Caverns

Caverns occur under the surface of the Earth wherever soluble rocks allow them to develop large openings. Limestone or calcite-rich (CaCO3) bedrock is the most common host rock, as well as dolostone rocks composed of the mineral dolomite (CaMg(CO3)2). The evaporite rocks of gypsum (CaSO4) and rock salt (NaCl) are also subject to solution and can develop underground [Page 367]openings large enough to be considered caverns. In a few places, caverns have developed in other rock lithologies, such as where a calcite-cemented sandstone has also been subjected to solution. Several natural processes other than the dissolving of the bedrock can also produce caves, such as ocean waves and lava flows, but these are almost never large enough to constitute a true cavern, which has an implication of being of great size.

Caverns are almost always also of the solutional type, wherein the rock is dissolved, either partly in the upper vadose (undersaturated with free air surfaces) zone above the water table or more commonly beneath an unconfined water table in the bedrock as a water-filled phreatic cavern. Only when the water table is lowered, usually when the climate changes to a more arid one and the water table drops or when a nearby stream erodes downward enough to allow the water to drain out of the caverns, does air enter the opening and the cavern become part of the vadose realm. Vadose caverns can also pass from phreatic to vadose and back again many times in response to water table fluctuations associated with varying climates or tectonics. In addition to those formed in association with unconfined water tables, some caverns are formed entirely under conditions of confined or artesian water.

Speleogenesis, or the origin of caves and caverns, is a process wherein surface water (H2O) infiltrating through the soil reacts with rotting organic matter to gain a bit of carbon dioxide (CO2) and becomes carbonic acid (H2CO3). This weak acid in the groundwater trickles slowly along joints and fissures in the bedrock to come in contact with the calcium carbonate (CaCO3) or the calcite mineral of the limestone, thereby dissolving it in the carbonation reaction. Mammoth Cave in Kentucky is this kind of cavern. Gypsum is 10 to 20 times more soluble in water than is carbonate rock, so caverns in that rock can form more quickly. Similarly, rock salt is even more soluble than gypsum, so large caves can form in it. Neither gypsum nor rock salt are very common, except in arid areas, with the result that caverns developed in these [Page 368]rocks are far rarer than those in limestone. Furthermore, because rock salt is so mobile underground and moves upward in salt domes or diapirs, where it is loaded by overlying rock, any openings that have formed in it deep underground tend to anneal or be resealed and infilled by the flowing salt. Only where a surface salt deposit is subjected to a falling water table or where a salt diapir penetrates the surface into the vadose zone and forms an uplifted region do caverns form and remain open. In these cases, the rainwater simply dissolves the salt, so that the brine flows back out onto the surface at a lower level.

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