Study Guide For EESA05, Chapter 6

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19 Jul 2010

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Chapter 6: Subsidence
6.1 Introduction to Subsidence
- Subsidence is the slow or rapid downward movement of Earth’s surface; form of the subsiding area can
be circular, linear, bead-like or irregular
- It can involve an imperceptible lowering of the surface over a larger area
- Subsidence is associated with dissolution of limestone, gypsum or rock salt at depth
- Karst landscape is irregular in form and has closed depressions
- Natural causes of subsidence: thawing of frozen ground, compaction of recently deposited sediment,
shrinking of expansive soils, EQ and deflation of magma chambers
- Human induced subsidence can result from withdrawal of fluids from subsurface reservoirs, collapse of
soil and rock over mines and other subsurface excavations and draining of wetlands
- dissolution of limestone, gypsum, or rock salt occurs as groundwater moves through the rock. Rock salt
and gypsum dissolve when they come in contact with neutral waters, whereas limestone dissolves readily
only in acidic waters
- rock salt is 7500 times more soluble than limestone and gypsum 150 times more soluble than limestone
- percolating water becomes acidic when CO2 is dissolved in it. Acidification occurs in the atmosphere
where rainfall is in equilibrium with CO2 and in soil where CO2 is produced by bacterial decomposition
- respiration of most soil bacteria and humans involve consumption of O2 and release of CO2
- dissolving CO2 in water produces carbonic acid
- areas underlain by thin-bedded, fractured, or well jointed limestone are especially vulnerable to
dissolution; surface waters are easily diverted to depth in these rocks along fractures or cracks between
sedimentary layers
- acidic percolating waters enlarge fractures by dissolving rock
- the chemical reactions leading to limestone dissolution:
o water + CO2= carbonic acid (H2CO3)
o carbonic acid + limestone (CaCO3)= Calcium + Bicarbonate (2HCO3
- dissolution produces empty spaces, or voids, of ranges of sizes beneath the land surface. Voids are
produces preferentially in zones of mixing of waters near the water table. Over time, contiguous voids
join to produce caves and caverns
- where large openings close to the surface, pits known as sinkholes may develop, either individually or in
large numbers
- a surface pockmarked with a large number of sinkholes is termed a karst plain
- many karst areas are characterized by beautiful rolling hills separated by areas of subsidence underlain by
extensive cave systems
- cave openings can be sites of disappearing streams where surface water goes underground, or the place
where groundwater emerges at the surface in springs
- in humid regions, extensive dissolution removes most of the soluble bedrock, leaving behind a landscape
of steep helps known as tower karst
Sinkholes: in karst areas, they range from 1 to several hundreds of meters in diameter and are of two basic
1) solution sinkholes: these pits form by solution of buried bedrock. Acidic groundwater is concentrated
in holes that develop along bedding planes, joints and fractures
2) collapse sinkholes: develops by the collapse surface or near-surface rock or sediment into an
underground cavern system
- some sinkholes open into subterranean passage, through which water escapes during rainstorms. Other
are filled with rubble that blocks the flow of water to the subsurface. Blocked sinkholes may contain
small lakes, but most lakes eventually drain when the water seeps through the debris
- artificial ponds and lakes constructed over sinkholes may drain suddenly into subterranean cavities
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