Karst Landforms: Erosional and Depositional

Karst is a landscape which is underlain by limestone and has been eroded by dissolution, producing towers, fissures, sinkholes, etc. It is named after a province of earlier Yugoslavia on the Adriatic Sea coast where such formations are most noticeable.

The development of all karst landforms requires the presence of rocks such as limestone, dolomite, and gypsum, which are capable of being dissolved by surface water or groundwater.

In simple words, the term karst describes a distinctive topography that indicates the dissolution (also called chemical solution) of underlying soluble rocks by surface water or groundwater. Although commonly associated with carbonate rocks (limestone and dolomite), other highly soluble rocks, such as evaporates (gypsum and rock salt), can be sculpted into karst terrain.

Understanding karst is important because approximately ten per cent of the Earth’s surface is occupied by karst landscape and as much as a quarter of the world’s population depends upon water supplied from karst areas. Though most abundant in humid regions where carbonate rock is present, karst terrain also occurs in temperate, tropical, alpine and polar environments. It is a dry, upland landscape with underground drainage instead of surface streams.

Karst Topography

The degree of development of karst landforms varies greatly from region to region. Large drainage systems in karst areas are likely to have both fluvial (surface) and karst (underground) drainage components. As stated in the introduction, the term karst describes a distinctive topography that indicates the dissolution of underlying rocks by surface water or groundwater.

Water falls as rain or snow and seeps into the limestone through small channels, and moves underground through a network of subterranean channels and caves. The water becomes weakly acidic because it reacts chemically with carbon dioxide naturally occurring in the atmosphere and the soil. This acid is named carbonic acid and is the same compound that makes carbonated beverages taste tangy.

Rainwater seeps downward through the soil and fractures in the rock, responding to the force of gravity. The carbonic acid in the moving groundwater dissolves the bedrock along the surfaces of joints, fractures and bedding planes, eventually forming cave passages and caverns.

Limestone is a sedimentary rock consisting primarily of calcium carbonate in the form of the mineral calcite. Rainwater dissolves the limestone by the following reaction:

Calcite + Carbonic acid = Calcium ions dissolved in groundwater + Bicarbonate ions dissolved in groundwater

Cracks and joints that interconnect in the soil and bedrock allow the water to reach a zone below the land’s surface where all the fractures and void spaces are completely filled (also known as saturated) with water. This water-rich zone is called the saturated zone, and its upper surface is called the water table. The volume of void space (space filled with air or water) in soil or bedrock is termed porosity.

The larger the proportion of voids in a given volume of soil or rock, the greater the porosity. When these voids are interconnected, water or air (or other fluids) can migrate from void to void.

Thus the soil or bedrock is said to be permeable because fluids (air and water) can easily move through them. Permeable bedrock makes a good aquifer, a rock layer that holds and conducts water. If the groundwater that flows through the underlying permeable bedrock is acidic and the bedrock is soluble, a distinctive type of topography, karst topography, can be created.

Essential Conditions for the Development of Karst Topography or Landforms:

1. Presence of soluble rocks, preferably limestone at the surface or sub-surface level.
2. These rocks should be dense, highly jointed and thinly bedded.
3. The presence of entrenched valleys below the uplands is underlain by soluble and well-jointed rocks. This favours the ready downward movement of groundwater through the rocks.
4. The rainfall should be neither too high nor too low.
5. There should be a perennial source of water.

Karst Erosional Landforms

The landforms in the Karst landscape develop on three scales ranging from a scale of less than 10 m – these include varieties of solution pits, pans or grooves collectively known as “Karren”; to intermediate scale features ranging approximately from 1 to 1000 m, namely dolines or sinkholes to large scale forms where landforms are normally greater than 1 km in length e.g. poljes, dry valleys or gorges. Most of the karst landscapes are formed by combining all these landforms and sometimes may extend for thousands of square kilometres.

A great diversity of forms and combinations occur in the karst terrains found in different parts of the world. This diversity in the landforms is the product of physical and chemical variations in the rocks themselves; geologic structure; tectonic and geomorphic history; regional topography; and past and present climatic conditions.

Various scholars have studied this diversity at the regional level for many countries. Some prominent studies include the work of Gams (1974) for Yugoslavia, Jakucs (1977) for Hungary, Gvozdetskij (1981) for the USSR and Zhang (1980) for the People’s Republic of China.

Karren / Lapies

Karrens are the highly corrugated and rough surface of limestone lithology with low ridges and pinnacles. These are formed when rain falls onto bare limestone or waves break into it. Therefore, falling droplets, sheet and channelled runoff, film flow and ponded water all create small-scale solution forms also termed lapies.

The most common form is found in circular pits with rounded bottoms and pans with flat bottoms, sinuous channels and descending slopes where joints or dipping bedding planes have opened up.

Terra Rosa

These are red claystone up to several meters thick and kilometres across that occur at the earth’s surface. These are thought to be formed by the residual dissolution of limestones and/or by an accumulation of detrital mud, ash or dust on pre-existing karst terrain.

Limestone Pavement

These are only found in places that were covered by ice during the last ice age. They are formed in some beds of limestones where the following characteristics are found-

1. beds that do not contain many fractures
2. beds that are more resistant to dissolution by rainwater
3. mechanically stronger beds

Grikes

Grikes are vertical or near-vertical fissures in limestone pavement. In the initial stage, the cracks or fissures are only microscopic, but as rainwater seeps in and dissolves the limestone, the cracks become wider.

Solution Holes

Holes produced by the dissolution of limestones by chemically active standing water. These are small, shallow, round and flat-bottomed depressions or pools on the limestone pavement. They are usually 5 – 30 cm wide.

Cavern

This is an underground cave formed by water action by various methods in a limestone or chalk area.

There are differing views on the mode of formation of these caverns:

The Mechanical Action School, represented by Penck, Weller and Dane, considers mechanical action by rock debris and pebbles to be responsible for cavern excavation. This school argues that the water table is too low to have a solution effect.

The Chemical Action School, on the other hand, considers the solution action of water to be mainly responsible for cavern excavation. Davis and Piper represent this school. The largest cavern in Kentucky (USA) is 48 km long and 25 metres high. In India, such caves can be seen in Bastar, Dehradun, and Shillong plateau.

Arch / Natural Bridge

These are formed due to the collapse of the roofs of caves or due to the disappearance of surface streams and their reappearance, which keeps standing, forming an arch.

Sink Hole / Swallow Hole

Sinkholes are funnel-shaped depressions having an average depth of three to nine metres and, in area, may vary from one square metre to more. These holes are developed by the enlargement of the cracks found in such rocks as a result of the continuous solvent action of the rainwater.

The swallow holes are cylindrical tunnel-like holes lying underneath the sinkhole at some depth. The surface streams which sink disappear underground through swallow holes because these are linked with underground caves through vertical shafts.

Karst Window

When several adjoining sinkholes collapse, they form an open, broad area called a karst window.

Karst Plain

The upper surface having several sinkholes is called a karst plain.

Sinking Creek

Numerous sinkholes are located in a line.

Dolines

Dolines are bowl-shaped enclosed depressions in the Karst terrain that can be several metres to several hundred metres in range. The formation of dolines is associated with four distinct processes that usually operate alone. These are –

1. The solution, acting downward
2. Mechanical collapse
3. Subsidence without rupture into an inter-stratal solution cavity and
4. Sapping or seepage erosion into caves or adjoining dolines.

Mostly, two or three of the above-mentioned processes operate together to develop or enlarge a depression. These range from cylindrical shafts to shallow saucers, although intermediate funnel and bowl shapes are more common.

Karst Lakes

When Dolines plugged by clay are filled with water, karst lakes are formed.

Uvala

Several adjoining dolines may come together to form a large depression called uvala.

Polje

Several uvalas may coalesce to create a valley called polje which is a flat-floored depression. An ideal polje is an elongated, flat-floored, closed depression surrounded by limestone hills that are well karstified.

Sinking Creeks / Bogas

In a valley, the water often gets lost through cracks and fissures in the bed. These are called sinking creeks, and if their tops are open, they are called bogas.

Dry Valley / Hanging Valley / Bourne

Sometimes, a stream cuts through an impermeable layer to reach a limestone bed. It erodes so much that it goes very deep. The water table is also lowered. Now the tributaries start serving the subterranean drainage and get dried up. These are dry valleys or bournes. Lack of adequate quantities of water and reduced erosion leaves them hanging at a height from the main valley. Thus, they are also referred to as hanging valleys.

Blind Valleys

If the streams lose themselves in these valleys, then these are called blind valleys. These valleys may have surface streams and may be used for agriculture. In other words, when streams become deeply entrenched, creating valleys within the paleo valley, a blind valley is formed.

Karst Depositional Landforms

Water is also responsible for the formation of a large number of depositional landforms in a karst landscape. Chemically saturated water degasses as it drips, pools, flows and percolates in the below-the-ground karst environment. In the following section, some of the landforms formed due to the deposition process have been discussed.

Speleothems

Deposits in the karstic caves are collectively called ‘Speleothems’; these are crystalline deposits of calcium carbonate as a result of precipitation from dilute aqueous solutions entering the cave, typically as drips from the roof.

Travertines

Banded calcareous deposits are called Travertines.

Drip Stones

These are calcareous deposits formed by the dripping of water in dry caves or caverns. These can only form when a cave is at or above the water table where water can evaporate. When groundwater drips from the roof of a limestone cave, it slowly deposits calcite.

Several types of dripstones are found in karstic caves. Stalactites, stalagmites, cave pillars, drapes, helictites and heligmites are such drip stones found in a karst cavern.

i) Stalactites and Stalagmites

The water containing limestone in solution seeps through the roof of caverns in the form of a continuous chain of drops. A portion of the roof hangs on the roof, and evaporation of water, a small deposit of limestone, is left behind, contributing to the formation of a stalactite growing downwards from the roof.

The remaining portion of the drop falls to the floor of the cavern. This also evaporates, leaving behind a small deposit of limestone, aiding the formation of a stalagmite, thicker and flatter, rising upwards from the floor.

ii) Cave Pillars

Sometimes, stalactite and stalagmite join together to form a complete pillar known as the column.

iii) Drapes or Curtains

Numerous needle-shaped dripstones hanging from the cave ceiling.

iv) Helictites

Sideward growth from stalactites.

v) Heligmites

Sideward growth from stalagmites.

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