Formation and Features of Cave Networks
Movement of Groundwater Creates Systems of Caverns and Passages
Aug 17, 2009
Caves or caverns are beautiful by-products of naturally occurring geologic processes. Over time, the interactions between water and limestone, a sedimentary rock consisting mainly of the mineral calcite, form a small hollow in the bedrock, which can grow into a colossal system of caverns and passages. When groundwater naturally drains from these networks, the precipitation of calcite creates spectacular formations called speleothems that are a feast for the eyes.
Cave Formation
As rain falls, it reacts with carbon dioxide in the atmosphere to form carbonic acid. The acid rain dissolves limestone rock exposed at the Earth’s surface, but also percolates underground and dissolves the limestone below the surface.
Caves form at or near the water table, which Stephen Marshak defines in his book Earth: Portrait of a Planet as a “subsurface boundary between rock or sediment in which pores contain air and rock or sediment in which pores contain water.” Here, the acidic groundwater slowly dissolves the surrounding limestone over time. When the water table drops, the groundwater drains away, leaving behind an empty cave.
Marshak points out four main requirements for significant cave formation. The first requirement is a thick deposit of limestone bedrock. Without this, any caves formed will not be big enough to notice. The second requirement is significant rainfall, which supplies sufficient carbonic acid to the subsurface. Third, the land surface must be above sea level. Otherwise, the water table will not be underground. The last requirement is a temperate to tropical climate. Outside of a certain temperature range, calcite dissolution will progress too slowly to have any significant effect.
Development of Cave Networks
The continual movement of groundwater through the bedrock constantly changes the shape of the cavern and its speleothems, or cave formations, eventually resulting in a cave network. These networks are composed of large openings, called rooms or chambers, connected by tunnel-like passages. Large chambers form where the limestone bedrock is the most soluble and groundwater flow is the fastest. Passages generally form from the flow of groundwater along pre-existing joints, or naturally formed cracks in the rock.
Most calcite dissolution occurs just below the water table. This is where the groundwater’s acidity is highest and its flow the fastest. Therefore, openings in cave systems are often aligned along the same horizontal plane. The shape of the network itself is determined by variations in permeability and composition of the bedrock in which the caves have formed.
Growth of Speleothems
Groundwater will continue to move through the cavern even after changes in the water table leave an empty space. Carbon dioxide will evaporate out of the water as it drips from the cave ceiling or along its walls, making the water less acidic. As this happens, any calcite dissolved in the water will precipitate, creating speleothems. Speleothems, as defined by Marshak, are cave features formed by the precipitation of calcite.
Marshak points out several basic speleothems: soda straws, stalactites, stalagmites, limestone columns, and flowstone. A soda straw forms when calcite precipitates around the outside of a consistent water drip, creating a hollow tube. A stalactite forms as precipitated calcite adds to the tip of an icicle-shaped cone. Conversely, a stalagmite forms where dripping water deposits calcite on the cave floor, building an upward-pointing cone. When stalactites and stalagmites merge or when a stalactite reaches the cave floor, a limestone column forms. Flowstone develops as groundwater flows along cave walls, instead of dripping, depositing sheets of limestone.
AbsoluteAstronomy, an Earth and space science reference site, also defines several more unique and complex speleothems, such as cave popcorn, helictites, dogtooth spar, frostwork, and anthodites. Cave popcorn consists of small clusters of very tiny calcite crystals that resemble popcorn in appearance. Helictites are a special type of stalactite that feature a central canal with twig-like or spiral projections that can sometimes look like ocean coral. Dogtooth spar consists of large calcite crystals that resemble the shape of a dog’s tooth and often forms near pools of groundwater. Frostwork is a needle-shaped growth of calcite. Anthodites are clusters of calcite crystals that resemble the appearance of flowers.
Evolution of Cavern Systems
Cave networks are in a perpetual state of flux due to the geologic processes and climatic characteristics that work together to form them. Acid rain and groundwater are always moving through the subsurface, creating new caverns and reshaping existing ones. The slow movement of groundwater through empty caves constantly forms new and awe-inspiring speleothems. These are processes that can occur over tens of thousands of years or longer, creating vast cave systems.
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