When I was younger I spent many happy hours exploring the cave systems of England.  My explorations took me through tight tunnels and into vast caverns.  There was a great sense of discovery.  Only recently have I realised how little we know about these places.

I used to pothole in limestone districts.  At the time I did not fully appreciate the complexities of the process.  I understood the basics.  Raindrops, being slightly acidic, dissolve the rock.  The caves become accessible because of a fall in the water table.  The wonderful stalactites and stalagmites that made our trips worthwhile were formed when salts precipitated out of solution.  

Our knowledge of caving systems is very restricted.  This underground world is the last great unexplored region of our planet.  On the surface we know that vast areas of the world are dominated by limestone scenery.  These areas are dominated by dramatic gorges, barren rocks and limited surface water.  Geographers reserve the word karst to describe landscapes dominated by the way in which rainwater dissolves the limestone.  Surface water, such as it is, quickly finds a passage underground. Some karst landscapes in countries with strong caving traditions have been systematically explored.  Most have not.  China has particularly dramatic karst landscapes.  I wager that these contain the most extensive cave systems in the world.

The Verdon Gorge France showing the features of Karst Scenery. Source: Wikipedia

The chemistry behind cave formation is peculiar.  Limestone is predominantly calcium carbonate.  This mineral is not particularly soluble in pure water.  When carbon dioxide is allowed to dissolve into the water the calcium carbonate dissolves readily.  Under high pressure a considerable amount of carbon dioxide can be absorbed. When the pressure is removed, the carbon dioxide escapes and the calcium carbonate falls out of solution.

We know that raindrops are slightly acidic. They absorb some carbon dioxide from the atmosphere. When rain water drains through the thin peat soils found on the karst it becomes more acidic. This acidity allows the rainwater to burn a path through the limestone.  High water pressure within the underground stream allows the water to retain more carbon dioxide which increases the capacity to dissolve calcium carbonate.  These underground streams carve out caverns and passages that are sufficiently large for potholes to explore. Once the channel is established the erosive properties of flowing water help to expand the channel.  

Complicated cave networks arise through changes in the water table.  Active chemical and physical erosion takes place on or below the waterline.  As erosion continues the water table falls.  Passages that are left above the waterline are no longer subject to such intense erosive forces and are referred to as dry caves.  These are the caves that I liked to explore.  Exploration below the waterline is a much more dangerous sport that requires the skills of cave diving.  Sometimes there is a rise in the water table and old workings are reactivated.

When high pressure water effluxes from an underground watercourse the loss of pressure releases carbon dioxide.  The water no longer retains the ability to hold calcium carbonate in solution.  Calcium carbonate is precipated.  The process culminates in the wonderful cave formations that we all enjoy in the numerous show caves around the world.

Calcium carbonate is a white mineral.  Pure ivory white cave formations are particularly stunning.  Usually the water contains other minerals which stain the formation. Sometimes the minerals precipitate into particularly colourful formations. 

When cave formations contain other colours it indicates that other minerals have dissolved into the water. 

I believe that the causes of colouration are:

•  Yellow-tan-brown:arising from organic matter, dirt and impurities of tannic acid washed in from the surface.
• Orange-red: arising from cave clays
• Red-brown to chocolate brown: arising from iron oxides.
• Grey and black: arising from manganese, occasionally from bat guano stains, lead or other impurities.
• Green: arising from a clay shale containing potassium iron silicate, or close to a light source algae. 
• Pink:arising from magnesium carbonate
• Other colors: though seldom found arise from human or biological activity. 

Phong Nha Cave in Phon Nha-Ke Bank, Vietnam showing the various colours found in cave formations Source: Wikipedia

Having explained how caves form I feel I have just scratched the surface.  Caves that form through chemical action are known as solution caves. Solution caves based upon the chemistry of calcium carbonate and rainwater are by far the most common.  However, alternative chemistries and attacks by other acids are possible.  Furthermore, I remain puzzled. Why do caves form in some but not all limestone environments.  Just what is so special about the karst country?   

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