Karst

The term Karst is a German word derived from the Slovenian Kras, which refers to bare, stony ground. In the geomorphic sense, karst is a distinct topography arising from the dissolving effects of water on terrain composed of soluble rock types.

While karst-like features can develop on even the most chemically inert substrates under certain climatic conditions given a sufficient amount of time, the rock types most commonly associated with karst features include carbonate rocks (limestone, dolomite) and evaporites (halite, sulphates anhydrite, gypsum). Typical karst features include lengthy networks of underground caves and channels, sinkholes, and pavements.

Carbonate rocks are not very soluble in pure water, thus solution depends on the presence of carbon dioxide which can be acquired from both the atmosphere and the soil. As a result, a weak carbonic acid is formed and then subsequently dissociated into hydrogen and hydrogen carbonate ions. The amount of solution that can occur depends on the amount of available CO₂, which is in turn controlled by factors such as temperature and the partial pressure of the molecule in the atmosphere.

For limestone, which generally consists of around 90% CaCO₃ (calcite) and 10% impurities (typically silica and various clays) the solution reaction is as follows:

CaCO₃ + H₂O + CO₂ ↔ Ca⁺² + 2HCO^-3

Dolomite consists of around 90% CaMg(CO₃)₂ and typically dissolves at a slower rate than calcite, resulting in a lesser-developed karst topography. The reaction is:

CaMg(CO₃)₂ + 2H₂O + 2CO₂ ↔ Ca⁺² + Mg⁺² + 4HCO^-3

The character of karst development is determined by the permeability of the terrain and the amount of precipitation. Joints, bedding planes, and faults all provide lines of weakness for karstic processes if precipitation is sufficient. The physical strength of the rock is a factor of critical importance in evaporites, as these rocks often lack the mechanical support to develop extensive karst networks, although there are some notable exceptions. Evaporites are also far more soluble than carbonates and are thus best able to retain karst features in sub-humid and semi-arid climates where the evaporites are covered by sediments or other less-soluble rocks.

The extent of karst processes is additionally impacted by the rate of water flow, as the optimal flow rate is one that is slow enough to allow sufficient water-terrain contact time for solution to occur while being fast enough to evacuate solutional products. Although karst features are fairly widespread, the most striking examples can be found in the United States, East Central Europe, and Southeast Asia.

Typical karst landforms include:

Karst Pavement:

Extensive bare rock surfaces usually characterised by alternating clints and grikes. Glaciation assists in the development of karst pavement through the periodic removal of accumulated debris, thus the most widespread examples are located in Canada.

Karren:

Found in Karst pavements and other exposed surfaces, these features can form networks known as karrenfelder. Karren can be tiny pinnacles, ridges, pits, or spurs, or they can take the form of small stream channels. The type of karren that develops is a function of chemical reactions, precipitation regimes, substrate lithology and porosity.

Sinkholes:

Alternately referred to as dolines, these circular depressions that can range from a few metres to several hundred metres in diameter. They tend to coalesce in wetter regions, forming clusters of star-shaped sinkholes divided by rounded hills. In the tropics these contiguous depressions are referred to as cockpits*. They can also be found throughout Southeast Asia.

Poljes:

Extensive karst depressions, typical of the classic Slovenian karst territories. These are elongated, flat-bottomed basins surrounded by steep rocky walls. Non-calcareous materials tend to accumulate in the floors of the poljes, forming in a protective seal over the underlying limestone. These forms are typically aligned along tectonic lines, faults, or folds.

Allogenic Valleys:

Only certain streams have enough volume to cross karst regions. Smaller streams tend to lose their flow and form blind valleys, while the larger streams tend to form allogenic valleys in the form of steep gorges.

Tower Karst:

Includes isolated vertical stacks and residual conical hills. The well-known karst towers in South China range from 100 to 200 metres in height with slopes between 80 and 90 degrees.

Caves:

Karst caves range from short passageways, open shafts, and singles rooms to extensive networks which are comprised of all three. It is not unheard of for these caves to contain large lakes or canal systems. These caves house striking features such as stalagmites and stalactites, and are scientifically significant since they tend to trap large amounts of sediment which can be extremely informative when subject to archaeological and paleontological analysis.

* as a small point of clarification, cockpit is a Jamaican term for the depressions themselves, whereas cockpit karst is the term used when referring to the entire assemblage including the surrounding hills. Many thanks to Gorgonzola for pointing that out.