Sulfuric acid cave

Sulfuric acid caves are caves formed when groundwater containing sulfuric acid (H2SO4) comes into contact with carbonate bedrock, creating a void in the rock.

Geochemistry and formation

The sulfuric acid that causes sulfuric acid speleogenesis is mostly derived from the oxidation of dissolved hydrogen sulfide (H2S). This H2S is in turn mainly derived from thermal or microbial sulfate reduction, though it is sometimes magmatic in origin. Thermal sulfate reduction is typical of deep environments (2.5 to 6 km deep, requiring temperatures between 100°C to 140°C)

while microbial sulfate reduction is more prevalent in shallower, cooler (up to 80°C) environments, where it is cool enough to be hospitable to sulfate-reducing microbes. Both forms of sulfate reduction rely on the presence of sulfates and on the presence of organic materials (frequently hydrocarbon deposits) from sediment to act as electron donors.

When groundwater carrying dissolved hydrogen sulfide encounters oxidizing conditions (oxygenated groundwater, preexisting caves), the H2S is oxidized into sulfuric acid. This is a mostly oxic process, although in low-oxygen environments, alternate electron acceptors like nitrate may allow the reaction to proceed. Sulfide minerals like pyrite and galena may be present in the carbonate host rock, and are frequently oxidized into sulfuric acid along with or instead of H2S.

Once sulfuric acid is present, it begins dissolving the carbonate host rock:

CaCO3 + H2SO4 + 2H2O ⇌ CaSO4 • 2H2O + CO2 + H2O

The CO2/carbonic acid released by this reaction may continue eroding the host rock on its own, as in carbonic acid speleogenesis. The dissolved sulfate anions and metal cations then begin precipitating into sulfate minerals such as gypsum.

Sulfuric acid speleogenesis occurs very quickly; large caves can form this way in a few thousand years, 1000 to 2000 times faster than carbonic acid speleogenesis.