Rock gate

This article is about a naturally formed geological formation. For the former railway tunnel near Etterzhausen, which is often only called Felsentor , see Felstortunnel .

Landscape Arch with a span of 92 meters in Arches National Park , Utah, USA

Rock gates , also known as arched rocks or surf gates , are rock formations that are pierced like a bridge or window and are created by erosion . Depending on the size, shape, origin and environment, other names come into play, such as rock bridge , stone or rock arch , natural bridge or surf gate . Its opening is completely surrounded by rock, and the thickness of the perforated rock formation is roughly the same as its width and height. These forms of erosion come in very different sizes, from holes barely the thickness of a thumb to openings that are more than a hundred meters thick and vaulted by rocks.

The largest concentration of over 2000 stone arches is found in Arches National Park in the US state of Utah . A heterogeneous sandstone layer was broken into parallel ribs by local bulges of a salt dome . In the harsh desert climate at over 1,500 meters above sea level, softer layers of stone eroded, leaving behind the stone arches made of harder stone.

Demarcation

The Pont d'Arc in southern France

The forms of erosion mentioned can arise in all types of rock. They are short-lived in directly erodible rocks or in environments with high erosion energy, such as the surf zone of cliffs . Permanent large forms are particularly common in sandstone and limestone , as these resistant rocks are prone to dissolution by water and weathering .

The designation of such natural openings depends on their size, shape, origin and environment. Rock or natural gate and rock window are of different sizes, a rock or natural bridge was created by water that often flows through it in the form of a stream or river. Rock , natural and stone arches are characterized by their shape and are often not mainly created by water. Surf gates are created by the effect of the ocean surf on cliffs.

In the following, all these forms are summarized under the term “rock gate”.

In English , such forms of erosion are called Natural Arch or Natural Bridge . According to the classification of the Natural Arch and Bridge Society , a natural arch is created without the involvement of running water. Such forms are common in the western United States, where they mainly arise in sandstone . The Natural Bridge , on the other hand, is created by the effect of flowing water and in many cases is continuously or temporarily traversed by a stream or river. A further subdivision of these main groups is possible among other things by the form or the origin.

Emergence

Natural Bridge in Bryce Canyon National Park , Utah, USA

Natural Bridge on the Dyrholaey Peninsula , Iceland

Stone arches and rock bridges are usually created by erosion, which removes parts of natural rock formations. On the microscopic level , this means the dissolution of cement , on the macroscopic level, the formation of fissures through various processes, such as pressure relief or tectonic processes such as uplift or expansion , and expansion through erosion or falling material. Both types of development can take place separately or in combination.

Rock gates only form under special conditions. The type of rock or rocks involved, the shape of the rock formation and the combination of erosion processes that affect it are decisive. The causes of the formation are the effects of water, temperature changes, gravity or tectonic forces. Wind usually does not play a direct role in the development, but it can be an important factor in further design, for example due to the abrasive effect of grains of sand carried along ( corrosion in combination with processes of deflation ).

In detail, among other things, the following processes favor or cause the formation of rock gates:

• tectonic uplift can lead to the formation of fissures and crevices as well as rivers cutting into the subsurface

• Glacier activity creates steep and vertical rock faces and rugged terrain, while the melting of glaciers promotes erosion from large amounts of water

• The cutting of river meanders into the subsoil can progress to the approach of river bends, so that only a relatively thin rock wall connects the future mountain with the valley walls and a rock fall can form a stone arch

• In the run-up to a river tap , a relatively thin rock wall is also formed in some cases, in which the formation of a rock gate is favored

• the formation of underground rivers through karstification and the collapse of parts of the caves formed thereby or their merging

• the formation of lava tubes and the collapse of parts of the tube

• the formation of fissures and crevices and their widening through erosion processes, as well as the removal of less resistant layers in a horizontal or inclined layer package

• the activity of the surf , which by carrying sand and stones leads to severe erosion in the surf area, while higher areas are only slightly affected. In addition, it leads to very different erosion of rock of different hardness

• Rock reinforcement through pressure stiffening. Directed pressure leads to a strengthening of a rock, for example through recrystallization of the rock components. The lateral deviation of the direction of tension in a rock formation above a crevice or an opening leads to a strengthening of the lateral areas and thus to a stronger resistance to erosion there

• Separation through pressure relief, as occurs, for example, when uncovering rocks previously under the earth's surface, favors the formation of crevices and crevices, often parallel to the surface of a rock wall ( desquamation )

• Chemical weathering , temperature weathering and the occurrence of freeze-thaw cycles lead to the widening of fissures and crevices or to the loss of rock strength

• different erosion of neighboring, differently resistant rocks.

literature

• James MacDonald / Christopher Burton: Collins Dictionary Geology. New Edition . HarperCollins Publishers, 2003, ISBN 0-00-714768-6 .
• Hans Murawski: Geological Dictionary . 8th edition. Enke, Stuttgart 1983, ISBN 3-432-84108-6 , pp. 64 .
• Radim Kettner: General geology. Volume 4 . 1st edition. Deutscher Verlag der Wissenschaften, Berlin 1960, p. 117-126 .
• Jay H. Wilbur: Natural Arch Information . In: naturalarches.org, The Natural Arch and Bridge Society . Retrieved April 26, 2009.
• Roland H. Winkelhöfer: Through the caves of Bohemian Switzerland. Cave guide and cadastral documentation. (2nd Edition). Verlag Der Höhlenforscher, Dresden 2009, ISBN 3-00-002317-8 .

Web links

Commons : Stone Arch  - collection of images, videos and audio files

Individual evidence

1. ↑ What is the difference between a natural arch and a natural bridge? . In: naturalarches.org, The Natural Arch and Bridge Society . Retrieved April 26, 2009.