page_title: Karst
section_title: Dissolution mechanism
hierarchical_section_title: Karst / Chemistry / Dissolution mechanism
caption_attribution_description: English: li jiang li river guilin yangshuo china 2011 karst raft
context_page_description: Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant rocks, such as quartzite, given the right conditions. Subterranean drainage may limit surface water, with few to no rivers or lakes. However, in regions where the dissolved bedrock is covered or confined by one or more superimposed non-soluble rock strata, distinctive karst features may occur only at subsurface levels and can be totally missing above ground.
  The study of karst is considered of prime importance in petroleum geology because as much as 50% of the world's hydrocarbon reserves are hosted in porous karst systems.
context_section_description: The carbonic acid that causes karstic features is formed as rain passes through Earth's atmosphere picking up carbon dioxide (CO₂), which dissolves in the water. Once the rain reaches the ground, it may pass through soil that can provide much more CO₂ to form a weak carbonic acid solution, which dissolves calcium carbonate. The primary reaction sequence in limestone dissolution is the following:
  In particular and very rare conditions such as encountered in the past in Lechuguilla Cave in New Mexico (and more recently in the Frasassi Caves in Italy), other mechanisms may also play a role. The oxidation of sulfides leading to the formation of sulfuric acid can also be one of the corrosion factors in karst formation. As oxygen (O₂)-rich surface waters seep into deep anoxic karst systems, they bring oxygen, which reacts with sulfide present in the system (pyrite or hydrogen sulfide) to form sulfuric acid (H₂SO₄). Sulfuric acid then reacts with calcium carbonate, causing increased erosion within the limestone formation. This chain of reactions is:
  This reaction chain forms gypsum.
caption: Li River, Guilin, China
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