Karst topography

Karst topography is a landscape of distinctive dissolution patterns often marked by underground drainages. These are areas where the bedrock has a soluble layer or layers, usually, but not always, of carbonate rock such as limestone or dolomite. In such places there may be little or no surface drainage. Some areas of karst topography, such as the region of southern Missouri and northern Arkansas in the USA, contain literally thousands of caves.

The word Karst is the German name for Kras, a region in Slovenia, partially extending to Italy, that rests on a limestone plateau. It was here that first scientific research of a karst topography was made.

Chemistry of karst landscapes

Karst landforms are generally the result of mildly acidic rainfall acting on soluble limestone or dolostone bedrock. The process of subsurface rock dissolution results in a topography with distinctive features, including sinkholes or dolines (closed basins), vertical shafts, disappearing streams, and springs. With a sufficiently low base level and sufficient time, complex underground drainage systems (such as karst aquifers) and extensive caves and cavern systems may form.

The carbonic acid that causes these features is formed as rain passes through the atmosphere picking up CO₂, which dissolves in the water. Once the rain reaches the ground, it passes through the soil, gathering up more CO₂ to form a weak carbonic acid solution: H₂O + CO₂ → H₂CO₃.

This mildly acidic water begins to dissolve any fractures and bedding planes in the limestone bedrock. Over time these fractures enlarge as the bedrock continues to dissolve. Openings in the rock increase in size, and an underground drainage system begins to develop, allowing more water to pass through and accelerating the formation of underground karst features.

Somewhat less common than this limestone karst is gypsum karst, where the solubility of the mineral gypsum provides many similar structures to the dissolution and redeposition of calcium carbonate.

Karst formations

Erosion along limestone shores, common in the tropics, produces typical karst topography, including a sharp makatea surface above the normal reach of the sea and undercuts that are mostly the result of biological activity or bioerosion at or a little above mean sea level. Some of the most dramatic of these formations can be seen in Thailand's Phangnga Bay and Halong Bay in Vietnam. Calcium carbonate removed by water may deposit elsewhere. In caves, stalactites and stalagmites are formed by deposition of calcium carbonate and other dissolved minerals as the water drips from above. An example is the Gruta Rei do Mato in the Lagoa Santa Karst formation around Sete Lagoas, Brazil with a stalactite of 20 meters height.

Other formations consist of shields (where the flow is from a fissure rather than from a point), and flowstone, which occurs when the flow of calcite-rich water is somewhat impeded and calcite is deposited in the flow. Helictites are curlicue-shaped formations associated with the roofs and walls of caves. Larger flow-type formations are rimstone pools and gours, which are bathtub-shaped and may contain large calcite or aragonite crystals as a result of slow evaporation. Rivers which emerge from limestone caves may also produce tufa terraces, consisting of layers of calcite deposited over extended periods of time as the water leaves the CO₂-rich cave environment.

Water drainage and problems

Farming in karst areas must take into account the excessive drainage. The soils may be fertile enough, and rainfall may be adequate, but rainwater quickly moves through the crevices into the ground, sometimes leaving the surface soil parched between rains.

Water supplies from wells in karst topography are inherently hazardous, as the well water may simply run from a sinkhole in a cattle pasture through a cave and to the well, bypassing the normal filtering that occurs in a porous aquifer. Karst formations are cavernous and therefore have high rates of permeability. This results in very little time for contaminants to be filtered out naturally through the pores of strata.

Groundwater in karst areas is just as easily polluted as surface streams. All too often, sinkholes have been used as farmstead or even community trash dumps. In karst areas where septic tanks are the main sewage disposal system, overloaded or malfunctioning systems dump raw sewage directly into underground channels.

The karst topography itself also poses some difficuties for human inhabitants. Sinkholes can develop gradually as surface openings enlarge, but quite often progressive erosion is unseen and the roof of an underground cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery.

The Driftless Area National Wildlife Refuge in Iowa protects ice age snails surviving in air chilled by flowing over buried karst ice formations.

Pseudokarst

Pseudokarst occurs where the primary erosive agent is not rainwater, but there is underground drainage. This can occur in basalt, where drainage is through lava caves, or among granite tors (for example Labertouche Cave in Victoria, Australia). Paleocollapse can resemble karst, but is formed by a different mechanism.