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| Sunday, 19 March 2006 |
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Junior Observer |  |
| News Business Features Editorial
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The mystery and wonder of caves In our first article on caves, which appeared on March 5, we discussed the different types of caves and how they are formed. Today we will tell you more about the various features of caves. The decorative dripstone features are called speleothems (from the Greek spelaion for cave and theme for deposit). When these structures are highlighted by lanterns or electric lights, they transform a cave into a natural wonderland. The most familiar speleothems are stalactites and stalagmites. Stalactites hang downward from the ceiling, and are formed as drop after drop of water slowly trickle through cracks in the cave roof. As each drop of water hangs from the ceiling, it loses carbon dioxide and deposits a film of calcite. Successive drops add ring below ring, the water dripping through the hollow centre of the rings, until a pendant cylinder forms. Tubular or "soda straw" stalactites grow in this way; most are fragile (breakable) and have the diameter of a drop of water, but some reach a length of perhaps a yard or more. The large cone-shaped stalactites begin as these fragile tubes, and then enlarge to cones when enough water accumulates to flow along the outside of the soda straws. Deposition of calcite on the outside of the tubes, most of which are near the ceiling and taper downward, results in the familiar cone shapes. Column formation Stalagmites grow upward from the floor of the cave, generally as a result of water dripping from overhanging stalactites. A column forms when a stalactite and a stalagmite grow until they join. A curtain or drapery begins to form on an inclined ceiling when the drops of water trickle along a slope. Gradually, a thin sheet of calcite grows downward from the ceiling and hangs in decorative folds like a drape. Sheets of calcite that are deposited on the walls or floor by flowing water are called flowstone. Rimstone dams are raised fence-like deposits of calcite on the cave floor that form around pools of water. Helictites are curious twisted or spiralling cylinders or needles. They apparently develop when water seeps through the ceiling so slowly that slight chemical or physical changes can cause reorientation of the crystal structure of the calcite or gypsum. Cave corals, also formed by slowly seeping water, are small clusters of individual knobs. Most cave passages contain deposits of material that have been washed into the cave. This material, known as cave fill, varies from sand and clay to stratified (arranged in layers) gravel. The pebbles in these deposits often are highly polished or frosted, and sometimes are as large as six inches in diameter. Cave fills are particularly noteworthy because they contain materials that reflect a geologic history and a record of past climates of the surrounding area. Rock material produced by the collapse of the ceiling or walls of a cave is called breakdown and may range in size from plates and chips to massive blocks. Most breakdown present in caves today appear to have occurred thousands of years ago. It is generally associated with the early history of cave development. Many minerals Many interesting minerals are found in caves, in addition to the calcite which forms the major features. Aragonite, a calcium carbonate mineral similar to calcite, but not as common, often occurs in intricate needles known as anthodites. Gypsum (calcium sulphate) and related calcium sulphate minerals are next to calcite in abundance. Some caves, although they are developed in limestone, have extensive passages lined with fine, curling growths of gypsum flowers. In other caves, selenite (a less common variety of gypsum) forms long transparent rods or nests of fibrous crystals. Sulphates of sodium and magnesium are also found in caves, although they are less conspicuous (easily seen) than gypsum. Iron minerals in the form of oxides (limonite) and hydroxide (goethite) occur in caves and in some places form stalactites. Manganese minerals in caves are commonly present as thin, sooty coatings on walls and ceilings and in earth fills. Nitrocalcite (calcium nitrate) is abundant in earth fills in many caves, but individual fragments are generally microscopic. Barite (barium sulphate) and celestite (strontium sulphate) also occur in earth fills. In some solution caves, clay minerals exist in relatively pure forms; these include the less common varieties, attapulgite and endellite. In deep caves encountered during mining operations, a number of ore minerals have been found in the decorative wall draperies. Most common are azurite and malachite (forms of copper carbonate). About 50 other minerals also have been reported in cave deposits. |
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