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> CONCRETION - Formation of concretions
The formation of concretions is related to water and its circulation. Without water, there can be no concretions. The water is involved not only in the process of dissolution, but also as a vehicle for the dissolved chemicals which it takes deeper into the empty spaces underground. Concretion is therefore a result of the operation of the karst. Like the hollowing out of the caves, it is one of the phases of the history of karsts, a natural stage in the evolution of the area of infiltration.
Location of concretions.
In the area of infiltration there are three types of empty spaces: the caves, the system of macrofissures and the system of microfissures. At the points where these meet, the water trickling from the surface towards the deeper areas of the karst system will create, in very precise conditions, the concretions that decorate the underground landscapes, true marvels of nature.
Circulations in the area of infiltration.
In the area of infiltration of a karst, water and air circulate, more or less rich in carbon dioxide. This carbon dioxide is in the air as a free gas, or in the water in uncombined (dissolved) or combined (carbonic acid) form.
Circulation of the water.
Water flows naturally by gravity, although locally surface tension creates a "force of suction" which acts against the downward flow. In the enlarged fissures and the galleries (the macrofissure system) the effect of suction is negligible and the water flows freely down. The speed of flow is directly related to the volume brought in from outside in precipitation (rain or snow).
In the narrow cracks of the microfissure system, the action of suction takes over. A phenomenon of self-adjusting retention occurs and as a first priority the water flows towards the fine fissures creating continuous seepage on the ceilings and walls.
Circulation of the air.
Inside the karst system, the air moves in two ways:
- Free air which flows in the large cracks and the caves.
Air flows both inwards and outwards according to two motive forces: atmospheric pressure and the differences of temperature between the outside and the inside. This is called single phase flow.
- Air is also drawn along by the water (diphase flow); This air rich in carbon dioxide flows combined with water, in the system of microfissures. When this system comes across an empty space (a hollow or a macrofissure) the air and the gas are partially released.
In this way the walls of caves are continually the site of major exchanges of gas.
The mechanisms of concretion.
It is now thought that the commonest mechanism is the result of a simple chemical reaction:
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This reaction shows the transformation of the calcium bicarbonate dissolved in the water into calcium carbonate rock: the concretion. This mechanism can be considered as the reverse of the dissolution of the limestone which hollows out the caves. The same chemical balance applies. It can be altered in either direction, when one of these components diminishes or disappears. Consequently, if the solution is degassed (loses part of its dissolved carbon dioxide) or if part of the water evaporates, some compensation will be necessary. This will be at the cost of the dissolved bicarbonate which, losing some of its own water or CO2, will transform into calcium carbonate to form a concretion.
So, for a concretion to form, it needs:
1) Dissolution of the limestone rock and formation of soluble bicarbonate. This means the production of CO2 in the ground, combination with rainwater, and then contact with the limestone because of the permeability and fissuring of the rock.
2) Transport of the bicarbonate dissolved by the water into the systems of cracks or the galleries.
3) Loss of the gas or the water, leading to the precipitation of the insoluble carbonate. This happens, when the water loaded with bicarbonate meets a well-ventilated area, allowing the carbon dioxide to escape or the water to evaporate.
These conditions are met in the area of infiltration. There the water, loaded with limestone from running through the fissures of the epikarstic area, meets large empty spaces (galleries and caves), which have been left dry by a lowering of the base level. |
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