|
|
|
| Sunday, 05 March 2006 |
 |
|
 |
Junior Observer |  |
| News Business Features Editorial
|
The mystery and wonder of caves A cave is a natural opening in the ground, extending beyond the zone of light, and large enough to permit the entry of a human. Occurring in a wide variety of rock types, and caused by widely differing geological processes, caves range in size from single small rooms to interconnecting passages many miles long. The scientific study of caves is called speleology (from the Greek words spelaion for cave and logos for study).
Caves have been natural attractions since prehistoric times. Evidence of early man's interest in them has been discovered in caves scattered throughout the world. Fragments of skeletons of some of the earliest man-like creatures have been discovered in cave deposits in South Africa, and the first evidence of primitive Neanderthal Man was found in a cave in the Neander Valley of Germany. Cro-Magnon Man created his remarkable murals on the walls of caves in southern France and northern Spain, where he took refuge more than 10,000 years ago, during the chill of the ice age. A simple classification of caves includes four main types, and several other relatively less important types. Solution caves are formed in carbonate and sulphate rocks, such as limestone, dolomite, marble, and gypsum by the action of slowly moving ground water that dissolves the rock to form tunnels, irregular passages, and even large caverns along joints and bedding planes. Most of the caves in the world - as well as the largest - are of this type.
Solution caves, however, have always been a source of wonder to man. How do these extensive, complex, and in some places, beautifully decorated passageways develop? Solution caves are formed in limestone and similar rocks by the action of water; they can be thought of as part of a huge subterranean plumbing system. After a rain, water seeps into cracks and pores of soil and rock, and percolates (filters) beneath the land surface. Eventually, some of the water reaches a zone where all the cracks and pores in the rock are already filled with water. This is known as saturated zone. The term water table refers to the upper surface of this saturated zone. Calcite (calcium carbonate), the main mineral of limestone, is barely soluble in pure water. Rainwater, however, absorbs some carbon dioxide as it passes through the atmosphere, and even more as it drains through soil and decaying vegetation. The water, combining chemically with the carbon dioxide, forms a weak carbonic acid solution. This acid slowly dissolves calcite, forms solution cavities, and excavates passageways. The resulting calcium bicarbonate solution is carried off in the underground drainage system. Lava caves are tunnels or tubes in lava, formed when the outer surface of a lava flow cools and hardens,while the molten lava within continues to flow and eventually drains out through the newly formed tube. Sea caves are formed by the constant action of waves, which attack the weaker portions of rocks, lining the shores of oceans and large lakes. Such caves testify to the enormous pressures exerted by waves, and to the corrosive power of the sand and gravel carried by waves. Glacier caves are formed by melted water which excavates drainage tunnels through the ice. Of entirely different origin, and not to be included in the category of glacier caves are the so-called "ice caves", which usually are either solution caves or lava caves within which ice forms, and persists through all or most of the year. In desert areas, some shallow caves may be formed by the sandblasting effect of silt, or fine sand being blown against a rock face. These eolian caves, some of which are spectacular in size, are surpassed in number by caves of other origins in most deserts. More common even in the driest deserts are sandstone caves, eroded in part by water, particularly if the sandstone is limy. Caves commonly known as wind caves are named not for the mode of origin of the cave, but for the strong air currents that alternately blow in or out of the cave, as the atmospheric pressure changes. Most wind caves are, in fact, solution caves. The streams of melted water draining out along the floor of a glacier cave or the surging, pounding waves at the mouth of a sea cave, offer immediate evidence of the origin of these caves. More on caves later. **** How caves developed It was once believed that caves formed near the Earth's surface - above the saturated zone - where the water moved downward through the cracks and pore spaces. This view, however, left many cave features unexplained. Why, for instance, are cave passages nearly horizontal, in places crossing folded or tilted rock structures? How would horizontal passages form at several different, but persistent levels? Recent studies of the movement and chemistry of ground water have shown that the first stage in cave development - the dissolving of carbonate rocks and the formation of cavities and passage-ways - takes place principally just below the water table, in the zone of saturation, where continuous mass movement of water occurs. A second stage in cave development occurs after a lowering of the water table (the water table normally sinks, as the river valleys deepen). During this stage, the solution cavities are stranded in the unsaturated zone where air can enter. This leads to the deposition of calcite, which forms a wide variety of dripstone features. The chemical process causing deposition of calcite is the reverse of the process of solution. Water in the unsaturated zone, which dissolved some calcite as it trickled down through the limestone above the cave, is still enriched with carbon dioxide when it reaches the ventilated (airy) cave. The carbon dioxide gas escapes from the water (just as it escapes from
an opened bottle of soda pop). The acidity of the water is thereby
reduced, the calcium bicarbonate cannot remain in solution, and calcite is
deposited as dripstone. |
|
| News | Business | Features
| Editorial | Security
| Politics | Produced by Lake House |
