stalactite and stalagmite

stalactite and stalagmite , mineral forms often found in caves; sometimes collectively called dripstone. A stalactite is an icicle-shaped mass of calcite attached to the roof of a limestone cavern. Groundwater trickling through cracks in the roofs of such caverns contains dissolved calcium bicarbonate. When a drop of water comes in contact with the air of the cavern, some of the calcium bicarbonate is transformed into calcium carbonate, which is precipitated out of the water solution and forms a ring of calcite on the roof of the cavern. By repetition of this process the length and thickness of the stalactite is increased. A stalagmite is a cone of calcite rising from the floor of a cavern. Stalagmites and stalactites are often found in pairs, the stalagmite being formed as a result of further evaporation and precipitation from solution after the trickle of water falls from the stalactite. Stalactites and stalagmites often meet each other to form solid pillars. Curtains of dripstone sometimes form when water drips from the ceiling of a cave along joint planes. Since stalactites, stalagmites, and curtains of dripstone form only in the presence of air, their existence in a cave indicates that the cave was above the water table while the dripstone was forming. The many colors often seen in these formations are caused by the presence of impurities. Celebrated caverns that owe much of their beauty to their stalactites and stalagmites are Mammoth Cave, Ky.; the Luray Caverns, Va.; and the Carlsbad Caverns, N.Mex. Onyx marble (Mexican onyx, Egyptian alabaster, or Oriental alabaster), used as a decorative stone, is derived from stalagmites and stalactites, as well as from similar deposits.

stalactites and stalagmites Stalactites and stalagmites are secondary carbonate deposits found in caves in limestone areas. They can be classified together as speleothems (from the Greek words for ‘cave’ and ‘deposit’). Stalactites hang down from the ceilings of caves while stalagmites grow up from the floor. The majority of stalactites and stalagmites are composed of calcite (calcium carbonate), although in certain circumstances they may consist of other minerals, such as aragonite or gypsum. They are deposited from supersaturated waters and their formation is favoured during periods of relative warmth and wetness. Therefore, their rate of growth is controlled climatically. In addition to climatic controls, local chemical and hydrological factors may determine whether one individual speleothem grows faster or slower than another. Estimated rates of stalagmite growth range between 0.2 and 36 cm per 1000 years, while stalactites may grow significantly faster. Some stalactites and stalagmites contain annual bands, analogous to tree rings, which are in some cases visible to the naked eye and in others visible only under ultraviolet light. In the future, it may be possible to reconstruct interannual climate variations from stalactites and stalagmites in the same way that tree rings have been used. The age of stalactites and stalagmites can be determined precisely by measuring certain radioactive isotopes of uranium and thorium. When a stalactite or stalagmite is deposited it contains a large amount of uranium and no thorium because of differences in the chemical behaviour of these elements. Uranium decays radioactively to produce thorium-230 at a known rate. By measuring the amount of thorium-230 that has been produced relative to the amount of uranium, an age can be calculated. These ages have provided geologists with insights into climate change over the past 350 000 years (the limit of this dating technique), because periods when there was minimal formation of stalactites and stalagmites can be interpreted as being relatively cold and dry. Stalactites and stalagmites from caves which are now submerged beneath the sea demonstrate that sea levels have changed in the past. By dating such stalactites and stalagmites (which can form only when they are exposed by a fall in sea level), constraints can be placed on the extent and timing of these sea-level changes. Stalactites and stalagmites can also be used to provide constraints on rates of landscape evolution by dating valley incision or tectonic uplift. In addition, ages from stalactites and stalagmites associated with archaeological and palaeontological material from cave sites may provide valuable chronological control on that material. It may be possible to obtain further information about past climates by determining the ratios of different oxygen isotopes in the calcite. Although stalactites and stalagmites are scientifically valuable, it is also important to conserve them as they are an integral and beautiful component of the cave environment.

M. S. Roberts