Aninselberg ormonadnock (/məˈnædnɒk/mə-NAD-nok) is an isolated rock hill, knob, ridge, or smallmountain that rises abruptly from a gently sloping or virtually level surroundingplain.[A]If the inselberg is dome-shaped and formed fromgranite orgneiss, it can also be called abornhardt, though not all bornhardts are inselbergs. An inselberg results when a body of rock resistant to erosion, such as granite, occurring within a body of softer rocks, is exposed bydifferential erosion and lowering of the surrounding landscape.[2]
The wordinselberg is aloan word fromGerman, and means "island mountain". The term was coined in 1900 by geologistWilhelm Bornhardt (1864–1946) to describe the abundance of such features found in easternAfrica.[3] At that time, the term applied only to arid landscape features. However, it has since been used to describe a broader geography and range of rock features, leading to confusion about the precise definition of the term.
In a 1973 study examining the use of the term, one researcher found that the term had been used for features insavannah climates 40% of the time, arid or semi-arid climates 32% of the time, humid-subtropical and arctic 12% of the time, and 6% each in humid-tropical and Mediterranean climates.
Monadnock is derived from anAbenaki term for an isolatedhill or a lone mountain that stands above the surrounding area, typically by surviving erosion. Geologists took the name fromMount Monadnock in southwesternNew Hampshire.[4] It is thought to derive from eithermenonadenak (transl. smooth mountain) ormenadena (transl. isolated mountain).[5] In this context,monadnock is used to describe a mountain that rises from an area of relatively flat and/or lower terrain. For instance, Mount Monadnock rises 610 metres (2,000 ft) above its surrounding terrain and stands, at 965 m (3,165 ft), nearly 300 m (1,000 ft) higher than any mountain peak within 48 km (30 mi).[6]
The classification of Anthony Young (1969) distinguishes six types of inselbergs:buttes,conical hills, convex-concave hills, rock crest over regolith-covered slope,rock dome (sugarloaf) andkopje or tor.[7]
A 1972 paper defined inselbergs as "steep-sided isolated hills rising relatively abruptly above gently sloping ground". This definition includes such features asbuttes; conical hills with rectilinear sides typically found in arid regions;regolith-covered concave-convex hills; rock crests over regolith slopes; rock domes with near vertical sides;tors (koppies) formed of large boulders but with solid rock cores. Thus, the terms monadnock and inselberg may not perfectly match,[8] though some authors have explicitly argued these terms are completely synonymous.[9]
TheSpitzkoppe of Namibia, a 670-metre (2,200 ft)granite peak formed by earlyCretaceous rifting and magmatism.
Inselbergs are common in eroded and weatheredshields.[10] The presence of an inselberg typically indicates the existence of a nearbyplateau or highland, or their remnants. This is especially the case for inselbergs composed ofsedimentary rock, which will display the same stratigraphic units as this nearby plateau. Once exposed, the inselbergs are destroyed by marginal collapse ofjoint blocks and exfoliation sheets. This process leaves behindtors perched at their summits and, over time, atalus-bordered residual known as acastle koppie appears.[11][12] By this association various inselberg fields in Africa and South America are assumed to be the vestiges of erodedetchplains.[13][14]
Summarizing the understanding on the origin of inselbergs in 1974, geomorphologist Michael Thomas writes "Hypotheses for the development of inselbergs have been advanced, refuted and reiterated over a period of more than seventy years."[24]Volcanic or other processes may give rise to a body ofrock resistant toerosion, inside a body of softer rock such aslimestone, which is more susceptible to erosion. When the less resistant rock is eroded away to form aplain, the more resistant rock is left behind as an isolated mountain. The strength of the uneroded rock is often attributed to the tightness of itsjointing.[25][D]
Inselbergs harbor unique and oftenendemic species from many taxonomic groups and can serve as refugia for animal species living in the surrounding matrix.Plant communities in these ecosystems are often adapted to extreme conditions such as high solar radiation and water scarcity due to the shallow and rocky soils. Since these species have a restricted distribution, they may also become severely threatened byinvasive species.[29]
The inselbergs of Eastern Africa tend to be a refuge for life in theSerengeti ofTanzania and in theMasai Mara ofKenya. Where the soil is too thin or hard to support tree life in large areas, soil trapped by inselbergs can be dense with trees while the surrounding land contains only short grass. Hollows in the rock surfaces provide catchments for rainwater. Many animals have adapted to the use of inselbergs, including thelion, thehyrax, and an abundance of bird and reptile life.[citation needed]
On many tropical inselbergs, bare granite and gneiss surfaces are denselycolonised bycyanobacteria andcyanobacterial lichens, which form darkbiofilms and crusts that can be more conspicuous thanvascular plants at ground level; a study in theIvory Coast recorded 23 cyanobacterial species from 10genera and 17 cyanobacterial lichen species from six genera on exposed rock across savanna, transitional andrainforest zones, with severalPeltula species forming extensive brown crusts. In drier savanna regions with a several-month dry season,Peltula lichens reach their greatest abundance and species richness and largely cover the rock, whereas towards more humid savanna and rainforest climates the number of lichen species decreases and filamentous and sheathed cyanobacteria such asScytonema,Stigonema andGloeocapsa become dominant, producing a blackpatina on the rock; thesepoikilohydric organisms tolerate prolonged desiccation and resumephotosynthesis only when liquid water is present on the surface. Cyanobacteria and cyanobacterial lichens promote physical and chemicalweathering (including surface alkalisation) and generate loose mineral material that accumulates with deadthalli around the base of the outcrops, and measurements on inselberg–savanna transects in tropical South America have shown higher nitrogen concentrations intopsoils adjacent to inselbergs than in the surrounding savanna, consistent withnitrogen fixation by the rock-dwelling cyanobacteria and with the relatively luxuriant growth of trees and shrubs in the immediate vicinity of some inselbergs.[30]
^Cliff Ollier has noted that inUganda inselbergs are commonly made of granite rock, sometimes ofgneiss and never ofamphibolite orvolcanic rock.[2] According to Ollier protuding quartzite hills tend to form ridges rather than "true inselbergs".[2]Dundret in northern Sweden is made ofgabbro.[23]
^Twidale (1981) "Granitic Inselbergs: …"[26] is a review that follows the Willis 1936 works and Twidale 1971, a series of papers available in 1970 and rock weathering strata and structure reviewed U.C.W. well worth reading as they show by theory and materials the importance of preceding structures, internal solution, subsurface weathering, slips, exfoliation, basal weathering (Young, A.Soils), biological effects, plants, solutes and salt plain catena associations, possible lake rise, but mainly the stripping of rock mass leaving resistant units, sometimes volcanic plugs.
^García, Carolina; Hermelin, Michel (2016). "Inselbergs Near Medellín". In Hermelin, Michel (ed.).Landscapes and Landforms of Colombia. Springer. p. 219.ISBN978-3-319-11800-0.
^Sundborg, Å.;Rapp, A. (1986).Erosion and Sedimentation by Water: Problems and prospects.Ambio. pp. 215–225.
^Ebert, Karin; Hall, Adrian M.; Hättestrand, Clas (2012). "Pre-glacial landforms on a glaciated shield: The inselberg plains of northern Sweden".Norwegian Journal of Geology.92:1–17.
^Thomas, Michael (1974).Tropical Geomorphology. The Macmillan Press Ltd. p. 136.
^"A Dictionary of Ecology" (2004). Encyclopedia.com. Retrieved November 28, 2009
^Büdel, B.; Becker, U.; Porembski, S.; Barthlott, W. (1997). "Cyanobacteria and cyanobacterial lichens from inselbergs of the Ivory Coast, Africa".Botanica Acta.110 (6):458–465.doi:10.1111/j.1438-8677.1997.tb00663.x.
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