Stone Waterfalls and Cave Carrots--The Story of Speleothems

Much less important to the geologist, but much more fascinating to the tourist are the speleothems, commonly called "cave formations." (We try to use "speleothem" or its proper name, to avoid confusion with the geologic rock formations like the Eminence.)

Speleothems form as long as rainfall enters the cave after passing through plant debris (humus) and carbonate bedrock. The rate at which speleothems form depends upon the amount and rate of water entering the cave, the amount of acidity and minerals in the water, and the temperature and humidity conditions in the cave when it enters. If someone persists in wanting to know, explain that if it is a dry year, speleothems may not grow at all, and that if we have optimum conditions they may grow a lot. (I heard the 1 inch per century rate for calcite deposition in a commercial cave once while my tour was standing on 3 to 4 inches of flowstone over a concrete walkway. If that were true, the walkway would be 3 to 4 hundred years old.)

Stalactites: StalaCtites are the granddaddy of speleothems. They form when that first drip of water leaves a bathtub ring of calcite as it falls. Drop by drop, the calcite collects, forming a hollow tube, or soda straw, of crystallized calcite. Soda straws are quite common up to about a foot in length, and (in other caves with the right conditions) have been known to grow to tens of feet. Usually, though, the tube becomes plugged, and the water flows down the outside of the soda straw, forming the icicle looking stalactite. A stalactite with water flowing on just one side may become a thin drapery blade or bacon rind.

Stalagmites: StalaGmites form when calcite laden water splashes on the floor. The shape of stalagmites depends on gravity (they are round tipped, whereas stalactites are pointy) and often have a splash cup at the end or otherwise resemble candles from the spatter of the mineral laden water. Stalactites stick tight to the ceiling; stalagmites grow mightily from the ground--a silly but very effective way to keep these two names attached to the proper formation.

Stalactites and stalagmites often grow in pairs, and sometimes grow together forming columns. But sometimes, they grow singly, or in very uneven sizes. Stalactites over water will not form stalagmites. A slow drip on a stalactite may deposit all its calcite on the ceiling and leave little to form a stalagmite. With a fast drip, or steady stream, little calcite may stick to the ceiling, but instead build up a large stalagmite.

Soda straws, stalactites, stalagmites and columns are called dripstone, because they are deposited by dripping water. Flowing water can also deposit calcite down a wall or across a floor. This is called (surprise!) flowstone. Water in a cave stream can build up flowstone, called rimstone dams, across the stream, as the tumbling action of the water shakes loose carbon dioxide (just like shaking a can of soda pop does) and causes calcite to be deposited. Rimstone dam pools are often good aquatic habitat.

Cave coral or "popcorn" is unusual in that it can be deposited either above or below water. If an area is uniformly coated with coral, it is a good bet that it formed underwater. Sometimes the water in cave pools becomes so saturated with calcite that it simply falls out of solution because of its release of carbon dioxide gas, and the inability of the water to hold any more mineral in solution at that temperature.

Just as rock candy forms from evaporating a saturated sugar solution, some cave coral, and botyroidal (grape-like) speleothems grow underwater in this fashion."Lily-pads" began as shelfstone, accumulating around existing stalagmites and shore flowstone, just like those sugar crystals eventually crust the sides of the pan, and the string suspended in it. The varying levels of the pools allow new deposits to grow, dry out, and grow again, eventually growing upward, as well as horizontally.

The Lily Pad Room, Onondaga Cave State Park

Sub-aerial (formed in air) cave coral is actually more common than that formed in water. The easiest way to tell them apart is: Sub-aqueous cave coral often shows a ring of shelfstone, or a distinct "waterline" below which is covered with the knobby stuff, and above which there is little if any. Cave Minerals lists 5 ways cave coral may form in the air: 1) water seeping through the bedrock and the crystal structure of the knobs itself; 2) thin films of water flowing over irregular surfaces; 3) splash from dripping water; 4) water moving up walls by capillary action; and 5) condensation from humid cave air.

All result from thin films of water moving over rock, and the subsequent evaporation deposition of the calcite, in the pattern mentioned above. Cave drafts seem to be a controlling factor in this deposition, more than just simple gassing off of carbon dioxide. Such drafts result in "photo-tropic (light-loving) cave coral," such as that in Cathedral Cave in Onondaga Cave State Park, where the coral deposits are largely on the side of the speleothem facing the entrance. The theory here is that the colder, drier air on the "out" side of the cave causes evaporation to occur much more rapidly than the steady state of a still cave. Or, perhaps the cave, barometrically breathing "in" during Missouri humid and wet summers, drew more moisture into it at these times, causing water to condense along the stalactites and stalagmites, a condensation which evaporated, whilst drawing mineral out of the existing speleothem for surface deposition.

Sometimes, when the water is quite still, calcite crystals form on the surface, making calcite ice or calcite rafts. These are quite fragile, and will sink at the slightest stirring of the water.

Helictites are crazily growing calcite structures, most often resembling snarled roots or potato sprouts. It is believed they grow as the result of capillary action, as the water seeps from the rock and is then pulled along through tiny, internal canals within the depositing crystal structure instead of yielding to gravity. Nearly freestanding speleothems called shields are supposed to grow in a similar manner, but along hairline cracks in the bedrock, instead of circular pores. The cave shield in the Onondaga Cave Rock of Ages room is one of only a few in the United States.

Cave pearls, are a fairly rare speleothem, formed similarly to a pearl, as a piece of sand or grit is rolled underwater, and coated on all sides with calcite. Constant movement of the pearl in a waterfilled depression keeps it from becoming attached to the bottom of the pool. Cave pearls can grow to several inches in diameter.

Some photos this page courtesy of Missouri Dept. of Natural Resources

2003 Jo Schaper.


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