A cave is defined as a natural void in the ground that is large enough for human entry. The sheer variety of environments in which they form makes classifying them challenging for scientists. Caves are generally categorized based on the dominant geological process responsible for their creation.
Caves Formed by Chemical Dissolution
The most numerous and extensive cave systems on Earth are formed by the process of chemical dissolution, often known as solutional or karst caves. These caves are created when slightly acidic water dissolves soluble bedrock over vast periods of time. The primary rock type involved is limestone, but they also form in dolomite, gypsum, and halite.
The formation process begins when rainwater absorbs carbon dioxide from the atmosphere and the soil. This absorption creates a weak carbonic acid, a chemical agent that is powerful enough to react with the rock. The acidified water then percolates down through the earth, exploiting existing weaknesses in the bedrock, such as joints, faults, and bedding planes. Over thousands to millions of years, this chemical action gradually enlarges the microscopic cracks into expansive conduits and chambers.
The scale of these systems can be immense, often leading to distinct surface features known as karst topography, including sinkholes and underground rivers. While most form from surface water, solutional caves are sometimes formed by water that rises up from deep within the earth, known as hypogene caves. These hypogene systems can have different chemical compositions, sometimes involving sulfuric acid, and often result in intricate, branching patterns.
Caves Formed by Volcanic Activity
Another distinct category of cave forms concurrently with the rock itself through thermal processes. These are known as primary caves, with lava tubes being the most common example. Lava tubes are found in areas of volcanic activity where highly fluid, low-viscosity lava flows across the landscape.
The formation mechanism begins when the outer layer of a lava flow is exposed to the air and begins to cool and solidify, creating a hardened crust over the still-molten lava underneath. This solid outer layer acts as an insulator, allowing the fluid lava within to travel great distances from the volcanic vent. When the supply of molten lava ceases, the remaining liquid material drains out, leaving behind a hollow, horizontal conduit that can sometimes extend for miles. These caves are characterized by smooth, often glazed walls and features like lavacicles, which are formed by dripping lava rather than water-borne minerals.
Caves Formed by Physical Erosion and Earth Movement
A third group of caves is created by direct physical forces like abrasion, melting, or crustal shifting, rather than chemical change or molten rock. Erosional caves, such as sea caves, are shaped by the relentless mechanical power of water. Sea caves form along coastlines where ocean waves crash against cliffs, exploiting weaknesses like faults or softer layers of rock.
The waves use hydraulic pressure and the abrasive action of sand and pebbles hurled against the rock face to gradually widen cracks into chambers. This process of mechanical erosion is responsible for the formation of sea caves in various rock types, including basalt, sandstone, and granite. They are typically found near sea level, their size and shape constantly modified by tidal and wave action.
Glacier caves, which are distinct from rock caves containing ice, are another type of erosional cave. These are formed entirely within the ice of a glacier, not the surrounding bedrock. They are created when meltwater streams enter the glacier through crevasses and carve channels by both eroding and melting the ice.
Finally, tectonic caves are formed purely by the movement of the Earth’s crust. They result from faulting, folding, or landslides that cause large rock masses to shift apart, creating open voids or fissures. Tectonic caves are characterized by their irregular, jagged passages that align with the geological stress fractures of the region.