sinkholes - Sinkholes Land Subsidence and Swelling Soils...

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Sinkholes, Land Subsidence and Swelling Soils Types of Ground Movement Sinkholes: overlying ground collapses into underground cavities Land subsidence: sediment becomes more closely packed, through groundwater or petroleum extraction, or earthquake shaking Swelling soils: from alteration (weathering) of volcanic ash to clays Cause of soil creep, but create problems away from slopes Sinkholes Ground may suddenly collapse into sinkholes tens to hundreds of meters across Can damage houses and roads Can drain streams, lakes and wetlands Can channel contaminants directly into underground aquifers Processes That Cause Sinkholes Some common sedimentary rocks are soluble in water -Salt, gypsum, limestone, other carbonates Areas with underlying limestone can have caves, springs, streams that sink into ground, sinkholes Caverns form where carbonate rocks near water table dissolve in groundwater Roof of cavern collapses if left unsupported where water table has dropped Limestone dissolves in slightly acidic rainwater, at rate of millimeters per thousand years Slightly faster rate in humid areas sinkholes and caverns more common in tropical climates Air pollution makes rain more acidic, increases rate of dissolution Highest rate of dissolution occurs at level of water table Landscape of limestone dissolution: karst (Kentucky) Formation of Calcium Bicarbonate H 2 O (rain water) + CO 2 (carbon dioxide) = H 2 CO 3 (carbonic acid in water) Carbonic acid reacts with limestone to form calcium bicarbonate: H 2 CO 3 + CaCO 3 = [Ca ++ + 2HCO 3 - ] Types of Sinkholes (Dis)Solution : acidic groundwater seeps through soil, dissolves underlying limestone along fractures, which widen and create holes- slow process- small (Cover) Subsidence : where deep, permeable sediment sits atop limestone, numerous
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sinkholes - Sinkholes Land Subsidence and Swelling Soils...

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