Brief notes on Karst Topography

The most distinctive type of landforms associated with a specific rock type is that which develops on limestone at or near the earth surface. Such landscapes are termed Karst after a region in former Yugoslavia.

The term Karst comes from the narrow strip of limestone plateau in Yugoslovakia and adjacent portions of Italy bordering the Adriatic Sea. This region is characterized by features of limestone solution on a large scale.

Landforms developed anywhere by solution in limestone are called karst landforms. It may be pointed out that not all the names of individual karst landforms have come from Yugoslavia. Every country has its own peculiar set of karst terms.

Notable karst areas:

There are many areas where karst topography is found, even though these areas do not possess all of the possible karst forms.

Such areas are: New South Wales, Western Australia, Southeast Asia, Central New Gwinea, Celebes, Southern France, Spanish, Andalusia, northern Puerto Rico, Western Cuba and Mexico. In addition to these countries, there are four areas in the United States which have major karst topography.

These areas comprise the Great Valley region of Pennsylvania, Maryland, Virginia and Tennessee; a belt extending from south-central Indiana into western Kentucky; Central Florida; and the Salem-Springfield plateaus area of Missouri.

However, the karst landscapes in these areas are different from the karst topography as are found in the tropical countries.

It may be pointed out that there are some other areas where karst features are found, but they are different from ideal karst landscape present in the karst region of Yugoslavia.

It may be stated that wherever soluble rocks such as limestone, dolomite, rock salt or gypsum are found at or beneath the surface, some solution may be expected and there may be a few features of karst landscape, but they do not dominate the landscape.

Conditions favourable for development of ideal karst topography:

There are four prerequisites for the maximum development of karst topography. First, there must be the presence of limestone strata at or near the surface. Secondly, the soluble rock should be dense, highly jointed, and thinly bedded.

The third condition is the existence of entrenched valleys below uplands underlain by soluble and well jointed rocks. The fourth condition demands that the region must receive moderate to abundant rainfall.

That is why the arid and semi-arid areas do not have fully developed karst, even though other three conditions as mentioned above are fulfilled. However, some arid regions show evidence of the work of solution dating from a period when the climate was more humid.

Mechanics of groundwater erosion in Karst region:

Water being the universal solvent dissolves carbon dioxide absorbed from the air, so that the rainwater contains carbonic acid (H₂C0₃). This acid reacts with many minerals, especially limestone, in a process called carbonation.

Carbonation means that carbon combines and trans­forms minerals having calcium, magnesium, potassium and sodium into carbonates. When rain- water falls on the formations of limestone which is calcium carbonate, CaC0₃, the constituent minerals are dissolved and washed away with the rainwater.

Thus, ordinary rainwater containing carbon dioxide can dissolve an appreciable quantity of carbonate of lime. While passing through a bed of limestone, the percolating water removes a small amount of material in solution, and the resultant effect, acting over a long period of time, is of great significance.

Water follows the easiest paths and flows through cracks, which are gradually enlarged by the process of solution and become still easier channels than before.

As time passes on, the water concentrates in these enlarged channels and in a limestone formation the underground water often forms streams of considerable size.

These underground channels become wider and wider to such an extent that they form of a series of caverns running for several kilometers into the ground.

The carbonate of lime is dissolved and is carried away in solution. The impurities that are insoluble are left behind to form a covering to the rock. The soil is very thin excepting in the valleys where the soil may attain relatively greater thickness.