A water-filled subterranean void is classified based on its formation process and geographic location. These features are described by specific nomenclature reflecting the forces that carved the rock, such as mechanical wave action or chemical dissolution. Scientific classification separates these features into two main categories: those formed along coastlines by physical erosion and those formed inland by chemical weathering of bedrock.
Coastal Caves Formed by Erosion
Coastal caves containing water are primarily known as sea caves or littoral caves, defined by the mechanical power of wave energy. These formations occur along coastlines where the ocean impacts a cliff face, exploiting natural weaknesses like joints, faults, or softer rock layers. The process is distinct from chemical weathering, relying instead on physical forces to hollow out the rock.
The primary mechanism is hydraulic action, which involves the sheer force of water crashing into the rock and compressing the air trapped within cracks. This rapid compression and decompression of air exerts immense pressure, progressively widening the fissure until rock fragments break away. This action is most effective near the water line, often resulting in caves that are wide and low.
Abrasion is another significant factor, where rocks, sand, and other sediments carried by the waves are hurled against the cliff face, grinding and scouring the bedrock. This effect, combined with the hydraulic pressure, continually deepens the initial notch or fissure into a full-fledged cave.
Prominent examples include the Sea Lion Caves in Oregon and the Algar de Benagil in Portugal. Continued wave action on these formations can eventually lead to the collapse of the roof, leaving behind a sea arch. Sea caves can form in almost any type of rock, as their creation depends on physical force rather than chemical composition.
Inland Caves Formed by Dissolution
The most common type of water-filled cave inland is the solution cave, which forms through the chemical dissolution of soluble bedrock, a process known as karstification. This process primarily acts on carbonate rocks like limestone and dolomite, which are susceptible to a weak acid formed when rainwater absorbs carbon dioxide from the atmosphere and soil. The resulting carbonic acid reacts with the calcium carbonate in the rock, slowly dissolving it and creating a network of underground passages.
The resulting topography is known as karst, characterized by features like sinkholes and underground drainage systems. Cenotes are a specific example, prevalent on Mexico’s Yucatán Peninsula, representing water-filled sinkholes. These natural pits form when the roof of a subterranean cavern collapses, exposing the groundwater below and providing access to a vast network of flooded cave systems.
A blue hole is essentially a submarine sinkhole, a type of water-filled dissolution cave formed on land during periods of lower sea level, such as the last Ice Age. Groundwater dissolved the exposed limestone, creating deep vertical shafts and caverns that were later flooded when sea levels rose. The deep blue color of these features, like the Great Blue Hole in Belize, results from the extreme depth and high transparency of the water.
Flank Margin Caves
Another specialized dissolution feature is the flank margin cave, which forms in coastal carbonate islands and coasts at the boundary between fresh groundwater and denser marine water. Dissolution is maximized in this mixing zone, or halocline, resulting in large, globular chambers that are horizontally extensive but vertically constrained. These caves are formed by mixing dissolution and can be found in places like the Bahamas.
Specialized Aquatic Cave Life
The stable, lightless environment of aquatic caves supports a unique collection of specialized organisms collectively known as stygofauna. These species inhabit the underground water systems, including groundwater, submerged passages, and the water-filled sections of solution caves. Stygofauna that are entirely dependent on this subterranean habitat for their entire life cycle are specifically termed stygobites.
Stygobites exhibit remarkable biological adaptations, most notably the loss of pigmentation, giving many species an albino appearance, and the reduction or complete loss of functional eyes. Since light is absent, natural selection favors the development of other sensory organs. Many species possess enhanced chemosensory and mechanosensory systems, such as elongated antennae and specialized hairs, to navigate and locate scarce food resources in total darkness.
Other aquatic cave inhabitants include stygophiles, which can live both in and out of the cave environment, and stygoxenes, which use the cave water only temporarily, often to forage or seek shelter. The energy source for these ecosystems is typically limited to organic matter washed in from the surface, creating an ecosystem where organisms are adapted to low-nutrient conditions and slow metabolic rates. This results in stygobites often having longer lifespans and lower reproductive rates than their surface-dwelling relatives.