Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterizes by underground drainage systems with sinkholes and caves. Groundwater also creates a few erosional landforms such as sinkholes, doline, uvalas, lapies, and caves. Groundwater is the portion of rain or snowmelt water that accumulates in the rocks after seeping through the surface.
- Limestone, which has been erodes by dissolution, underlies the landscape of Karst, producing towers, fissures, sinkholes, etc.
- It is so named after a province of Yugoslavia on the Adriatic sea coast where such formations are most noticeable.
- The dissolution of soluble rocks such as limestone, dolomite, and gypsum forms Karst topography.
- Karst is not typically well developed in chalk, because chalk is highly porous rather than dense, so the flow of groundwater is not concentrates along with fractures.
- It is characterizes by underground drainage systems with sinkholes, caves etc..
Conditions Essential for Full Development of Karst Topography
- Presence of soluble rocks, preferably limestone at the surface or sub-surface level.
- These rocks should be dense, highly jointed and thinly bedded.
- In India karst topography is present in the Vindhya region (mainly southwestern Bihar), the Himalayas (parts of Jammu & Kashmir, Robert Cave, Sahasradhara, the eastern Himalayas, areas near Dehradun), Pachmarhi in Madhya Pradesh, Gupt Godavari Cave in Chitrakoot (U.P.), the surrounding coast near Vishakhapatnam(Borra Caves), and Bastar in Chhattisgarh.
- Borra Caves – Borra Caves, also called Borra Guhalu, are located on the East Coast of India, in the Ananthagiri hills of the Araku Valley.
Characteristics of Karst Landforms
- Generally, Karst regions have a bleak landscape, occasionally broken by precipitous slopes.
- General absence of surface drainage as most of the surface water percolate underground, hence surface valleys are generally dry.
- Streams generally cut their way along the joints & fissures of the rock wearing out a system of underground channels.
- When the water penetrates to the base of the limestone & meets the non-porous rocks, it reemerges onto the surfaces as a spring or resurgence.
- Rainwater finds its way into the underlying rock through the well-jointed limestones and their joints and cracks.
- Progressive widening by the solution enlarges these cracks into trenches & a most intriguing feature called limestone pavement is developed.
- The enlarged joints are called Grikes & the isolated, rectangular blocks are termed as clints.
- The surface of limestone abounds with swallow holes, which are small depressions carved out by solution where rainwater sinks into limestone at the point of weakness. These depressions are known as sinkholes.
- Once water has sunk into limestone, it etches out caverns & passages along joints.
- When a number of swallow holes coalesce, a larger hollow is formed & is called a Doline.
- Several dolines may merge as a result of subsidence (gradual caving) to form an even larger depression called an Uvala
- In Yugoslavia, some very large depressions called Polje, may be as large as 100 square miles, but produced partly due to faulting.
- Subterranean streams which descent through swallow holes to the underground passes leads to the formation of caves & caverns which may contain ponds or lakes.
- The most spectacular underground features that adorn the limestone caves are
- Calcite pillars
- Water carries calcium in solution & when this lime charged water evaporates, it leaves behind solidified crystalline calcium carbonate.
- Stalactites are sharp, slender, downward growing pinnacles that hang from the cave roofs.
- When moisture drips from the roof, it trickles down the stalactites and drops to the floor, where it deposits calcium to form stalagmites, which are shorter, fatter, and more round.
- Over a longer period, the stalactite hanging from the roof eventually joins with the stalagmite growing from the floor to form a pillar.
Erosional landforms of Karst topography
- A steephead valley, steephead or blind valley is a deep, narrow, flat bottomed valley with an abrupt ending.
- Karst valley abruptly terminated by the passage underground of the watercourse which has hitherto resisted the karst processes and remained at the surface.
- A sinkhole, also known as a cenote, sink, sink-hole, swallet, swallow hole, or doline (the different terms for sinkholes are often used interchangeably), is a depression or hole in the ground caused by some form of collapse of the surface layer.
- Most are caused by karst processes – for example, the chemical dissolutionof carbonate rocks or suffosion processes
- The surface streams which sink disappear underground through swallow holes.
- Local names:
- Black hole – Sea water
- Blue hole- deep under water
- Cenotes- Belize (British Honduras)
- Sotanos- Mexico
- Tomo- New Zealand
Depositional landforms of Karst topography
- All types of deposits in the caverns are collectively called ‘speleothems’ of which calcite is the common constituent.
- Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on their depositional history and environment.
Stalactites & Helictite
- The water containing limestone in solution, seeps through the roof in the form of a continuous chain of drops.
- As water evaporates, it leaves behind a small deposit of limestone on the roof, contributing to the formation of a stalactite, which grows downwards from the roof.
- Usually, the base is broader than the free end of the hanging stalactites.
- Formations known for descending vertically are stalactites, whereas formations known for extending horizontally or diagonally are helictites.
Stalagmites & Halagmite
- A stalagmite is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from the ceiling drippings.
- It is an upward-growing mound of mineral deposits that have precipitated from water dripping onto the floor of a cave.
- Halagmites are stalagmites that extend horizontally or diagonally.
- The combination or fusion of stalactites and stalagmites form the pillars
- The diameters of pillars vary
Human activities of Karst region
- Karst regions are often barren & at best carry a thin layer of soil.
- The porosity of the rocks & the absence of surface drainage make vegetative growth difficult, hence limestone can usually support only poor grass.
- Limestone vegetation in tropical regions is luxuriant because of heavy rainfall all the year around.
- The only mineral found in association of limestones is lead.
- The building industry uses good quality limestone as a construction material and quarries it for the cement industry.