Also known as speleotherms:
It’s all about water. . . shallow seas covered the land millions of years ago, depositing layers of sediment that eventually hardened into limestone. Rainwater, made slightly acidic by absorbing carbon dioxide from the air and soil, penetrated cracks in the earth and slowly dissolved passages in it. Later, lowering groundwater levels left behind vast, air-filled rooms. Water seeping down from the surface of the earth dissolved minerals on its trip through the limestone. Once it reached the cave, the trapped carbon dioxide escaped from the water. No longer able to hold the dissolved calcite, the drop deposited its tiny mineral load. Over time, minerals have created the beautiful speleotherms and variety of colors found in the cave. . . The form a speleotherm takes is determined by whether the water drips, flows, seeps, condenses, or pools. *
The following information on various cave formations is taken from a “Formations Glossary” found in the Kartchner Caverns State Park brochure. My comments are in italics, and are gleaned from a recent tour of the caverns.
Thin-walled hollow tubes about 1/4“ in diameter; soda straws grow from ceilings of caves as water runs down inside them, depositing rings of calcite at their tips. These incredible formations can be over 20,000 years old and reach lengths over 20 feet. Backlit by hidden lights in the cave, each appears to have a tiny diamond suspended from its tip. (If one of these drops of water falls on you, you are said to have received a "cave kiss"). Soda straws are so fragile that sound waves or movements of the air around them can cause them to break and fall.
Twisted formations projecting at all angles from ceilings, walls and over the floor of caves. The over-all formation seems to defy the laws of gravity. Many helicites were growing out from the sides of the soda straws; they look like tiny branches, and geologists cannot explain why they grow up or outward instead of down, the way normal cave formations do.
Formations that hang from cave ceilings and form as layers of calcite are deposited. This occurs when the water flows over the outside of soda straws after the hollow centers become partially, or fully, plugged. Our guide shone his laser pointer on a living, growing (wet) stalactite, and it was translucent; the whole cone-shaped rock lit up. Then he shone his pointer on a “dormant,” dry formation, and there was just a small red dot on the surface. The difference was quite dramatic.
Formations that rise from the cave floor and are often, but not always, formed by dripping water from stalactites above. Stalagmites are usually larger in diameter than stalactites, and more rounded on top. As a child, I learned the following mnemonic to keep the two straight: stalactites hold tight to the ceiling; stalagmites might eventually make it up from the floor.
This formation forms when, over time, water flows down walls, over floors and/ or other formations, and results in a build-up of calcite. The different colored strips are a result of the presence of different minerals, including iron oxide (red rust) in the calcite; other colors include a wide assortment of creams, yellows, and browns.
Formed by the joining of stalactites and stalagmites, or when a stalactite connects with the cave floor or a stalagmite connects with the cave ceiling
This formation is formed when, over a period of time, water flows down walls, over floors and other formations, and results in a buildup of calcite. In contrast to the "cave bacon" or draperies mentioned above, flowstone is usuall a thicker, heavier buildup. Occasionally water flows over a stalagmite or column, and the resulting formation looks like a tiered wedding cake or a series of trees on a steep hill.
A cave shield forms as calcite-rich seep water under hydrostatic pressure is forced from tiny cracks in a cave wall, ceiling or occasionally, floor. As this seep water loses carbon dioxide to the cave air, calcite is deposited as parallel extensions to the crack walls. The result: two thin, sandwiched disks separated only by a capillary, sheet-like void that feeds their growth. ** Often, soda straws or stalactites will grow down from shields on the ceiling or walls; sometimes, these formations become too heavy to support their own weight and fall to the cave floor.
* http://www.pr.state.az.us/parkhtml/kartchner.html , 5/23/02
** Shields were not mentioned in the brochure, but were on the tour; this info is from http://www.goodearthgraphics.com/virtcave/shield/shield.html , 5/23/02