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Amboni Caves Expedition: Part 2
PROCESSES OF SOLUTION AND PRECIPITATION
LEAD TO THE GROWTH OF FORMATION IN CAVES.

The air occupying the pore spaces in soil is usually much richer in carbon dioxide than in the ordinary atmosphere, since the respiration of plant roots and the bacterial decay of humans liberate this gas. Cave air has a lower partial pressure of carbon dioxide than soil air, because rain water in percolating through soil acquires a high content of carbon dioxide; if such water reaches a bed of limestone it takes calcium carbonate into solution.

If the water now penetrates through crevices in the rock to emerge into cavern, it meets air containing carbon dioxide at a lower partial pressure. It loses this gas to the air, and calcite is deposited. It is this deposition-taking place under varied conditions, which produces the manifold beautiful structures in caves. Evaporation of the solvent water is commonly believed to be the cause of deposition, but actually it plays a minor role, since the air in caves is practically saturated with water vapor, so that evaporation is inhibited.

At a given locality the subterranean temperature remains uniform and invariable below a certain depth (We're not dealing with depths so great that the rise of temperature towards the earth's center need be considered). Therefore there are no temperature changes to affect the growth of cave formations. Temperature is important only in that, together with partial pressure determines the carbon dioxide content of the ground water.

When a drop exudes slowly from the roof of a cave, some deposition takes place before it is large enough for gravity to detach it, and a succession of drops forming in the same spot results in a hanging formation, the well-known STALACTITE. Later exudation trickles down the stalactite losing carbon dioxide en route and adding to the girth of the structure before finally forming drops and causing growth at the tip. If a drop still contains some calcium bicarbonate after detachment, the solid is deposited when the drop splashes on the floor and a STALAGMITE grows upwards. Eventually a stalagmite and its companion stalactite may meet to form a column extending from floor to ceiling. Ultimately lateral growth of such a column may block the passageway or several columns may coalesce to form a single wall. All the structures described above bear the general name of Dripstone formation descriptive of their origin.

Where a trickle on a cave floor has to traverse a barrier it is spread out in a thin layer, which has a large ration of surface area to volume. This factor favor loss of carbon dioxide and causes deposition. Thus the barrier grows upwards with level edge and forms a rim stone dam enclosing a pool or gourd. Such gourd sometimes occurs in beautifully cascaded series. That formation you can see in the cave park at Amboni, Tanga.

A SCIENTIST'S DESCRIPTION OF CAVE
AND FORMATION IN CAVES.

Carbon dioxide + H2O = H2C02
(Carbon dioxide) + (water) = (Carbonic acid)
Carbonic acid attacks the limestone rock
CaCO3 + H2CO3 = Ca2 + 2(HCO3)
(Calcium Carbonate) + (Carbonic acid) =
(Calcium ions) + (bicarbonate ions)

Soluble Soluble
Carbon dioxide is given off into the cave air and Calcium Carbonate is deposited Ca2+ +2(HC03-) = CO2 + CaCO3 + H20

Those interested in visiting The Amboni Caves may contact the writer at e-mail: kisonipm@hotmail.com

Or Mr. Kassim or Tezani, Tour Guides Offices,
The Amboni Culture and Guiding Promoters, P.O. Box 1021
Tel: 255 741 211091 or 255 27 2643546
Fax: 255 27 2643546
Majestic Cinema, Mkwakwani Road,
Tanga, Tanzania, East Africa.

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