Sinkhole (geology)

A sinkhole (a word of Slovene origin meaning valley or depression) alludes to a special type of geological depression characteristic of karstic reliefs. In Spanish it is known as “torca”.

Sinkholes, very common in limestone massifs, may be occupied by lakes, as in the Adriatic region of Yugoslavia, the Balkan Peninsula, the Alps of central Europe, in the United States (in Indiana, Kentucky, Tennessee) or in the Yucatan Peninsula in Mexico, where they are called cenotes.

Morphology

Fertile doll in the of Sauveterre, Lozère, France.

Hundred of a karst duct.

Some dolinas are formed by the collapse of the roof of an underground cave. It is a common form of relief in limestone areas.

In general, they do not usually occur in exclusively calcareous regions, but in those made up of marl, rocks that are made up of limestone and clay in various proportions. The process is relatively simple: carbon dioxide from the atmosphere combines with rainwater to form carbonic acid, which attacks the calcium carbonate of the marls (which is not soluble in water) and converts it into calcium bicarbonate, which is soluble in water, so that the clay remains free, which is deposited in the lower areas of the relief forming terra rossa basins (Italian term meaning «red earth») called sinkholes, uvalas or karstic valleys such as poljés. The terra rossa is, therefore, a decalcification clay and presents a great fertility with regard to its agricultural use.

At first all sinkholes are absorbent (they lose water through infiltration), then it happens that wherever the runoff carries clay, it is deposited and waterproofs the bottom of the sinkhole, turning it into a pond that will be filled after the rains and It will slowly dry out due to the evaporation of the water. A sinkhole as a karstic formation that is produced by the dissolution of the rock or by the collapse of the roof of a cave, forming depressions of various sizes and circular contours. When these chasms have very abrupt steep walls they are called torcas and if, on the contrary, they are smoother and without scarps they are called hoyas. It usually appears as a wide well, with a more or less flattened bottom. If the water floods them, it can create lagoons.

Types

First Pozo de las Ánimas, in Malargüe, Argentina, dolina de hundimiento.

Second Pozo de las Ánimas, in Malargüe, next to the previous one.

Collapse in karstified plasters in the south of the province of Madrid.

The dissolution sinkholes are the consequence of a slow loss of soil matter, dissolved by rainwater that pools and then infiltrates into it. The subsidence sinkholes are formed when the ground subsides over a karstic cavity, due to the collapse of its roof.

If they have a flat bottom and are not very deep, it is said to be a trough, or saucer sinkhole, with a terra rossa bottom, which prevents the karstification processes from continuing. However, when the process continues, the sinkhole takes on the appearance of a funnel, we will then have a sinkhole. The sinkhole can communicate with a chasm: a cavity with subvertical walls that communicates with the surface. Normally the chasms communicate with an underground network.

When many sinkholes come together, an uvala can be formed and, on a larger scale, a poljé.