Abornhardt(/ˈbɔːrnˈhɑːrt/) is a dome-shaped, steep-sided, bald rockoutcroppingat least 30 metres (100 ft) in height and several hundred metres in width.[1]They are named afterWilhelm Bornhardt(1864–1946), a German geologist and explorer ofGerman East Africa,who first described the feature.[2][3]
Whilebornhardtwas originally used to sometimes denote a type ofinselberg(literally island mountain—an isolated dome in an otherwise flat landscape), the termbornhardtis used in modern literature to refer to domed hills and mountains regardless of isolation; thus, not all bornhardts are inselbergs and not all inselbergs are bornhardts.[4]Bornhardts are commonly composed ofigneous rocks,[4]oftengranites,[4][5]but examples ofgneiss,quartziteandarkosebornhardts exists.[5]
TheSugarloaf MountainofRio de Janeirois a typical example of this landform and is the origin of the common bornhardt nickname "sugar loaf".[6]
Bornhardts are most easily seen in arid and semi-arid regions, but occur over a wide range of climates. Found in diverse topographic settings, they mainly occur in multicyclic landscapes. Overgeological time,bornhardts may degrade to form other landforms such asnubbins(orknolls) andcastle koppies.[7]Bornhardts were previously thought of as characteristicallytropical landforms,but have been shown to be more related tolithologyandrock structurethan climate.[8]
Depending on their shape, they are also referred to as sugar loaves, whalebacks, turtlebacks or domes. In certain locales they may be known asdwalas,half-oranges,matopos,etc. Often, the underlying geological fracture pattern is shown by the surface arrangement of bornhardts, as can be seen in theKamiesbergeofNamaqualandand theEverard RangesofCentral Australia.[9]
Bornhardts commonly show extensivesheet jointing(fracture systems in the rock parallel to its surface). There are two main theories on the origins of bornhardts, each relating to jointing. Theexogenetictheory asserts that sheet jointing is formed in response to the emerging topography, while theendogenetictheory suggests that the topography in bornhardts reflects already existing sheet jointing.[10]
See also
edit- Granite dome– Rounded hills of bare granite formed by exfoliation
- Inselberg– Isolated, steep rock hill on relatively flat terrain
- Monolith– Stone block made of one single piece; object made of one single rock piece
References
editCitations
edit- ^Mayhew, Susan (2015).A Dictionary of Geography(5th ed.). Oxford University Press. p. 56.ISBN978-0-19-968085-6.
- ^"Glossery of Terms: Bornhardt".InfoCenter for Environmental Geology. Archived fromthe originalon 25 April 2012.
- ^Twidale & Vidal-Romaní 2005,pp. 109–116.
- ^abcMigoń, Piotr(2004)."Bornhardt".InGoudie, Andrew S.(ed.).Encyclopedia of Geomorphology.Vol. 1. London: Routledge. pp.92–93.ISBN0-415-32737-7.
- ^abKing, Lester(1975). "Bornhardt landforms and what do they teach".Zeitschrift für Geomorphologie.19(3): 299–318.Bibcode:1975ZGm....19..299K.doi:10.1127/zfg/19/1975/299.
- ^Allaby, Michael (2010).A Dictionary of Ecology(4th ed.). Oxford University Press. p. 53.ISBN978-0-19-956766-9.
- ^Twidale, C. R. (2007). "Bornhardts and associated fracture patterns".Revista de la Asociación Geológica Argentina.62(1): 139–153.hdl:2440/43513.ISSN0004-4822.
- ^Thomas, Michael F. (2004)."Tropical Geomorphology".InGoudie, Andrew S.(ed.).Encyclopedia of Geomorphology.Vol. 2. London: Routledge. pp.1063-1069.ISBN0-415-32738-5.
- ^Twidale, C. R.(1981). "Granitic Inselbergs: Domed, Block-Strewn and Castellated".The Geographical Journal.147(1): 54–71.Bibcode:1981GeogJ.147...54T.doi:10.2307/633409.JSTOR633409.
- ^Twidale & Vidal-Romaní 2005,pp. 32–48.
Sources
edit- Twidale, Charles Rowland;Vidal-Romaní, Juan Ramón (2005).Landforms and Geology of Granite Terrains.A. A. Balkema.ISBN0-415-36435-3.
External links
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