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Wednesday, November 29, 2006

I was just doing some housekeeping of my documents/photos.. Look what i found

Formation Of Karst Landscape in Humid Tropics

There are different karstic landforms that emerge from the humid tropics. It depends on the amount of rainfall as well as the intensity of the emission of carbon dioxide. When compared to the temperate regions, the humid tropics have higher amounts of both rain and emission of Co2. Due to a higher rate of evaporation, it results in the ground surface deposition of calcium carbonate. Also, stalactites are able to overhang cliff faces and cave openings. As a result of heavy rain, there is high surface run off. Surface streams are able to flow for some distance, before eventually plunging underground. As a result, joints are widened. All the above actions lead to the emergence of a landscape with rounded/conical top enclosing depressions that are of irregular shape.

Thus the difference in the climate and rock structure leads to the variety of karstic landforms in the humid tropics.

1) Cockpit, cone or kegel karst

The hills are cone in shape. Depressions which are star-shaped known as cockpit are formed along major joint intersections. The depressions would over time become flat alluvium-covered plains which are towered by steep conical hills

2) Tower or turn karst.

The hills are steeper. Cliff faces and jagged summits are formed from consolidated and massive limestone. Small shallow caves known as foot caves forms at the base.

3) Crevice Karst

The surface is alternated by joints which have been enlarged to as much as 6m wide and 20m deep. After repeated weathering, the main joints develop into deep chasms. A landscape consisting of jagged ridges and pinnacles are formed as the adjoining faces retreat.

Other conditions which are necessary for the formation of karst landscape includes soluble limestone or dolerite that must be jointed instead of being permeable. Karst features are poorly developed in chalk as water can percolate through. The water should also drain away as when there is presence of water, karst landscape would not be able to form.

There are 4 stages in the formation of karsts landscape. According to the Cvijic’s karst cycle, the cycle starts with Youth followed by Maturity, Late Maturity and lastly Old age.

When rainwater comes into contacts with limestone which are exposed, this causes dissolutions along the lines of weaknesses. Clints and grikes which are narrow vertical channels separated by sharp narrow ridges are formed as a result. Terra Rosa, a red clay residue may fill the grooves. Through solution, the joints and crevices are enlarged. Dolines or solution pans are able to develop into shallow funnel-shaped depressions which may become clogged with inwash clay. A karst lake is able to be formed when it holds water. There is limited underground drainage and no large caverns.

In the maturity stage, surface drainage flows into swallow holes which are actually enlarged openings. It disappears beneath the ground through sinkholes. It joins a developing subterranean drainage system. There are very little surface streams. These streams flow for only short distances, usually on clay-lined channels. An extensive cavern network is the result of water descending along bedding and joint planes which erodes and widens through solution as well as erosion.

The extensive cavern network formed in the previous stage causes a weaker rock structure. As a result, the surface collapses. Karst windows are formed on the surface. These features allows the underground streams to become visible. Long and narrow irregular depressions called solution valley develops. A few sinkholes merging together are able to form compound sinkholes and extensive depressions which are called uvulas. The features are formed when the roof collapses. Poljes are elongated basins which have flat floors and steep enclosing walls which are created by fault controlled erosion. It may sometimes contain large lakes.

Hills bears semblance to Iselbergs, called buttes remain behind. Also other features like hums and termoines scatter the landscape. They are separated by broad and flat basins which are covered with clay residues.

|| nobone ran out @ 11/29/2006 10:32:00 am ||