subsidence and fissures Subsidence is a process where the ground surface moves down. The surface either sags gently of drops catastrophically due to the closing of open space beneath the Earth's surface. The fluids within rocks help support the weight of overlying rocks - and once the fluids are removed the material collapses. Areas in Arizona that are experiencing subsidence due to the withdrawl of groundwater. Large fissures developing in the farming area 40 miles north of Tucson.
Catastrophic subsidence is the sudden collapse of the surface of the earth.
There are many causes of sinkholes, but most occur due to the collapse of underground caves. Limestone is a fairly common rock type. Calcium carbonate can be dissolved by many acids, and if ground waters contain much carbon dioxide, caves can be dissolved. Circulating waters dissolve caves in regions of limstone. The caves can be quite large, and the open space must support the weight of the overlying material.