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TITLE: Caves

A Subterranean Wonderland. Drop by drop, year after year, cave deposits slowly grow. Because these formations build up over many human lifetimes and are very fragile, they are irreplaceable. Today, both federal and state laws protect cave deposits.

An icicle-shaped formation that grew down from a cave roof. Early in their growth, stalactites resemble soda straws.

This special type of flowstone called drapery was deposited by water flowing down a cave wall.

Cave Popcorn
Formed on cave walls, the sides of other formations, and the edges of pools.

A pillar-shaped formation that grew up from a cave floor. The drip and flow of water, along with the varying rate of growth, produced this specimen's texture.

How to Make a Cave
Trickle rainwater through soil into cracks in a thick underground bed of limestone. The water will absorb carbon dioxide from the air and from microscopic organisms in the soil to become a weak acid.

H2O + CO2 = H2CO3

water + carbon dioxide = carbonic acid


Step 2
Let the acidic groundwater slowly dissolve calcite in the limestone. Tiny cracks will grow bigger as more rock is dissolved.

H2CO3 + CaCO3 = Ca2+ + 2 HCO31-

carbonic acid + calcite = dissolved calcium + dissolved bicarbonate


Step 3
Continue this for thousands of years. The cracks will slowly enlarge into underground tunnels.


Step 4
The acidic groundwater that creates caves can also build wonderful cave formations. Loss of carbon dioxide from the water causes calcite to crystallize.

Ca2+ + 2 HCO31- = CO2 + CaCO3 + H2O

dissolved calcium + dissolved bicarbonate = carbon dioxide + calcite + water

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits