> CONCRETION - The different types of concretion
Traditionally concretions are divided into two groups: Aerial concretions: formed out of water, in an airy, ventilated environment, in drained galleries and caves. Drowned concretions: developed in an underwater environment (drowned karst), or under the water of the troughs, basins and gour pools where the running cave-water will gather. There are different types of these according to the flow that gave rise to them.
AERIAL CONCRETIONS.
These concretions, known by cavers as “pretties”, are shaped by gravity. Due to gravity, they are essentially vertical shapes on the roof, the walls and the ground, and stratified horizontal shapes on the ground. Depending on the characteristics of the flow that gave rise to them, there are differences between concretions formed:
- Drop by drop, whether slow or abundant
- By trickling
- In a continuous flow
- In a flow under pressure
Drop by drop.
This type of water supply creates the two most widespread forms – "stalactites" which hang from the ceilings and upper parts and "stalagmites" which rise from the floors and flatter parts.
Stalactites.
As the drop of water emerges, degassing occurs: the carbon dioxide is given off and part of the mineral content is deposited. A little tube is formed, which grows longer and thicker as it is fed by the flow of water. The central axis of a stalactite generally consists of a little tube through which the water flows. This channel may become blocked. This does not however prevent the stalactite from growing; the water flows down the outside, continues to lengthen it, and also adds to its thickness which may become very great.
Stalagmites.
The drops that fall from the roof of a cave will not have lost all their mineral content in the form of a stalactite. Falling to the ground, it bursts into droplets which deposit some or all of the carbonate they still contain. A fairly thick stalagmite will begin to grow taller at the point of impact by mineral deposit on its tip as each drop arrives, and grow thicker as the drops trickle down its sides. In caverns with high ceilings and an abundant supply of water, the splashing due to the distance the drops fall will create stalagmites in the shape of "stacks of plates" or "palm trees". The growth of the "palm leaves" is due both to the splashes that land on them and to the effects of surface tension, which causes the water gathered in the hollows at the base of these "leaves" to migrate towards their outer edges. The size of stalagmites and des stalactites is directly related to the water flow. For instance on a ceiling, in the presence of a heavy flow, the deposit will not have time to build up; the concretion will be reduced and the stalactites small. The drop takes almost all of its mineral content to the ground, building up massive thick stalagmites.
Columns.
These are formed when a stalactite meets up with the stalagmite below.
Pillars.
As columns grow thicker they can form pillars of sometimes imposing size.
Soda straws, Macaroni.
These small diameter tube-shaped concretions are the result of a very slow flow of water. At the point of emergence, the weakness of the flow prevents the drop from growing quickly and surface tension prevents it from falling. Degassing slowly occurs and a circular film of calcite forms around the surface of the drop. The process continues slowly and the tube grows more or less regularly. In this way certain soda straws can grow to several metres long for just a few millimetres in diameter. Sometimes the tube becomes blocked and the flow is diverted or even prevented. The soda straw will then be fed laterally or by its surface and create strange shapes: knife-blades, sabres, spurs or spheres. These unusual soda straws are classified as "eccentrics".
Trickling water.
This slow, sometimes discontinuous flow through cracks in ceilings or ledges of walls will result in concretions in often translucent sheets: more or less wavy draperies, pendants, elephant’s ears, crenulations or saw-blades.
Continuous flow
This is an abundant flow emerging from cracks and faults located on the walls, or else from large channels opening from the sides of chambers and galleries.
The corresponding forms may be draperies, rivers and cascades of stalagmites.
In close detail, the surface of these concretions is formed of a succession of small hollows with water overflowing from one to the other, known as "micro gour pools".
Often these concretions form excrescences with rounded surfaces, on the walls. The flow of the water from their edges builds laminations and draperies under these shapes, and the completed dome and draperies looks like a jellyfish.
When these concretions join up they form "walls", "organ-pipes", "fences" or "colonnades".
On the ground, a continuous flow may build up in layers into well stratified "stalagmite floors".
Flow under pressure.
In very narrow fissures, it may happen that the flow of water is greater than the opening can naturally cope with. The water will then emerge under pressure.
The result is concretions with flat surfaces, presenting concentric rings of growth; these "shields" may reach several metres across.
If the emergence is very fine or occasional and the flow less abundant, rosettes or bouquets of fine crystals may decorate the walls.
DROWNED CONCRETIONS.
In the lower parts or in hollows of caves, water may gather and sometimes form very deep pools: troughs and gour pools.
Under the effect of evaporation they become supersaturated in calcite, and its crystallisation, under very different forms, gives a great variety of concretions.
Gour pools
If the supply of water is weak or intermittent, calcite may be deposited on the edge of pools and gradually build up a little dam. This will hold back the water forming a basin known as a "gour pool".
The water backs up behind the dam, and finally overflows in a cascade. On its way the calcite continues to be deposited and the threshold builds up, the level of the water rises again, and the pool becomes deeper.
These gour pools, whose outlines are often scalloped, frequently build up into a succession of pools with water running down from one to another in very beautiful arrangements.
Miscellaneous concretions
In these pools, if the supply is spasmodic the supersaturation rises quickly and the calcite crystallises into crystals of all sizes: large rhombohedra, "Pigs’ teeth", fine "sticks", "chalices", "corollas", "hollow triangles", etc.
These crystals sometimes combine to form "bouquets" and dense "pin-cushions", spherical shapes clustering into in "bunches of grapes" (botryoidal concretions) and a multitude of extravagant forms.
Sometimes when the flow is very slow or non-existent, the calcite forms a veil of crystals on the surface of the water: "floating calcite".
Sometimes this floating calcite forms around a nucleus such as a floating object to produce cupules, corollas or very fragile floors.
Sometimes it builds up around a concretion that breaks the surface of the pool, forming amazing candlesticks.
Cave pearls
"Cave pearls" or "pisoliths" also form in troughs or small depressions fed by a trickle or a permanent dripping.
The calcite is deposed around a seed (debris or a grain of sand) which forms the nucleus. The flow of the moving water causes the germ to rotate and calcite is deposited around it in concentric layers forming a small spherical or ovoid bead. If the water supply and the agitation continue, the "pearl" will grow and take on a fine polish. |