GEOL 118 Lecture Notes - Lecture 23: Storm Drain, Karst
5 Apr 2016
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Geol 118 - lecture 23 - subsidence ii. Creation of underground cavity removes support for rock above, which collapse over cavity (collapse sinkhole) Downward dissolution of soluble rock (limestone) starting from surface (solution/subsidence sinkhole) Coal mining- involves human-constructed cavity (underground coal mine) After underground roof supports are removed, subsidence occurs in days or weeks. Indicated by circular or elongate depressions, ground fractures. Room and pillar mining (~50% coal removal) Subsidence may not occur (if rock is strong) Pillars = unmined areas that support overlying rock. Subsidence is worst for underground mines of low grade coal. Sinkholes- circular depressions (often water-filled) usually in limestone, bedrock, and other soluble bedrock. Karst topography- landscape dominated by underground caves + surface sinkholes, lakes, and disappearing streams. Form during periods of fluctuating water table (rainy/dry seasons or light/heavy groundwater removal) Step 1- need source of acid to dissolve limestone h2co2 produced when co2 dissolves in water.
