How Are Caves and Karst Landscapes Formed?
Caves and karst form when slightly acidic groundwater dissolves limestone along cracks, enlarging them over thousands of years into passages and sinkholes.
Caves and karst landscapes form when slightly acidic groundwater dissolves soluble rock, mainly limestone, and widens the cracks it flows through. Over thousands to millions of years the dissolved material is carried away, leaving passages, sinkholes and underground rivers. Karst is the name for terrain shaped this way, taken from the Kras plateau in what is now Slovenia.
What makes water dissolve rock?
Rainwater is slightly acidic. As it falls it absorbs some carbon dioxide from the air and forms carbonic acid (H₂CO₃), a weak acid of the kind described in pH and acid–base chemistry. Water that soaks through soil is more aggressive still, because soil air holds far more carbon dioxide than the open air, released by plant roots and decaying material.
Limestone is a sedimentary rock made mostly of calcium carbonate, which forms part of the rock cycle. Gypsum and rock salt dissolve more easily still, so similar landscapes form in those rocks too. Carbonic acid reacts with it to form calcium and bicarbonate ions that stay dissolved in the water. The overall reaction is CaCO₃ + CO₂ + H₂O ⇌ Ca²⁺ + 2HCO₃⁻, and it runs faster in cold water, which holds more carbon dioxide.
The process is slow. Measured lowering of karst surfaces is typically about 10 to 100 millimetres per thousand years, so most caves take a very long time to form. Rates vary with rock type, climate and water flow, so these figures are averages.
Why do caves follow cracks and layers?
Groundwater does not dissolve rock evenly. It moves through joints (cracks), fractures and the bedding planes between rock layers, which are all part of the water cycle underground. Along these openings the flow is concentrated and dissolution is fastest, so a crack becomes a channel, and a channel becomes a conduit.
Caves are shaped mainly by where the water table sits. Below it, passages are full of water, and flow carves them into rounded tubes. Above it, air-filled passages are cut by water flowing down. Many caves have both: a flooded lower level, and a dry upper level left behind when the water table dropped. Mammoth Cave in Kentucky, the longest surveyed cave system in the world with more than 400 miles of passages, formed in this way.
Stalactites and stalagmites form in the air-filled part. Dripping water loses carbon dioxide to the air, and the dissolved calcium carbonate comes back out as calcite, adding a thin layer with each drop. Growth is usually well under a millimetre a year. The passage is the space the water took away, and the decorations are the calcite it put back.
What features make up a karst landscape?
Karst is recognised by a set of surface features, each produced by the same dissolution.
| Feature | How it forms |
|---|---|
| Sinkhole (doline) | Soil sinks or collapses into a dissolved cavity below |
| Sinking stream | A river disappears underground into a cave or swallow hole |
| Karst spring | Groundwater emerges where a conduit meets the surface |
| Tower karst | Steep towers are left standing as the surrounding plain dissolves away, as at Guilin in southern China |
Sinkholes come in two kinds. Dissolution sinkholes form slowly as a funnel. Collapse sinkholes form suddenly when a cave roof gives way, often after heavy rain, or after pumping lowers the water table and loosens the soil above a cavity.
In the Yucatán Peninsula of Mexico, water-filled sinkholes called cenotes expose the groundwater below. A ring of them follows part of the buried Chicxulub crater, the impact that ended the dinosaurs.
Why does karst matter to people?
Karst aquifers supply a large share of drinking water in many regions. Water moves through conduits quickly and is filtered little, so pollution from the surface can reach springs and wells within hours or days. The same openings make karst hard to build on, because collapsing ground can damage roads and buildings. Caves also hold fossils, archaeological records and animals found nowhere else.
Tags
caves earth science geology hydrology karst
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