Caves are formed under the action of geological processes. They include combinations of chemical processes, erosion, tectonic forces, microorganisms, pressure and atmospheric effects. Most caves are formed in limestone due to dissolution.
Caves can be formed wherever there is soluble rock, and are most prevalent in limestone, but can also be formed in other rock, including chalk, dolomite, marble, granite, salt, sandstone, hardened coral and gypsum.
The longest and most abundant caves are located in limestone. Limestone is dissolved by rainwater and ground water with the participation of free carbon dioxide H2CO3 or other mineral and organic acids. In the process of dissolution, a characteristic relief form known as karst is formed, which is characterized by funnels, flows and underground drainage. Limestone caves are often decorated with formations of calcium carbonate formed due to slow cooling of rocks. They include: stalactites, stalagmites, draperies and columns. These secondary placers in caves are called leaky formations.
One of the most decorated caves in the world is Lechuguilla Cave (New Mexico, USA). Lechuguilla and neighboring Carlsbad Caves are now examples of another type of caves. They were formed with H2S (Hydrogen Sulfide) gas rising from below, where oil reservoirs emit sulfur soot. This gas is mixed with groundwater to form H2SO4 (sulfuric acid). The acid then dissolves the limestone from below rather than from above with acid water soaked from the surface.
Some caves are formed at the same time as the surrounding rocks. These caves are sometimes called “primary” caves. Lava tubes are formed by volcanic eruptions and are most often concentrated in “primary” caves. The lava flow, cooling down, is covered with a hard crust, forming a lava tube, inside which still flows molten rock. After the eruption is actually over, lava flows out of the tube from the lower end, and inside the tube there is a cavity. Examples of such caves can be found in Tenerife, Great Island (Hawaii), and many other places.
Sea caves are found along coasts around the world. A special question – these are the coastal caves, which are formed on the coast of the sea in weakened areas under the influence of surf. Somewhere else, in other places, such as Phang Nga Cove in Thailand caves are flooded by the sea and now subject to coastal erosion. Caves of the sea have sizes in total from 5 meters (16 feet) to 50 meters (160 feet) in length, and sometimes they can exceed 300 meters (980 feet).
Glacier caves: Caves formed in the body of glaciers melted water. Such caves are found on many glaciers. Melt glacier water is absorbed by the glacier body through large cracks or at the intersection of the cracks, forming a course sometimes passable for humans. Characteristic lengths are the first hundreds of meters, depths – up to a hundred meters or more. A special type of glacier caves – caves formed in the glacier in the place of underground thermal water outlet. Since the water is hot, it can make volumetric galleries, but such caves do not lie in the glacier itself, but under it, because the ice melts from below. Thermal glacial caves are found in Iceland, Greenland and reach significant sizes.
Volcanic caves. These caves occur during volcanic eruptions. The flow of lava, cooling down, is covered with a hard crust, forming a lava tube, inside which still flows molten rock. After the eruption is actually over, lava flows out of the tube from the lower end, and inside the tube there is a cavity. It is clear that the lava caves lie on the surface itself, and often the roof collapses.
Tectonic caves. Such caves can appear in any rock as a result of the formation of tectonic faults. As a rule, such caves are found on the sides of deeply cut river valleys in the plateau, when huge massifs of rocks break off from the sides, forming cracks. The cracks are usually wedged to the depth. Most often they are filled with loose deposits from the surface of the massif, but sometimes form rather deep vertical caves, up to 100 m deep. The cracksi are widespread in Eastern Siberia. They are relatively poorly studied, and are probably very common.