Cave Evolution

Limestone Cave Evolution, by Matt London

Limestone is composed of the mineral calcite (CaCO3). The limestone that caves form in today was created in ancient seas millions of years ago by marine animals and plants that extracted calcium carbonate from the seawater. Sand grains composed of the skeletal remains of these organisms, together with smaller grains produced by micro-organisms, were later compacted under the pressure of the oceans and great seas above and cemented into firm rock.

The limestone removed in producing a cave is not simply dissolved in water. In fact, limestone is only slightly more soluble in pure water then quartz, the chief mineral in sand stone, and is less soluble than the minerals of such rocks as granite and basalt, neither of which ever contains solution caves!

Limestone and marble caves (the later being Limestone that has re-crystallized due to extreme pressure and heat) are formed when carbonic acid attributed to rainwater mixing with carbon dioxide contained in minute amounts in the atmosphere and then much larger amounts in the soil layer or blanket of soil laying above the limestone to form carbonic acid. The rich tropical soils that cover the limestone landscape found in the south of Thailand for example provide the catalyst for the dissolution process that dissolves the limestone to form Thailand’s caves.

The Four Stages of Cave Development:

  • The initial enlargement of joints and partings by ground water in the water saturated zone;
  • The development of master channels directly below the water table during a period when altitude of the water table is relatively stable and a high partial pressure of carbon dioxide exists at the top of the water-saturated zone;
  • A transitional stage, in which nearby steams have cut down to the point where their seasonal fluctuations strongly affect the level of the water table in the cave, sometimes introducing river silt into the cave system; and
  • The further lowering of the water table and down cutting of the surface until an opening to the surface is created, the carbonic acid content of the cave water consequently becoming so low that the water ceases to dissolve the limestone. Meanwhile, the acid content in the aerated zone between the soil and the cave remains high. In the final phase of this stage, therefore, surface sinkholes enlarge by dissolution until some of them join, and the roof of the cave progressively collapses and is eroded away.

REFERENCES: Speleology The study of caves. âGeorge W. Moore, G Nicholas Sulivan, Annals of the former world, John McPhee