Caves are dark, mysterious geological structures that develop over millions of years. Some can be as small as a room, while others extend for miles. Mammoth Cave is one example of an incompletely charted cave. It contains 350 miles of known passageways, and an estimated 600 miles of unexplored surfaces. All caves have certain features in common, but they may form differently.
Defining a Cave
A cave is an underground cavern with an opening at the surface that is large enough for a person to enter. There are several types of caves including solution caves, lava caves, sea caves, and glacier caves. Lava caves are formed as a result of lava flow and sea caves develop as a result of waves. Glacier caves and solution caves form as water drains from the surface, through the Earth, and into the underground water table.
The geology of a cave consists of several structures. Mineral deposits, rock slabs, cave fill, underground rivers, and waterfalls are some of the features of cave geology. Mineral deposits take the form of large, sharp spikes that hang from the ceiling of the cave or protrude from the ground. It forms as a result of dripping water, which leaves behind minerals on the surrounding structure as it flows through the Earth.
Stalactites are the vertical formations that hang from the ceiling, and stalagmites are the similarly shaped mineral deposits that rise from the ground. Eventually, the stalactite and stalagmite will meet. Another type of structure that develops from dripping water is helictites. Helictites consist of mineral deposits, but the direction of the formation is not uniform. It usually appears as a spiral and expands in any direction.
The cave itself may have a layer of sand, silt, clay, gravel, or a mix. Collectively, this substance is described as cave fill, and it is usually deposited after the cave is hollowed out. The small stones within this mix are very smooth. An occasional block of rock dislodged from the ceiling may occupy a cave.
Mineral Composition of Caves
The most common mineral found in caves is calcite, which is made of calcium carbonate. The majority of helictites, stalactites and stalagmites are made of calcite. There are several other minerals that are found in cave geology including:
- Aragonite – It is made of calcium carbonate and found in needle-like crystals called anthodites.
- Gypsum – It is made of calcium sulfate and forms flower like structures aptly named gypsum flowers.
- Selenite – It is a form of gypsum that develops into long transparent rods.
- Limonite – An iron oxide mineral.
- Goethite – An iron hydroxide mineral.
- Nitrocalcite – It is made of calcium nitrate and is found in cave fill materials.
- Barite – It is made of barium sulfate and is found in cave fill materials.
- Celestite – It is made of strontium sulfate and is found in cave fill materials.
How Caves Form
Different types of caves form differently. A lava cave is basically a tunnel that forms around flowing lava. The exterior of the lava cools first and hardens, creating a tube within which hotter lava flows through. After the lava drains out, the remaining structure is hollow. Sea caves are carved out of the coastline by waves. Both the force of the water and the sand and gravel beating against the rocks eventually wear away the surface and form a cave. Glacial caves form when melt water burrows through the ice and creates hollowed out caverns.
Solution caves also form from draining water, but in a different way. The rocks submerged within the water table are generally limestone, which consists of calcite. The water is slightly acidic, which dissolves the calcite and forms small open spaces within the limestone. These spaces eventually become the cavern or passageways of a cave when the water table drops. During this period, the spaces are exposed to air, and through a series of chemical processes, receives mineral deposits in the form of stalactites and stalagmites.
“Mammoth Cave National Park Information Page”
“Geology of Caves.” – US Geological Survey
“Cave.” – Merriam Webster Visual Dictionary Online