Cave

A cave is a naturally occurring hollow area inside the earth. Most caves are formed by some type of erosional process. The most notable exception is hollow lava tubes such as those in Hawaii. The formation of caves depends upon geologic, topographic, and hydrologic factors. These factors determine where and how caves develop, as well as their structure and shape. The study of caves is called speleology. Some caves may be small hillside openings, while others consist of large chambers and interconnecting tunnels and mazes. Openings to the surface may be large gaping holes or small crevices.

Caves have provided shelter to prehistoric, ancient, and primitive contemporary people such as the Tasadays of the Philippine Islands. Human remains, artifacts, sculptures, and drawings found in caves have aided archeologists to learn about early humans. Caves are sites of many important archeological discoveries such as The Dead Sea Scrolls. Many religious traditions have regarded caves as sacred and have used them to perform rituals, ceremonies, and sacrifices. Some ancient traditions felt that caves led to the underworld. Caves have fascinated poets, artists, philosophers, and musicians.

Solution caves

Solution caves form by chemical weathering of the surrounding bedrock as groundwater moves along fractures in the rock. These caves produce a particular type of terrain called karst. Karst terrain primarily forms in bedrock of calcium carbonate, or limestone, but can develop in any soluble sedimentary rock such as dolomite, rock gypsum, or rock salt. The host rock extends from near the earth's surface to below the water table. Several distinctive karst features make this terrain easy to identify. The most common are sinkholes, circular depressions where the underlying rock has been dissolved away. Disappearing streams and natural bridges are also common clues. Entrances to solution caves are not always obvious, and their discovery is sometimes quite by accident.

Formation of karst involves the chemical interaction of air, soil, water, and rock. As water flows over and drains into the earth's surface, it mixes with carbon dioxide from the air and soil to form carbonic acid (H 2CO ₃). The groundwater becomes acidic and dissolves the calcium carbonate in the bedrock, and seeps or percolates through naturally occurring fractures in the rock. With continual water drainage, the fractures become established passageways. The passageways eventually enlarge and often connect, creating an underground drainage system. Over thousands, perhaps millions of years, these passages evolve into the caves we see today.

During heavy rain or flooding in a well-established karst terrain, very little water flows over the surface in stream channels. Most water drains into the ground through enlarged fractures and sinkholes. This underground drainage system sometimes carries large amounts of water, sand, and mud through the passageways and further erodes the bedrock. Sometimes ceilings fall and passageways collapse, creating new spaces and drainage routes.

Not all solution caves form due to dissolution by carbonic acid. Some caves form in areas where hydrogen sulfide gas is released from the earth's crust or from decaying organic material. Sulfuric acid forms when the hydrogen sulfide comes in contact with water. It chemically weathers the limestone, similar to acid rain.