Karst Topography – Limestone & Chalk

by Devender

0 1260

Limestone and Chalk are derived from the accumulation of corals & shells in the sea and are formed of sedimentary rocks of organic origin.

Karst Topography – Limestone & Chalk

In its purest form, limestone is made up of calcite or calcium carbonate, along with magnesium present in form of dolomite.

Chalk is white & soft and is a pure form of limestone
Limestone is soluble in rainwater
It forms a weak acid, with carbon dioxide from the air

A region with a large stretch of limestone possesses a very distinct type of topography, termed as Karst region.

Features of Karst Topography

The Karst regions have a bleak landscape, occasionally broken by precipitous slopes and there is an absence of surface drainage as most of the surface water percolates underground, hence surface valleys are generally dry.

Streams generally cut their way along the joints & fissures of the rock
It wears out a system of underground channels
When the water penetrates to the base of the limestone, it meets the non-porous rocks
When it meets with these rocks, it re-emerges onto the surfaces as a spring or resurgence

The limestones are well jointed and rainwater finds its way into the underlying rock through these joints & cracks. These cracks enlarge into trenches by progressive widening by the solution & a most intriguing feature called limestone pavement is developed.

The enlarged joints are called Grikes & the isolated, rectangular blocks are termed as clints. There are numerous swallow holes on the surface of limestone that are small depressions carved out by solution where rainwater sinks into the limestone at the point of weakness, also known as sinkholes.

Once the water has sunk into limestone, it etches out caverns & passages along joints
When a number of swallow holes coalesce, a larger hollow is formed called a Doline
Several dolines merge as a result of subsidence to form an even larger depression called an Uvala
Polje are very large depressions as long as 100 square miles produced partly due to faulting

Caves & caverns that may contain ponds or lakes are formed by subterranean streams that descent through swallow holes to the underground passes. The most spectacular underground features that adorn the limestone caves are -

Stalactites
Stalagmites
Calcite pillars

Solidified crystalline calcium carbonate is left behind when the lime-charged water evaporates as water carries calcium in the solution.

Sharp, slender, downward growing pinnacles that hang from the cave roofs are called Stalactites
Calcium is deposited to form Stalagmites when moisture drips from the roof, it trickles down the stalactites & drops to the floor
These are shorter, fatter & more round

Over a longer period, the stalactite hanging from the roof is eventually joined to stalagmite growing from the floor to form a pillar.

Human activities of Karst region

Karst regions carry a thin layer of soil and are often barren. Limestone can usually support only poor grass as the porosity of the rocks & the absence of surface drainage makes vegetative growth difficult.

Limestone vegetation in tropical regions is luxuriant because of heavy rainfall all year round
Lead is the only mineral found in association with limestones
Good quality limestone is used as building materials & quarried for the cement industry

Chalk

The landforms of chalk are different from other limestones. There is very little or no surface drainage. The valleys which once contained rivers are now dry often called Coombes.