This page last updated on 09-Jul-2012 EENS 3050 Natural DisastersTulane University Prof. Stephen A. NelsonSubsidence: Dissolution & Human Related CausesSurface Subsidence and CollapseSubsidence hazards involve either the sudden collapse of the ground to form a depression or the slow subsidence or compaction of the sediments near the Earth's surface. Sudden collapse events are rarely major disasters, certainly not anywhere near the scale of the earthquake, volcanic, tsunami, or landslide disasters, but the slow subsidence of areas can cause as much economic damage, although spread out over a longer period of time. The most common type of sudden collapse is due to erosion of underground soil or rock caused by leaking human-made sewer pipes or water mains. This rarely destroys large areas, but commonly swallows up vehicles.The second most common type of sudden collapse involves dissolution of carbonate rocks (limestones) beneath the surface.This is the topic we will cover first.Carbonate Dissolution Carbonate rocks such as limestone, composed mostly of the mineral calcite (CaCO3) are very susceptible to dissolution by groundwater during the process of chemical weathering. Such dissolution can result in systems of caves and sinkholes.DissolutionWater in the atmosphere can dissolve small amounts of carbon dioxide (CO2 ). This results in rain water having a small amount of carbonic acid (H2CO3) when it falls on the Earth's surface. As the water infiltrates into the groundwater system and encounters carbonate rocks like limestone, it may start to dissolve the calcite in the limestone by the following chemical reaction:CaCO3 + H2CO3 = Ca+2 + 2HCO3-2, which states that calcite reacts with carbonic acid to produce dissolved Calcium ion plus dissolved Bicarbonate ion.This reaction takes place as the water moves along fractures and other partings or openings in the rock. This results in dissolution of much of the limestone if the reaction continues to take place over a long period of time.Subsidence7/9/2012Page 1 of 10Caves & Cave Formationz Caves are large underground open spaces. If there are many interconnected chambers in a cave system, it is called a cavern. Most caves are formed by the chemical dissolution process described above, as a result of circulating groundwater. The dissolution begins along fracture systems in the rock, widening the fractures and connecting them to other fractures, until a cave is formed. Most caves are thought to form near the water table (the surface below which all open space in rock is filled with water) , and thus the openings are initially filled with water.z After the water table is lowered due to changing geologic conditions, further seepage of water into the now open cave system results in the deposition of stalactites (icicle like stones) where the water drips into the cave. If water is absent from the floor of the cave stalagmites form where the water drips on the floor of the cave. Both stalactites and stalagmites are composed of newly precipitated calcite, initially dissolved from the limestone above, carried in the groundwater, and re-precipitated when the water reaches a low pressure area like a cave.z The rate at which caves form depends on such factors as the acidity of the water and the velocity at which the water moves through the rock. Highly acidic water and high flow velocity increase the rate of dissolution, and thus the rate at which a cave forms. Based on observations of the rate of dissolution from currently forming caves, it appears that the cave formation process can take anywhere between 10,000 years and 1 million years.Sinkholesz A sinkhole is a large dissolution cavity that is open to the Earth's surface. Some sinkholes form when the roofs of caves collapse, others can form at the surface by dissolving the rock downward. Because we are here concerned with subsidence disasters and hazards we will concentrate on the formation of sinkholes by collapse.z Sinkholes are common in areas underlain by limestone. Central Florida is such an area, and in one small area of about 25 km2, over 1000 sinkholes have formed by collapse in recent years.z Sinkholes may form as a result of lowering the water table by excessive pumping for Subsidence7/9/2012Page 2 of 10human use of the water. This appears to be responsible for sinkhole formation in Florida. Caverns that were forming just below the water table were filled with water. The water table was lowered over the years resulting in the level of groundwater in the caverns to become lower. While the water table was high, the water in the cavern helped to support the roof of the cavern. This support is removed when the water table is lowered, and thus the unsupported roof eventually becomes unstable and collapses to form a sinkholez .Sinkholes may also form by slow enlargement of caverns by continued dissolution of the limestone. This may no matter what the level of the water table.z When sinkholes collapse to expose the water table at the surface, the sinkhole will be filled with water forming small circular lakes.z Although common in areas underlain by limestone, sinkholes can form in any area where highly water soluble rocks occur close to the surface. Such rocks include rock salt made of the mineral halite, and gypsum deposits, both of which easily dissolve in groundwater. Removal of Solids and Mine Related CollapseHumans can play a large role causing collapse of the surface. Mining activities that remove material from below the surface can result in collapse if precautions are not taken to ensure that the there is adequate support for the overlying rocks.Removal of Saltz Salt occurs beneath the surface in areas that were once below sea level in restricted basins where extensive evaporation caused the concentration of salt in seawater to become so high that the salt was precipitated on the bottom. This occurred, for example during the Jurassic Period (about 150 million years ago) in the area of the Gulf of Mexico. Later deposition of other sediments on top of this salt, resulted in low density salt underlying higher density sediments. Since salt is rather ductile, it began to flow upward toward the surface, and in many cases became detached from the original layer of salt at depth to form what is called a salt dome. Since the salt now occurs close to the surface, it can dissolve and collapse to form sinkholes. z The salt can also be mined to produce salt for