Karst topography is named after the typical topography developed in limestone rocks of Karst region in the Balkans adjacent to the Adriatic Sea. Karst topography includes typical landforms in any limestone or dolomitic region, produced by the action of groundwater through the processes of solution and deposition.
Limestone is an organically formed sedimentary rock. In its pure state, limestone is made up of calcite or calcium carbonate but where magnesium is also present it is termed as dolomite. Limestone is soluble in rainwater.
Conditions for the formation of Karst Topography
- A region with a large stretch of water-soluble rocks such as limestone at the surface or sub-surface level
- Limestones should not be porous
- These rocks should be dense, thinly bedded and well jointed
- A perennial source of water and a low water table to allow the formation of conspicuous features.
- Moderate to abundant rainfall to cause the solvent action of water i.e. solution of rocks
Mechanism of erosion in Karst region
- In Karst regions, rocks are permeable, thinly bedded and highly jointed and cracked.
- Thus there is the general absence of surface drainage as the surface water has gone underground
- After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves.
- Rocks are eroded due to this downward and horizontal movement of water.
- It is through the chemical process of solution and precipitation deposition by surface water and groundwater, varieties of landforms are developed in rocks like limestones or dolomites rich in calcium carbonate.
- Small to medium-sized round to sub-rounded shallow depressions called swallow holes form on the surface of limestones through solution where rainwater sinks into the limestone at a point of weakness
- They are also known as sinkholes
- Sinkholes are a common feature in limestone/karst areas.
- A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom
- There is a great variation in sizes of Sinkholes with areas from a few sq. m to a hectare and with depth from a less than half a metre to thirty metres or more.
- These holes grow in size through continuous solvent action
- They are also referred to as solution sinks
- They are also referred to as Collapse sinks .
- They are less common than sinkholes
- They might start as solution forms first, and if the bottom of a sinkhole forms the roof of a void or cave underground, it might collapse leaving a large hole opening into a cave or a void below
- They are long, narrow to wide trenches, also referred to as Valley sinks .
- Several sinkholes and dolines may merge together as a result of subsidence to form a large depression called an Uvala.
- These are grooved, fluted and ridge-like features in an open limestone field.
- These ridges or lapies form due to differential solution activity along parallel to sub-parallel joints.
- Eventually, the lapie field may transform into smooth limestone pavements.
- A limestone pavement is a natural karst landform consisting of a flat, incised surface of exposed limestone that resembles an artificial pavement.
- These are formed by the solvent action of underground water in the limestones, causing progressive widening and enlargement of joints and cracks in the trenches.
- The enlarged joints are called grikes and the isolated, rectangular blocks are termed as clints.
- Cave formation is prominent in areas where there are alternating beds of rocks (sandstone, shale, quartzite) with limestone or dolomite in between or in areas where limestones are dense, massive and occurring as thick beds.
- Water percolates down either through the materials or through cracks and joints and moves horizontally along bedding planes.
- Gradually, the limestone dissolves along these bedding planes resulting in the creation of long and narrow gaps called caves.
- A polije is a very large, flat-floored depression in the karst region.
- They are often formed by merging of several uvalas or partly due to faulting
- They are commonly found in subtropical and tropical latitudes
- Some of these may also appear in the temperate region. They may also be found in boreal regions, though very rarely.
- During the rainy season, parts of the floor which are at or near the water table may become temporary lakes
- Drier areas are fertile. Usually covered with thick sediments, they are used extensively for agricultural purposes
- A ponor is a natural surface opening in the karst regions
- They are found directly underneath the sinkholes
- A ponor is kind of a portal where a surface stream or lake flows either partially or completely underground into a karst groundwater system.
Landforms due to depositions
Depositional landforms in karst region are developed due to the deposition of calcium carbonate. The calcium carbonate dissolved during the erosional process starts to precipitate when the water evaporates or when the solution is super-saturated.
Stalactites, Stalagmites and Pillars are the most spectacular underground features, found in the limestone caves.
- Stalactites are the sharp, slender, downward-growing icicles of different diameters that hang from the cave roofs.
- Stalactites have a variety of forms
- Their bases are normally broad which taper towards the free ends
- The water carries calcium in solution and when this lime-charged water evaporates, it leaves behind the solidified crystalline calcium carbonate.
- Stalagmites form due to dripping water from the surface or through the thin pipe, of the stalactite, immediately below it
- Moisture dripping from the roof trickles down the stalactite and drops to the floor where stalagmites are formed due to deposition of calcium
- Stalagmites may take the shape of a column, a disc, with either a smooth, rounded bulging end or a miniature crater-like depression.
- Over a long period, the stalactite is eventually merged with the stalagmite
- Thus, the pillars or columns of different diameters are formed.