Caves are cavities in Earth that typically form over thousands of years through natural mechanical and chemical processes. Most underground caves form as slightly acidic groundwater dissolves limestone rock. The result is a subterranean environment of caverns and, in some places, vast networks of smaller, interconnecting chambers. This interactive activity, adapted from The Virtual Cave, by Dave Bunnell, provides a detailed map of a solution cave system with links to images and descriptions of its many formations.
Underground caves, also known as solution caves, form from a chemical reaction between water and rock. Limestone rock is composed primarily of the mineral calcite, a crystallized form of calcium carbonate. Pure water by itself has little effect on calcite. Rainwater, however, which absorbs carbon dioxide (CO2) as it passes through the atmosphere and percolates through soil, becomes a slightly acidic solution called carbonic acid.
Carbonic acid is capable of dissolving calcite. When it does, it forms a solution called calcium bicarbonate. As the calcite is slowly dissolved, cavities and passageways form in the limestone rock. Most initial cave development occurs just below the water table in the zone of saturation.
A secondary phase of development occurs after the water table falls and the cavities fill with air. As soon as calcium bicarbonate solution enters a ventilated cave through cracks in its ceiling, CO2 gas begins to evaporate out of solution. Calcium bicarbonate only exists in solution, and the loss of CO2 prompts calcite to be deposited and the remaining water to run off. The general term for all such secondary cave formations is speleothems, from the Greek words spelaion, meaning cave, and thema, meaning deposit.
Speleothems form at varying rates, depending on how fast the calcite residue builds up. Although it takes an average of 120 years for a cubic inch of calcite to form, environmental factors can influence the rate of growth. For instance, higher ground temperature speeds the decay rate of plants and animals. This, in turn, adds more organic material to groundwater and raises its calcium bicarbonate concentration. Because this fortified water is more acidic, the rate of speleothem growth increases.
The location and unique shapes of speleothems depend on whether the depositing water source is flowing, dripping, seeping, splashing, still, or heated, among other possibilities. The most commonly known speleothems are stalactites and stalagmites. Stalactites project down from the ceiling and resemble slowly dripping icicles. Their droplets give rise to stalagmites, which grow up from the cave floor.
A delicate soda straw represents the earliest growth phase of stalactites. Imagine a thin tube attached to the ceiling. As a drop of calcium bicarbonate solution accumulates and runs down the inside of the tube, it hangs momentarily -- and sometimes much longer -- before falling to the floor. During this hesitation, CO2 escapes into the air, precipitating a very thin film of calcite. Over successive drops, the straw lengthens ever so slightly.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.