Also for a geologist mysterious and up to now
not seen are the discovered and photographed structures in the Jebel Uweinat
by Andras Zboray, Hungary, in 2008.
"After having read your article on mysterious columnar sandstone structures in Jebel Uweinat I realised that I found a similar structure in Jabal Arkenu, when I was in the mountain with Andras Zboray in November 2010. Also in 2009 we found the same phenomenon on a low sandstone hill north-east of Jebel Uweinat and also on the flank of Uweinat when we went up to the top of the mountain in the same trip."
More columnar-jointed sandstones
around the world
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Paraguay |
Quote: "The columnar joints of Aregua, located in the western segment of the Asuncion Rift, Eastern Paraguay, occur in reddish eolian sandstones and are disposed orthogonally at both sides and terminations of the nephelinite dyke. Column ranges from 3 to 10 cm in diameter, reaching 15 m in length, and the polygonal forms have four to seven sides, being the pentagonal predominant. Parallel and regularly spaced one bands with relief in steps occur perpendicularly to the columns elongation. In comparison with the basaltic joints, the sandstone columns of Aregua show a moderately mature pattern, far from the typically hexagonal pattern. The textural characteristics of the sandstones near the nephelinite dyke, including the abundant presence of fluid trapped as inclusions, suggest that initial silica-rich cement is composed of a low crystallinity phase, formed quickly in loco under conditions of intense dissolution and reprecipitation of quartz. A hydrovolcanic event seems to be the most probable process to promote a rapid and localized dissolution of quartz in addition to the abrupt increasing of temperature and/or pH in the pore waters. The interstitial aqueous solutions in the sandstones were heated as consequence of magma injection causing partial dissolution of quartz grains, addition of silica to the interstitial liquid phase and further reprecipitation during cooling. Rapid cooling and severe thermomechanical contraction lead to nucleation and propagation of the cracks. Increasing of internal tensions in the sandstone has caused the nucleation of polygons and consequent individualization of prisms interconnected by a system of joints. Joints acted firstly on isotherm surfaces of less temperature and later on successive adjacent planes towards the dyke heat source. Cracking initiated on the surface of lower temperature isotherm and propagated towards the dyke following heat flow lines. The proposed mechanism for the origin of columnar joints in sandstones of Aregua differs in terms of velocity from those admitted for the formation of similar features in igneous rocks, where the cells of heat convection can act for a long timeinterval over all the extension of the magmatic body." |
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Scotland |
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Quote: "Columnar joints, resembling those in igneous rocks, are present in Devonian and Early Carboniferous sandstones at two localities on the Island of Bute in southwestern Scotland. New data concerning field relations and geochemical changes constrain processes responsible for generation of columnar fracture patterns in sedimentary rocks. The columns are mostly normal to bedding and occur at several discrete stratigraphic levels. The host rocks are cut by Early Carboniferous volcanic necks or plugs, which acted as heat sources, but development of columnar jointing was strongly controlled by small sills and dikes of a recessive, purple, fine-grained rock. Geochemistry of these rocks points to an igneous origin. Where contacts are exposed, columnar joints appear above subconcordant intrusions and disappear upward. The lowest parts of the columnar-jointed sandstones are Si depleted and enriched in Ca, Mg, Fe, Al, rare earth elements (REEs), and some base metals. REE patterns from columnar-jointed sandstones near contacts with intrusions resemble those of the igneous rocks, probably as a result of hydrothermal activity. Because most of the columns are approximately normal to bedding, intrusion probably took place before folding of the host rocks and possibly before complete consolidation." |
Germany | |
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Quote: "In the area of the Zittau Mountains (southeast corner of Germany), the geological feature of columnar sandstone is known. For the sandstone atypical column-structures have a diameter up to 15 centimeters and stand perpendicular bundled on a massive sandstone block. Cause is the widespread Tertiary volcanism. In its wake the habitus of the Cretaceous sandstones was changed. The material was exposed thermal processes (contact with magma and thermal solutions), which led to a fritting (marginal melting of mineral grains). The sandstone retained its porosity, but its hardness was increased." ?? Not far, in the Czech Republic, another locality of such columnar-jointed sandstone is known (Cvikov region). |
Australia |
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Alexander Glass, Lecturer and Director of Undergraduate Studies, Duke University, Durham, USA, sent me just this photos. Both show a columnar sandstone formation from the Bondi Beach, near Sydney, Australia. This Triassic sandstone is placed near a volcanic neck. |
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India |
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The contrary: A sample of columnar jointed sandstones with strong pyrometamorphism |
Sandstein-Säulen im Basalt-Steinbruch bei
Biebergemünd im nordwestlichen Spessart (Germany) |
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