Speleothems - Caves and Karst (U.S. National Park Service) (2024)

Classifying speleothems is tricky because no two speleothems are exactly alike. Nevertheless, speleologists have taken three basic approaches: classification by morphology, classification by origin, and classification by crystallography. All three of these approaches have their problems (Hill, 1997), so cavers often take a more practical approach that primarily uses morphology (e.g., cave pearls) but includes whatever is known about origin (e.g., geysermites) and crystallography (e.g., spar) when needed.

Cave Minerals of the World (Hill, 1997) refers to 38 different types of speleothems and numerous subtypes and varieties.

Stalactites are the most common and most familiar of all speleothems; they resemble icicles or carrots hanging from cave ceilings. Stalactites range in size from small, slender, soda straws to thick, massive pendants tens of feet long and wide.

Stalactites are usually composed of calcite, but they may consist of other minerals. All stalactites, whatever their composition, begin their growth as hollow soda straws. At first a water droplet collects on the cave ceiling by condensation or by water coming through a fracture in the rock. With the loss of carbon dioxide, a thin film of carbonate material precipitates and covers the surface of the drop. Similarly with evaporation in arid cave environments, a thin film of sulfate or other noncarbonate material precipitates over the surface of the drop. As the drop accumulates more water and becomes heavier, it begins to oscillate. This causes the film of material to move up toward the ceiling and to adhere there by surface tension. When the drop falls to the floor, this film is left on the ceiling as a round rim of material—the initial growth ring of a soda straw (Hill, 1997). As drop after drop follows a similar path, an infinitesimal trace of material is left behind, and a hollow tube is created and eventually enlarged, as long as water continues to drip. It is not surprising, then, that the word stalactite is Greek for “oozing out in drops.”

Helictites are contorted speleothems that twist in any direction, seemingly in defiance of gravity. The term “helictite” comes from the Greek root helix, meaning to spiral. Helictites have been compared to “the horrible, snaky tresses of Medusa” (Hill, 1997). They have been described as threads, beads, worms, and antlers or twigs.

Helictites grow on cave ceilings, walls, and less often on cave floors. They typically grow on other speleothems, such as carbonate coatings, crusts, and sometimes on soda straws.

Regardless of size or shape, all helictites have one thing in common: they possess tiny central channels through which their extremities and diameters are fed and increased by seeping capillary water.

Stalagmites are convex floor deposits built up by water dripping from an overhead stalactite or from the cave ceiling. Because falling water droplets tend to splash, stalagmites spread out as they gradually build up from the floor. Hence, they do not have central, hollow tubes like stalactites. Stalagmites are usually larger in diameter than the stalactites above them and they generally have rounded tops instead of pointed tips.

“That which drops” is the Greek meaning of stalagmites. When a drop of water falls from the ceiling or stalactite, it still has some material left in solution. When the drop hits the floor, carbon dioxide is given off and carbonate material is precipitated as a mound below the point of dripping; or, if a noncarbonate mineral, evaporation causes precipitation of mineral material.

Stalagmites can assume a fascinating variety of shapes and people have compared them to broomsticks, totem poles, toadstools, bathtubs, Christmas trees, beehives, coins and buttons, and even fried eggs!

Columns are not stalactites nor are they stalagmites; they are both, together. When a stalagmite grows together with its counterpart feeder stalactite, a new speleothem is formed: a column or pillar.

Columns can reach gigantic proportions, sometimes over 65 feet (20 m) in height and diameter. Typically the largest columns are aligned along ceiling joints, where the greatest amount of water is dripping into the cave.