How do caves form? Caves: Windows into the

The term “cave” refers to a natural opening, usually in Caves allow to access and investigate rocks, that is large enough for entry (Gunn, subterranean processes up close. 2004). There are several different types of caves: volcanic, glacier, crevice, erosion, and solution. Geology and tectonic activity, or folds and faults Solution caves occur in and and are within the ’s crust, is immediately obvious in the most common type in central Texas. Although caves. Cave walls, ceiling, and floor can show bends, dissolution is the dominant process, fractures, and folds in rock layers. other processes such as erosion and Cave passages cut through the rock gravitational breakdown, or layers and make rock identification collapse, can contribute to cave easier. development (Palmer, 1991). Aquifer Recharge becomes an Minerals vary widely in their observable process in caves. An chemical response to groundwater. aquifer is a body of rock capable of , for example, is soluble in a containing and transmitting large weak acid such as H2CO3 - carbonic quantities of water (Gunn, 2004); acid. Thus, limestone which consists caves are the largest pathway within primarily of calcite (CaCO3) is the aquifer. The water supply within vulnerable to chemical attack by the aquifer is slowly replenished with groundwater. Carbonic acid can each water droplet that falls from the form by a reaction between water cave ceiling to the floor below. and carbon dioxide. As rainwater, already slightly acidic (pH~6), Water flow paths are recorded by passes through the atmosphere and the location and patterns of the soil layer, it mixes with carbon formations throughout the cave dioxide and forms carbonic acid passage (Musgrove et al., 2001). (Fig. 1). The acidic water passes Fig. 1: Rainwater mixes with carbon Curtains of , sinuous dioxide in the atmosphere and soils to through fractures, crevices, and draperies, or a line of can form carbonic acid (H2CO3), which acts cavities and dissolves the limestone to dissolve away limestone. form as water passes through very slowly, enlarging the network fractures. A tight cluster of soda of passageways. Most solutional straws can indicate diffuse flow— caves require more than 100,000 years to grow large water passing through minute void spaces within enough for a human to be able to pass through (Palmer permeable limestone. 1991). References Most caverns are created at or just below the water Gunn, J., 2004, Encyclopedia of Caves and Science. New table in the zone of saturation in limestone. If the water York: Fitzroy Dearborn, 902p. table is stable, large openings can be created because Musgrove M., Banner J.L., Mack L.E., Combs D.M., James E.W., Cheng H., Edwards R.L., 2001, Geochronology water would contact all surfaces of the cave, dissolving of late to from the limestone at a large scale. If the water table drops, central Texas: Implications for regional paleoclimate: the area of active cave formation will move lower into Geological Society of America Bulletin, v. 113(12), p. the bedrock and the upper openings are left in the zone 1532-1543. of aeration, only subject to dissolution from running Palmer, A.N., 1991, Origin and morphology of limestone water. Water dripping through these dry passages may caves, Geological Society of America Bulletin, v. 103, p deposit CaCO3 in various forms collectively referred to 1-21. as speleothems.

Produced by the Environmental Science Institute, University of Texas at Austin Caves as a Window into the Edwards Aquifer project For more information: www.esi.utexas.edu/outreach/caves