The term “cave” describes any natural void in rock, ground, or ice large enough for a person to enter, meaning not all caves are underground. While many of the world’s largest caves are deep beneath the surface, a cave is defined by its void space, not its depth or formation process. The diverse geological forces that create these hollows determine their environment, resulting in formations that range from expansive subterranean networks to small coastal openings. The three primary mechanisms that shape these natural spaces are chemical dissolution, mechanical erosion, and igneous activity.
The Primary Mechanism: Solution Caves
Solution caves are the most common type worldwide, created by dissolution where acidic water slowly eats away at soluble bedrock. The process begins as rainwater absorbs carbon dioxide from the atmosphere and soil, forming a weak carbonic acid solution.
This slightly acidic water seeps through fractures in rocks like limestone, dolomite, or marble. The carbonic acid reacts with the calcium carbonate, gradually dissolving the rock and enlarging the initial cracks over vast periods. This process creates karst topography, characterized by surface features like sinkholes and disappearing streams.
The deepest cave systems, such as Mammoth Cave, typically form below the water table in the phreatic zone, where the rock is saturated with groundwater. As water tables lower, these passages drain and are exposed to the air. Once air enters, secondary mineral deposits known as speleothems (like stalactites and stalagmites) begin to form from the precipitation of dissolved minerals.
Caves Formed by Surface Erosion
Many caves result from mechanical forces acting directly on the Earth’s surface, rather than internal chemical processes. Sea caves, for example, form along coastlines and lakeshores where wave action exploits weaknesses in cliff bedrock. The primary erosional forces are hydraulic action, where the physical force of waves compresses air into rock fissures, and abrasion, which involves the grinding of rock by sand and pebbles.
This constant, high-energy impact wears away the rock, typically along faults or joints, creating a surface opening. Continued erosion of a sea cave can eventually break through a headland to form a sea arch, or its roof may collapse to leave an isolated sea stack. Other near-surface features, such as rock shelters, are often created by weathering or differential erosion that hollows out a shallow space at the base of a cliff.
Caves Formed by Igneous Activity
Lava tubes are the most common cave type created by thermal processes associated with volcanic activity. They form during the flow of low-viscosity lava, such as the basalt found in Hawaii or New Mexico. As the molten lava flows from a vent, its outer surface is exposed to cooler air and begins to solidify, forming an insulating crust.
The hot, molten lava beneath this hardened shell continues to flow downhill in a channel. If the lava source subsides or is diverted, the liquid interior drains out, leaving behind a long, hollow, cylindrical tunnel. These caves form rapidly during the eruption and cooling, and are generally near-surface features, often with collapsed roofs known as skylights.