Furna (geomorphology)

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Furnas are geomorphological landforms that are created by underground erosion . The inventory includes large funnels in the earth's surface (up to several hundred meters in diameter, up to over a hundred meters deep), caves, tunnels and crevices. They are thus similar to the karst forms , which, however, arise in limestone through solution. The term was coined by Maack in 1946.

Furnas in Brazil

The name of the Devonian Furnas Formation in the Paraná Basin in southern Brazil, in which these hollow forms often occur, is derived from the Furnas . The formation consists mainly of sandstones, predominantly arkose , with clayey and marl-like binders. Weathering and leaching of the clayey components by groundwater flows, especially in the area of ​​faults, loosen the rock structure and erosion can start at these points. Initially, underground tunnel systems are created, and the collapse of the rock above finally creates huge erosion craters several hundred meters in diameter and well over 100 m deep. These craters are connected by tunnel or crevice systems.

Use of the term

In the geoscientific literature, the term Furna has remained almost exclusively restricted to the area of ​​the Furnas Formation in Brazil, although such subterranean forms of erosion also occur in other regions of the world.

In the area of ​​distribution of the Furnas Formation, Furnas are also often important archaeological sites of human remains (e.g. cave paintings). That is why the term can also be found in archaeological literature.

However, he was on volcanic cave systems z. B. transferred to the Azores, so that the term is used there. Typical volcanic furnas in the Azores are long tunnels with a connection to the sea. They were created by expanding existing volcanic cave systems (e.g. empty underground lava tubes ). At the other end there is often a narrow connection to the earth's surface, the so-called "fumarole". When the waves are heavy, the sea water spurts out "fumarole-like" from this opening. However, the term "fumaroles" in the geological sense is limited to volcanic exhalations and should not actually be used in the above sense.

  • View into a Furna in Vila Velha

  • Platform at the end of an elevator at a depth of approx. 70 meters in a Furna in Vila Velha

  • View into a large furna in Vila Velha, Paraná, Brazil

literature

  • R. Maack: Geologia e geografia da regiao de Vila Velha e consideracoes a glaciacao carbonífera do Brasil. In: Arquivos do Museu Paranaense. Vol. 5, 1946, ZDB -ID 301803-9 , pp. 1-302.

Individual evidence

  1. ↑ Mário Sérgio de Melo, Paulo César Fonseca Giannini, Luiz Carlos Ruiz Pessenda, M. Brandt Neto: Holocene paleoclimatic reconstruction based on the Lagoa Dourada deposits, southern Brazil. In: Geologica Acta. Vol. 1, Nº 3, 2003, ISSN  1695-6133 , pp. 289-302, online (PDF; 519 kB) .

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