What Is Karst? Landscape Made by Water Dissolving Rock

Rainwater absorbs carbon dioxide from the atmosphere and more from soil, which makes it a weak acid, and that acid dissolves limestone and comparable rocks slowly but continuously. Water following cracks widens them, and a widened crack carries more water, which widens it further, so the process concentrates into a network of enlarged passages rather than spreading evenly. Over long periods that network becomes a drainage system beneath the surface, and once it exists, water reaching the ground goes into it rather than flowing across the surface. That transfer of drainage from above to below is what defines the landscape and produces everything else about it, since a surface without flowing water develops differently from one with it.

The features are distinctive and mostly concern water going missing:

• •Sinkholes, where the surface collapses into a void beneath or where dissolution forms a closed depression
• •Sinking streams, which flow normally and then disappear into the ground at a definite point
• •Dry valleys, cut by water that no longer flows there because the drainage has gone underground
• •Springs discharging large volumes where the underground drainage reaches the surface again
• •Caves, which are the passages themselves once air has replaced the water
• •Bare fissured rock pavements where soil has been washed into the cracks

The landscape creates specific practical problems that are well understood and frequently ignored. Water supply is unreliable and unpredictable, since surface water is absent and groundwater moves through open conduits rather than seeping through pores, which means it travels fast, arrives in pulses and carries whatever entered the system. Contamination is the serious consequence, since a pollutant entering a sinkhole reaches a spring kilometres away within hours with essentially no filtration, and the usual assumptions about soil and rock cleaning water do not apply. Construction faces the risk of undetected voids beneath foundations, and sinkhole collapse damages buildings and roads regularly in such regions. Dams and reservoirs leak. Agriculture sends fertiliser and waste directly into the water supply. Managing these requires tracing where the water actually goes, which is done with dyes.

The passages themselves preserve records that surface environments destroy. Formations built by dripping water accumulate in layers over tens of thousands of years, and their chemistry records rainfall, temperature and vegetation above at the time each layer formed, which makes them among the most precisely datable climate archives available, since the layers can be dated directly by radioactive decay. Sediments washed in preserve pollen and bones from the surface. Animal remains accumulate where creatures fell in or were carried in by predators, producing assemblages that record local fauna over long periods. Human occupation left material in cave entrances across the world, and the deepest painted chambers preserve art that could not have survived anywhere exposed. The isolation that makes caves difficult to reach is exactly what preserves their contents.

Soluble rock underlies a substantial share of the land surface and such landscapes are correspondingly widespread. The classic examples give the phenomenon its name from a region at the head of the Adriatic, where the features were first studied systematically. Extensive terrains occur across southern China, where towers of residual limestone rise from flat plains and form one of the most photographed landscapes anywhere. The Yucatan peninsula is almost entirely such terrain, with its water supply held in flooded cave systems accessed through collapse features that were central to the settlement and the religion of the civilisation there. Large areas of the eastern United States, of the Balkans, of southeast Asia and of Australia are the same. Comparable processes affect gypsum and salt, which dissolve far faster and produce features developing within a human lifetime.

Slightly acid rainwater dissolves limestone along cracks, and a widened crack carries more water and widens further, so drainage concentrates into underground passages and abandons the surface. Sinkholes, vanishing streams, dry valleys and large springs follow. Contamination is the practical danger, since a pollutant entering a sinkhole reaches a spring kilometres away within hours and unfiltered.