Processes in underground karst regions

There are two processes involved in creating underground features in karst regions:

(The sand and pebbles that are carried by rivers may cause erosion by scouring, but this third process has only a minor impact compared to the other two.)

Solution underground

Ingleborough Cave

Acidic water percolates through joints, bedding planes and faults, widening them by dissolving the limestone.

Rivers flowing through the widened joints form passageways and caverns, predominantly by solution.

The carbon dioxide (CO2) dissolves in the water (H2O), forming a weak acid that converts the calcite (CaCO3) into calcium bicarbonate (Ca(HCO3)2), which is then carried away by the water.

The chemical reaction that occurs can be written like this:

CO2 + H2O + CaCO3 Ca(HCO3)2

More about underground solution features including caves and resurgence features (springs).

Precipitation in caves

Sword of Damocles stalactite

So far we have discussed surface features and caves as sites of solution and erosion, but in some cases, caves are areas of precipitation.

This takes place only in vadose caves and is characteristic of areas of massive limestones.

Calcite in the form of tufa or travertine is precipitated from mineral-rich waters (e.g. at Mother Shipton's Well).

In caves, precipitation results in the formation of straws, stalactites, stalagmites, columns, curtains, etc.

These features are together known as 'speleothems'. ('Speleo-' comes from the Ancient Greek word for cave.)

We have already seen how acidic rainwater takes the calcite that makes up limestone into solution. Speleothems form when the chemical reaction above is reversed.

Ca(HCO3)2 CO2 + H2O + CaCO3
       
    lost to the air   drops to the cave floor   deposited on the cave roof

Rain water (H2O) takes the calcite (CaCO3) into solution as calcium bicarbonate (Ca(HCO3)2) when it flows over exposed surfaces and seeps through joints and fissures of a karst region.

The rain water holds the dissolved Ca(HCO3)2 and carbon dioxide (CO2) in equilibrium and it will stay that way until the equilibrium is upset.

Eventually the mineralised water reaches the roof of a cave where it forms a droplet. The droplet will be suspended from the cave roof for a few seconds before dropping off onto the floor below.

The water droplet contains a higher concentration of dissolved CO2 compared to the air in the cave and in those few seconds some of the CO2 is released. This upsets the equilibrium. The water becomes 'supersaturated', which means it cannot hold on to all of the Ca(HCO3)2, and deposits a tiny amount of CaCO3 onto the cave roof.

The amount of CaCO3 gradually increases and in this is way all the different kinds of speleothems form. Their shape depends on where they form in the cave: on the roof, on the floor or on a wall.