GEOMORFOLOGIA OF CAVES AREAS

           Caves develop in the subsoil; yet this does not make of them isolated phenomena. They undergo the effects of a number of the geological and climatological process which model the surface relief, mostly composed of carbonate rocks, which are soluble rocks represented basically by the calcareous: calcium carbonate (CaCO3) and dolomites (calcium carbonate and double calcium carbonate, and magnesium – MgCa(CO3)2)).

           In these areas, because of the acid water action, through the chemical factor (corrosion and dissolution) and also physical (erosion) and with a predominantly subterranean drainage, it is made this relief. The landscape is ruined, pitted, mostly in blind valleys where we can see the macro-forms (colines, stone towers, needles and archs, great vertical walls, canyons, sinks and resurgences of rivers, grottoes and abysses), and the micro-forms (lapies and speleothems). To this singular relief is given the name KARST which means originally “field od calcareous stones” and the group of macro and microforms are known as “karstic features”.

           Caves are a form of karst, but they are not always present. We can find a large amount of macro and microforms, formed as we already said, by chemical action, principally subterranean of the acid water and the erosion of the carbonate rock that forms galleries, chambers, and more or less ample spaces. It is also formed by the irregular dissolution and descontinuos of the acid water which invades the soil and rocks, for their cracks, fissures, laminations, forming specially the micro-forms, such as stalactites, stalagmites, columns, drapes, volcanoes, flowers, helictites, lapies etc. The climate and the composition of the soil intervene in the karst formation, because the type of vegetation (tropical vegetation, roots, amount of remains), climate (rainfall indicator, temperature, wind) and others (different amount of minerals, different intensity of chemical and physical action of the climate and the water).

           Sometimes, we can find a similar relief, but without the calcareous rock, or just a small amount of it. In this case we have a pseudo-karstic relief, for example, what occurs in the quartzítico soil of São Tomé das Letras and Ibitipoca, or areníticos and quartzítico of Vila Velha (state of Paraná), Chapada dos Guimarães or, the eruptivas rocks in Itatiaia.

MACRO AND MICROFORMS IN KARST RELIEF

KARSTIC FEATURES

As has already been mentioned, the karst displays a peculiar form of relief characterized by features at times on a large scale, at times on a very small scale, which combine to give the landscape a ruined and pitted appearence. These features may be gathered togheter in two major groups:

1. Primary Karstic Forms – destructive – theses include superficial forms: dolines, poljes, lapies, towers, canyons, bridges, archs, sinks etc, and subterranean forms: caves, in all their multiplicity of form. These features are mostly macroforms.
2. Secondary Karstic Forms – constructive – although not exclusively so, these are typically subterranean, then so-called speleothem: stalactites, stalagmites, columns, helictites, draps etc.

          The mechanical means to the formation of the karstic features can be described as: physical and chemical actions: corrosion, dissolution, erosion, transportation, abatement       

v      Erosion – is the attack upon, and carrying away of, rock material by mechanical means (physical), by water, wind, snow, plants, animals.

v      Abrasion – type of erosion produced by the action of waves bearing gravel and sand or the action of wind-borne hard particles, which are blown against rock.

v      Corrosion – is the attack by chemical agents which modify the rock, making it liable to the process of dissolution and transport. This factor is considered the principal form of action to the formation of karstic relief and it may be understood through two simplified equations:

a)      Water (H2O) in form of rain, dissolves carbonic gas (CO3) in the atmosphere or soil, forming an acid solution – carbonic acid (H2CO3).

[HO + CO3 = H2CO3]

b)      Acid water (H2CO3), sliding over the surface of the calcareous rock – calcium carbonate (CaCO3), and modifying the rock forming the calcium bicarbonate (CO(HCO3)2) which is soluble and easily carried off by water.

[H2CO3 + CaCO3 = Ca (H CO3)2]

          The superficial karstic features can be subdivided into re-entrant features and residual features.

v     Re-entrant forms – corresponds to the formations produced by the dissolution of the rock. They are usually concave features or pitted: dolines, poljes, karstic lakes, uvalas, blind valleys, canyons, sinks, ressurgências, caves;

v      Residual Forms – corresponds to the prominent features produced by the most resistent areas of the original model, representing the areas that resisted to the action of the time after the process of rescue: rock towers, mount, archs;

v Mixed Forms – corresponds to the features formed by the areas of dissolution and residual forms called generically by lapies.

ü     Doline – is a closed depression that is formed in soluble rocks, having many forms in its opening, it can variate from few centimetres to dozen of metres, in general, larger than deep.

For its genesis, it can be:

1. By Dissolution – when carbonate rocks sofrem, in the surface, corrosion and dissolution by acid water provocating the formation of depressions normally larger than deep . In some cases, a course of water is formed into depht, called alluvial dolines;
2. By Collapse  – when for the presence of a deeper cavity, occurs the crumble of its roof, coming up a depression in the surface, which might comunicate with the inner cavity.

Of course, the type one and two can be associated.

For their shape, they can be:

1. Bowl Dolines – the breadth is much greater than the depth, in a proportion of 10:1. The bottom is flat and the edges are sloped.
2. Funnel Dolines – have a diameter equal to two or three times the depth. The base is narrow.
3. Basin Dolines – generally have a diameter less than the depth, and the rocky walls are vertical, straight down to the base.
4. Caudron Dolines – width, depth and depht almost with the same size.
5. Trench Dolines – singular type with length greater than the width provocated by a crumble of a great way of caves and galleries, when it is very large, it can form a canyon of crumble.

For the presence of water, it can be:

1. Dry;
2. Umid.

ü      Uvalas – coalescência of closed colines.

ü      Poljes – are great closed depressions, reaching many kilometres, abrupt walls with flat basin, mostly raincoat. When inundated they form a karstic lake.

ü      Dry Valleys – are those which have no surface water. They are originated because of the water.

ü      Blind Valleys – closed valleys where the watre infiltrate in the soil by sinks.

ü      Karstic Canyons – great and large valleys of dissolved calcareous rock and resistent residual rocks.

ü      Wall ou Rampart or Falésias Rochosas – steep side walls, ample of  bulks and mixed calcareous, where many formations are noted – towers, vertical estrias, archs, lapies or karstic anfiteatros (when the water dissolves its base forming a concavity until tunnels and passages) etc.

ü      Lapies – great group of mixed features presented, which include structures in depression (re-entrants features) and structures in relief (residual features), and, according to its mophological aspect, it can be called of: canaletas, canelura, meandro, furo, pit, footstep, marmita , draps, alveolar , cell, neddle, plates, cálices and others, they can be found in the surface or in the subterranean spaces.