Karst

Karst is characterized by the dissolution of soluble rocks such as limestone, dolomite, and gypsum. Karst areas are typically characterized by subsurface drainage systems with sinkholes, dolines, and dry valleys at the surface and caves underground.

Karst is a geological phenomenon, first described in Slovenia. This area is called Kras in Slowenian or Karst in German, which means rocky place. Since then all areas with a similar geological situation are called Karst Areas.

The basic aspects of karst areas are soluble bedrock, cracks and water. The rain water is able to dissolve small amounts of rock and carry them away. Most rocks are not permeable to water, but sedimentary rocks have horizontal layers called bedding with so-called bedding planes. Additionaly, during times of up and down movement, the rocks are bent and get mostly vertical cracks. When water follows this cracks, it dissolves the rock and forms caves and caverns.

Common rocks for karst areas all over the world are:

Limestone (calcium carbonate CaCO₃)
Dolomite (magnesium calcium carbonate CaMg₂CO₆)
Gypsum (CaSO₄ + H₂O)
Salt (NaCl)
Sandstones to a limited extent. They are soluble depending on the chemical composition. It is often referred to as pseudokarst.
Quartzite (SiO₂). However, this is a special case, since quartzite is soluble only under extreme temperature conditions and quartzite karst is therefore known only from some regions in the tropics.

There are caves in other kind of rock, but they are not karst features! Typical features of karst areas are:

Underground drainage: no standing or flowing water on the surface. The water seeps away immediately and drains underground.
Large karst springs, which often change their yield very rapidly. Since the water is not stored in the pores of a porous rock, but flows through large cave passages, it remains only a few days underground and the variations in precipitation are passed on very quickly.
Sinks or swallows, where the water of streams and rivers disappears. They usually occur at the boundary of non-karstable rock to karstable rock.
Dolines or sinkholes, which are formed by solution or by the collapse of caves.
Polje, which are depressions (valleys) without surface inflow and outflow. Streams originate in karst springs on one side and disappear in sinks on the other side.
Many polje contain karst lakes which are fed by groundwater and have no surface inflows or outflows.
Dry valleys, which are former river valleys which have become dry due to karstification and the lowering of the karst water table.

The type of erosional forms which the karst takes, depends on many variables:

The mechanical structure and chemical composition of the rock. (limestone, dolostones, gypsum,...)
The local climate and temperature range.
The amount of rainfall.
The amount of vegetation. (Bare Karst verus Covered Karst)
The up and down movement of the rocks and the hydrogeological situation.