Caverns are natural underground voids large enough for human entry that form as a result of geological processes acting over vast timescales. These subterranean spaces offer a window into Earth’s history and house specialized ecosystems. The formation and characteristics of these underground systems are the focus of speleology. Caverns are active, evolving features that reflect the delicate interplay between rock, water, and time.
The Science of Formation
Most large caverns are created through dissolution, a chemical process that occurs most effectively in soluble rock formations like limestone or dolomite, forming karst topography. This process begins when rainwater absorbs carbon dioxide (CO2) from the atmosphere and the soil, creating a weak carbonic acid (H2CO3). This slightly acidic water then seeps into cracks and fractures in the bedrock.
As the acidic water moves through the rock, it reacts with the calcium carbonate (CaCO3) that constitutes limestone, slowly dissolving it over thousands to millions of years. This chemical weathering gradually enlarges the initial fractures into a network of passages and chambers. In some cases, such as the formation of Carlsbad Caverns, a more potent sulfuric acid (H2SO4) is the dissolving agent, created when hydrogen sulfide gas from deep underground petroleum reservoirs reacts with oxygenated groundwater.
While chemical dissolution is the dominant mechanism, other forces also create underground voids. Mechanical processes, such as tectonic movement along fault lines, can cause large blocks of rock to shift and create open fracture caves. Erosion from flowing water carrying abrasive sediments can also carve out passages in any type of rock, though these erosional caves are generally less extensive than those formed by chemical action. These secondary processes contribute to the diverse array of subterranean landscapes.
Classifying Caverns by Origin
Caverns are categorized based on the specific geological mechanism responsible for their origin. Solution caverns, formed by the dissolution of soluble bedrock, are the most widespread and are responsible for the largest cave systems known today. They are characterized by horizontal passages, vertical shafts, and large, irregular chambers that follow the bedding planes and joints of the rock.
Lava tubes are primary caves formed during volcanic activity. As molten lava flows, its surface cools rapidly upon contact with the air, forming a solid crust that insulates the hot lava still flowing beneath. When the eruption ceases, the molten lava drains out, leaving behind a hollow, smooth-walled, tubular passage. These tubes are found in volcanic regions, such as Hawaii and Iceland.
Mechanical erosion by water causes sea or littoral caves, which are carved out along coastlines by the pounding of ocean waves. The hydraulic pressure and abrasive power of the water exploit weaknesses and fractures in sea cliffs, gradually hollowing out chambers that rarely extend far inland. Another mechanically formed cave is the ice cave, which can refer either to a bedrock cave containing perennial ice formations or to a glacier cave formed directly within the ice by meltwater flowing through it.
Unique Features and Structures
The interior of many caverns is defined by mineral deposits known as speleothems, which are secondary formations. These structures result from the reverse chemical reaction that created the cave: the precipitation of calcium carbonate. Water saturated with dissolved minerals seeps into the cave atmosphere, where it releases excess carbon dioxide, causing the minerals to solidify.
The most recognized speleothems are stalactites, which are icicle-shaped deposits that hang from the ceiling, and stalagmites, which are mounded formations that grow upward from the cave floor. When a stalactite and its corresponding stalagmite merge, they form a unified column extending from floor to ceiling. Other common forms include flowstone, which are sheet-like deposits that coat walls and floors, and draperies, which are thin, curtain-like sheets of calcite.
Caverns host a biological community adapted to permanent darkness and stable temperatures. Strict cave dwellers, known as troglobites, have evolved characteristics for life without light, such as the loss of eyes and skin pigment. These animals rely on food sources washed in from the surface. Other species, called troglophiles, can complete their life cycle both inside and outside the cave, retaining the ability to survive elsewhere.