In Deutsch
In Deutsch
In Deutsch
In Deutsch

Shield


en: shield
de: Schild (Sinter-) (s)
es: colada
fr: disque, couche (de) calcaire (f)
hu: cseppkõdob; pajzs
it: colata (sf)(-concrezionale)


Shields or Discs are rather rare, only few caves have one, very few have more than one. In very few caves they are abundant, like in Lehman Cave, U.S.A., where more than 100 can be found.

Shields look like huge discs, although they are generally an incomplete circle. At their back end they are attached to the cave wall. They stick out of the wall upwards, in an angle between 10° and 60°, common are about 30°. Almost circular, sometimes elliptical, plates of about two centimers thick and a diametre of up to one metre are found. They have concentric rings similar to the layers of other dripstones. Some have flowstone growing downwards from the shield to the floor, similar to common waterfall-like flowstones. But there are never flowstones on top of the shield.

Somtimes a shield collapses, when the weight of the stalactites growing downwards overstrains the stability of the the plate. Such a collapse, or a close look at the disc's rim, reveals a characteristic of shields. They always consist of two parallel plates, each one centimetre thick, separated by a medial crack.

It forms by water seeping through the medial crack powered by capilar forces. The shield is formed by an almost horizontal fissure in the rock, where water emerges. Water standing in the fissure fills it to the surface of the rock, probably forming a dam of water, hold together by the surface tension. Carbon dioxide is released into the cave air, then the water starts to vaporize slowly. A general necessity of the formation of Shields is an extremely slow but almost constant flow of water. The limestone in the water is deposited on the hard surfaces, which are the rims of the fissure. Two parallel dams are formed along the edge, the water fills the limestone dams too.

Later, when the dams become higher and higher, they start to become circles, as the water flows from the source to the full lenth of the rim. It flows for a certain distance until it reaches the rim, and where it reacheds first, the growth rate is highest. As a result the shield always becomes circular, with the rim always the same distance from the water source. If the amount of water is a little higher, the water flows out of the rim of the shield and growth of limestone beneath the shield starts. Thus typical flowstones are formed below the Shield.


See also


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