> CONCRETION - Conditions of formation of concretions
The process may appear simple, but it is complicated in nature by many factors influencing the conditions of deposit. To the chemical processes of the formation of concretions are added the physical laws of crystallisation. And chemical reactions and crystallisation are very sensitive to the constraints of the surrounding environment. The combined effects of method of flow, surface tension and temperature give rise to the great diversity of concretions which decorate the caves.
In the chemical process:
- The reactions involved are not instantaneous and do not have the same speed. So the deposition of carbonate will spread out over time.
- Studying the speed and the quality des deposits shows that temperature plays an important role. Cold water hinders dissolution and reduces concretion and conversely warm water encourages deposits.
- So concretions reflect the climatic conditions existing when they were formed.
As for the environmental constraints:
The flow and the conditions of circulation of the water which carries the carbonates also have a major role.
- The water comes from precipitation, so again its flow depends directly on the climatic conditions at the surface.
- It has to percolate through the ground to acquire the carbon dioxide necessary for the dissolution process. So its dissolving power will depend on the covering vegetation, itself related to climatic conditions.
- The permeability and the porosity of the rock will influence the flow and the speed of percolation of the water.
These characteristics depend on the nature of the limestone surrounding the cave. More generally, the flow and the speed of water through the caves are not regular, so the deposits will be spread out along the watercourse and separated. From these observations we can define a scheme of how concretion in caves is organised, based on the flow of water percolating into them. |