school energy&environment

Different zones, different forms

Cave morphologies are controlled by the zones in which they have been formed : in the vadose zone mechanical erosion features prevail ( such as canyons, gorges, meanders and vertical shafts) and collapses, such as rooms; while in the phreatic zone corrosion features prevail. Finding typical phreatic zone features in a vadose zone (or more rarely vice versa) is a precious clue in order to reconstruct the evolution and the geological history of a cave. Most underground caves form in the phreatic zone, in particular at its top (near to what is called the water table, or, less correct, the piezometric surface), where galleries are permanently flooded, but where there is a certain mixing of water and a merging with meteoric water, which periodically renews the corrosive capacity. Therefore caves are not formed starting from the surface but from inside, because flowing water must be concentrated in order to give rise to galleries and conduits of a certain size. The shapes of galleries and conduits in the phreatic zone are peculiar. Water occupying uniformly the entire section of a gallery leads to corrosion over the entire surface, thus giving origin to conduits with a circular cross section (both vertical and horizontal ). If the rock contain portions that are easier to corrode or to erode, such as bedding planes or layers where the rock is particularly frail, fracures, etc, cross sections will elongate along these layers and elliptical or more complex galleries will form. Galleries of this type are known as phreatic galleries, and form in the first phases of the birth and evolution of a cave. If the water flow is very slow, on the bottom of the conduits fine sediments that “protect” the rock may accumulate. In this case dissolution only takes place on the ceiling, and galleries, known as paragenetic galleries, are formed, usually characterized by a flat roof. Together with large-scale phreatic morphology, small-scale corrosion forms may be present, and when these are found they are precious evidence in the reconstruction of the history of a cave. For example it is possible to see ceiling half tubes or anastomoses (remains of the most ancient conduits of a karst system, which can be seen winding on the ceilings of larger galleries), dissolution pockets and boneyards where water mixes (at the confluence of conduits, where the chemical composition of water is transformed by mixing), or scallops. The latter are small asymmetric scoops, with an elongated tip indicating the direction of flow, formed by the presence of eddies in non laminar water flow. At times these may be seen also in inactive caves and can be examined by speleologists. They are extremely precious clues regarding water flow in the past. In fact is it possible to recognize the direction of flow and in addition the scallops size can help evaluating the speed of flow in flooded conditions. In fact scallops sizes are inversely proportional to the speed of flow. Small, tightly packed forms indicate a rapid flow, large well separated forms indicate slow flows.