II.

Formation of Caves

Naturally formed caves evolve in various ways, in many cases as a result of the solvent action of water and compounds in it. Known technically as caves of solution, such chambers are most common in limestone formations, particularly in regions that have ample rainfall. The surface water in such regions contains carbon dioxide and acids derived from the organic constituents of the soil. Attacking the soluble limestone, this acidic water dissolves and carries the limestone away in solution. Over long periods of time, such action results in the formation of subterranean chambers. The depth of such chambers depends on the depth of the water table (see Water).

If after several unusually wet years the water table is rising, old cave chambers become flooded and new ones begin forming at higher levels. Likewise, during a long dry spell, chambers will begin forming at lower levels, closer to the declining water table. Over thousands of years, fluctuations such as these produce multi-level cave systems, as in Mammoth Cave National Park in Kentucky, where a subterranean stream flows through the lowest level. Underground rivers erode and transport sediments and rock fragments in a manner analogous to that of surface streams. If such action has been predominant, the cave is said to have been formed by mechanical abrasion.

Another way that limestone caves are thought to form is from the action of sulfuric acid produced by bacteria. Recent research indicates that some large caves such as Carlsbad Caverns formed in this way instead of by the slow dissolving effects of carbonic acid from carbon dioxide and water. Oil deposits deep beneath limestone formations released hydrogen sulfide gas. Underground microbes that derive energy from hydrogen sulfide helped create sulfuric acid in combination with oxygen and water. The strong acid solution dissolved the limestone, hollowing out the chambers and leaving gypsum formations, which are not found in caves formed mainly by water. Other caves where this sulfuric acid process is still taking place often have coatings of bacterial slime on the walls and ceilings.

River action forms still another type of cave, commonly with a very large opening that gives it the appearance of a natural amphitheater. A river entrenched in a steep-walled canyon most actively erodes that portion of the canyon wall against which the current is strongest, as at a bend or in a meander. By erosion, solution, and quarrying, the river excavates a large quantity of rock, forming a large undercut area in the side of the canyon. With the passage of time the riverbed is lowered, and eventually the cave is left high in the side of the canyon. Such rock shelters were used extensively in what is now the southwestern United States by the prehistoric Native Americans known as Cliff Dwellers, who built their homes within them (see Cliff Dweller).

Wind action (eolian action) is partly responsible for the formation of small caves that are confined mostly to desert or semidesert regions. The action of windblown sand is one of several forces involved in the formation of these grottoes and caves in rock ledges and cliffs.

Lava caves can form during volcanic eruptions when molten rock runs beneath the surface of a lava flow that has hardened. When the liquid lava drains away, it leaves a hollow tube in the solid rock. Chambers or pockets of gas in lava can also form caves. Ice melting underneath lava can also hollow out caverns.

Ice caves sometimes occur in glaciers or in icebergs when meltwater drains through or under the ice, eroding it. Wave action against sea cliffs can erode the rock to form sea caves. Caves may also result from large boulders piled together by rockslides or when earthquakes create large cracks in rocks.