How are caves formed in the Earth's crust?

Answer 1

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 found in the Hawaiian Islands. 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 hosted in rocks other than limestone are usually formed by water erosional processes. For example, rivers running through canyons with steep walls erode the rock at points where the current is strong. Such caves usually have large openings and are not too deep. Caves of this type can be found in the southwestern United States and were at one time inhabited by prehistoric American Indians known as Cliff Dwellers. Sea caves are formed by waves continually crashing against cliffs or steep walls. Often these caves can only be entered at low tide. Ice caves are also formed in glaciers and icebergs by meltwater that drains down crevices in the ice.

Lava caves, which are often several miles long, form when the exterior of a lava flow hardens and cools to form a roof, but lava below the surface flows out, leaving a hollow tube. Wind or aeolian caves usually form in sandstone cliffs as wind-blown sand abrades the cliff face. They are found in desertareas, and occur in a bottleneck shape with the entrance much smaller than the chamber. Talus caves are formed by boulders that have piled up on mountain slopes. The most common, largest, and spectacular caves are 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 (H2CO3). 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 seen 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.

The deep cave environment is often completely dark, has a stable atmosphere, and the temperature is rather constant, varying only a few degrees throughout the year. The humidity in limestone caves is usually near 100%. Many caves contain unique life forms, underground streams and lakes, and have unusual mineral formations called speleothems.

When groundwater seeps through the bedrock and reaches a chamber or tunnel, it meets a different atmosphere. Whatever mineral is in solution reacts with the surrounding atmosphere, precipitates out, and is deposited in the form of a crystal on the cave ceiling or walls. Calcite, and to a lesser degree, aragonite, are the most common minerals of speleothems. The amount of mineral that precipitates out depends upon how much gas was dissolved in the water. For example, water that must pass through a thick layer of soil becomes more saturated with carbon dioxide than water that passes through a thin layer. This charges the water with more carbonic acid and causes it to dissolve more limestone from the bedrock. Later, it will form a thicker mineral deposit in the cave interior as a result.

Water that makes its way to a cave ceiling hangs as a drop. When the drop of water gives off carbon dioxide and reaches chemical equilibrium with the cave atmosphere, calcite starts to precipitate out. Calcite deposited on the walls or floors in layers is called flowstone.

Sometimes water runs down the slope of a wall, and as the calcite is deposited, a low ridge is formed. Subsequent drops of water follow the ridge, adding more calcite. Constant buildup of calcite in this fashion results in the formation of a large sheet-like formation, called a curtain, hanging from the ceiling. Curtain formations often have waves and folds in them and have streaks of various shades of off-white and browns. The streakiness results from variations in the mineral and ironcontent of the precipitating solution.

Often, a hanging drop falls directly to the ground. Some calcite is deposited on the ceiling before the drop falls. When the drop falls, another takes its place. As with a curtain formation, subsequent drops will follow a raised surface and a buildup of calcite in the form of a hanging drop develops. This process results in icicle-shaped speleothems called stalactites. The water that falls to the floor builds up in the same fashion, resembling an upside down icicle called a stalagmite.

Of course, there are variations in the shape of speleothems depending on how much water drips from the ceiling, the temperature of the cave interior, rates and directions of air flow in the cave, and how much dissolved limestone the water contains. Speleothems occur as tiered formations, cylinders, cones, some join together, and occasionally stalactites and stalagmites meet and form a tower. Sometimes, when a stalactite is forming, the calcite is initially deposited in a round ring. As calcite builds up on the rim and water drips through the center, a hollow tube called a straw develops. Straws are often transparent or opaque and their diameter may be only that of a drop of water.

Stalagmites and stalactites occur in most solution caves and usually, wherever a stalactite forms, there is also a stalagmite. In caves where there is a great deal of seepage, water may drip continuously. Speleothems formed under a steady drip of water are typically smooth. Those formed in caves where the water supply is seasonal may reveal growth rings similar to those of a tree trunk. Stalactites and stalagmites grow by only a fraction of an inch or centimeter in a year, and since some are many yards or meters long, one can appreciate the time it takes for these speleothems to develop.

The most bizarre of speleothems are called helictites. Helictites are hollow, cylindrical formations that grow and twist in a number of directions and are not simply oriented according to the gravitational pull of a water drop. Other influences such as crystal growth patterns and air currents influence the direction in which these speleothems grow. Helictites grow out from the side of other speleothems and rarely grow larger than 4 in (8.5 cm) in length.

Speleothems called anthodites are usually made of aragonite. Calcite and aragonite are both forms of calcium carbonate, but crystallize differently. Anthodites grow as radiating, delicate, needle-like crystals. Pools of seepage water that drain leave behind round formations called cave popcorn. Cave pearls are formed in seepage pools by grains of sand encrusted with calcite; flowing water moves the grains about and they gather concentric layers of calcite.