subsidence

subsidence - Subsidence EENS 204 Tulane University Natural...

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This page last updated on 11-Mar-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Surface Subsidence and Collapse Subsidence 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. Carbonate Dissolution and Karst Topography Carbonate rocks such as limestone, composed mostly of the mineral calcite (CaCO 3 ) are very susceptible to dissolution by groundwater during the process of chemical weathering. Such dissolution can result in systems of caves, sinkholes, and eventually to karst topography. Dissolution Water in the atmosphere can dissolve small amounts of carbon dioxide (CO 2 ). This results in rain water having a small amount of carbonic acid (H 2 CO 3 ) 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: CaCO 3 + H 2 CO 3 = Ca +2 + 2HCO 3 -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. Subsidence 3/11/2004 Page 1 of 9
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z 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
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This note was uploaded on 11/20/2010 for the course LIR 30 taught by Professor Thornley,k during the Spring '08 term at Santa Rosa.

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subsidence - Subsidence EENS 204 Tulane University Natural...

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