How Karst Landscapes Form Caves and Sinkholes

Pure water does not dissolve limestone very quickly. Rainwater becomes more effective after absorbing carbon dioxide from the air and soil. The gas reacts with water to form weak carbonic acid, creating a solution that can slowly react with calcium carbonate in limestone.

Water enters joints, bedding planes and other cracks in the rock. Each small reaction removes a tiny amount of material. Over thousands of years, narrow cracks can become wider passages that allow even more water to move underground.

Karst development is strongest where soluble rock is thick, fractured and exposed to plenty of moving water. Climate matters because rainfall affects how much water enters the ground, while soil and vegetation can add carbon dioxide that increases acidity.

As cracks enlarge, water begins to follow underground routes instead of staying entirely on the surface. Small passages can join into cave systems, and streams may disappear through openings called swallow holes. Water can later return to the surface at springs.

Caves often contain stalactites hanging from ceilings and stalagmites rising from floors. These features form when mineral-rich water enters an air-filled cave and loses some carbon dioxide. Calcium carbonate is deposited again, slowly building shapes from repeated drops.

The position of the water table influences cave development. Some passages form while completely filled with water, while others are modified after water levels fall. A cave can therefore record several stages of landscape change. Older cave levels can remain above active streams, giving geographers clues about earlier water tables and the long history of erosion beneath the surface.

Karst regions often contain sinkholes, also called dolines. Some develop gradually as soil and weathered material move into widening cracks. Others form when the roof of an underground cavity collapses. Their size and suddenness can vary greatly.

Because water travels through open fractures and caves, pollution can move quickly from the surface into groundwater. Karst aquifers can supply valuable drinking water, but they may have less natural filtering than water moving slowly through fine sediment.

Look for these clues on a karst map:

• •Areas of limestone or other soluble rock.
• •Streams that disappear underground.
• •Sinkholes and closed depressions.
• •Caves, springs and underground channels.
• •Limited surface drainage in strongly karstified areas.

Karst does not look the same everywhere. In cooler regions that were scoured by ice, such as parts of the Yorkshire Dales in England and the Burren in Ireland, glaciers scraped away soil and exposed flat beds of limestone. Rainwater then widened the joints into deep cracks called grikes, leaving blocks called clints between them. The result is a limestone pavement, a surface that looks almost like a giant, uneven patio. Plants that like shade and shelter often grow down in the grikes, out of the wind and away from grazing animals.

In warm, wet tropical regions, dissolution can work faster and for longer without interruption from ice. Over very long periods, the rock between hollows may be removed until isolated steep hills remain, a style often called tower karst. The landscapes around Guilin in southern China and the islands of Ha Long Bay in Vietnam are well-known examples, with the Vietnamese towers partly drowned by the sea. Comparing these regions shows that the same basic chemistry can produce very different scenery depending on climate, time and past glaciation.

Once a stream vanishes into a swallow hole, you cannot see where it goes, so hydrologists use tracer tests. A harmless dye, often a fluorescent one such as fluorescein, is poured into the sinking stream. Monitors are placed at springs across the area, sometimes small packets of activated charcoal that soak up the dye and are later tested in a laboratory. The time the dye takes to appear, and the springs where it emerges, reveal how the underground drainage is connected and how fast water moves through it.

The results can be surprising, because underground routes often ignore surface watersheds. A famous case is the upper Danube in southern Germany, where in dry periods much of the river disappears into its limestone bed. Tracer experiments in the nineteenth century showed that this water travels underground to the Aach spring and then flows towards the Rhine, so part of one great river secretly feeds another. Tracer tests are now routine for protecting drinking water, since they show which surface areas could pollute a particular spring.

Stalagmites grow upward in thin layers as each drip deposits a little calcium carbonate, rather like tree rings, and some keep growing for many thousands of years. Scientists can date these layers precisely using uranium. Drip water carries tiny amounts of dissolved uranium into the growing stalagmite but almost no thorium, because thorium barely dissolves in water. Once locked in the calcite, the uranium slowly decays into thorium at a known rate, so the ratio of thorium to uranium in a layer acts as a clock that can reach back hundreds of thousands of years.

The layers also record conditions above the cave. Oxygen in the calcite comes from rainwater that seeped down through the rock, and the balance between lighter and heavier oxygen isotopes in that water changes with temperature, rainfall amount and where the moisture came from. By measuring the isotopes layer by layer and pairing them with uranium dates, researchers have reconstructed how monsoon rains strengthened and weakened over long periods. Because cave interiors are stable and sheltered, stalagmites can preserve these records better than many surface deposits, and snapping one off destroys an archive that took ages to build.

Limestone is not the only rock that water can carve into karst. Gypsum, a soft mineral made of calcium sulfate, dissolves far more readily, and it does not need acidic water to do so. Ordinary groundwater is enough. That means gypsum karst can change on a human timescale rather than over many thousands of years. The city of Ripon in North Yorkshire sits above gypsum beds, and sudden subsidence hollows have repeatedly damaged buildings and roads there. Planning rules in the area require ground investigations before new construction, because a cavity can enlarge noticeably within a single lifetime.

Rock salt dissolves faster still, so it usually survives only where it is buried and shielded from fresh water. Along the shores of the Dead Sea, a striking example has unfolded since the 1980s. As the sea level has dropped, largely because water from the rivers that feed it has been diverted, fresh groundwater has moved in to replace the retreating brine. It dissolves layers of salt buried in the sediment, and thousands of sinkholes have opened, swallowing parts of roads, farmland and tourist sites. Chemistry that takes ages in limestone can reshape a coastline within decades when the rock is salt.

Karst landscapes form when slightly acidic water dissolves soluble rock along cracks and layers. The process creates underground drainage, caves, springs and sinkholes while also making groundwater especially important. Follow where the water enters, dissolves rock and returns to the surface, and the landscape starts to make sense.