Inselberg

Definition

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) and kopje or tor.

A 1972 paper defined inselbergs as "steep-sided isolated hills rising relatively abruptly above gently sloping ground". This definition includes such features as buttes; 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, though some authors have explicitly argued these terms are completely synonymous.

Geology

Geological and geographical patterns

Inselbergs are common in eroded and weathered shields. The presence of an inselberg typically indicates the existence of a nearby plateau or highland, or their remnants. This is especially the case for inselbergs composed of sedimentary rock, which will display the same stratigraphic units as this nearby plateau. Once exposed, the inselbergs are destroyed by marginal collapse of joint blocks and exfoliation sheets. This process leaves behind tors perched at their summits and, over time, a talus-bordered residual known as a castle koppie appears. By this association various inselberg fields in Africa and South America are assumed to be the vestiges of eroded etchplains.

Location

Clusters of inselbergs, called inselberg fields and inselberg plains, occur in various parts of the world, including Tanzania, the Anti-Atlas of Morocco, Northeast Brazil, Namibia, the interior of Angola, and the northern portions of Finland and Sweden.

The types of rock of which inselbergs are made include granite, gneiss and gabbro.

Origin and development

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." Volcanic or other processes may give rise to a body of rock resistant to erosion, inside a body of softer rock such as limestone, which is more susceptible to erosion. When the less resistant rock is eroded away to form a plain, the more resistant rock is left behind as an isolated mountain. The strength of the uneroded rock is often attributed to the tightness of its jointing.

Inselbergs can be reshaped by ice sheets much the same way as roches moutonnées. In northern Sweden, examples of this type of inselberg are called flyggbergs.

Ecology

Inselbergs harbor unique and often endemic 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 by invasive species.

The inselbergs of Eastern Africa tend to be a refuge for life in the Serengeti of Tanzania and in the Masai Mara of Kenya. 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 the lion, the hyrax, and an abundance of bird and reptile life.

On many tropical inselbergs, bare granite and gneiss surfaces are densely colonised by cyanobacteria and cyanobacterial lichens, which form dark biofilms and crusts that can be more conspicuous than vascular plants at ground level; a study in the Ivory Coast recorded 23 cyanobacterial species from 10 genera and 17 cyanobacterial lichen species from six genera on exposed rock across savanna, transitional and rainforest zones, with several Peltula 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 as Scytonema, Stigonema and Gloeocapsa become dominant, producing a black patina on the rock; these poikilohydric organisms tolerate prolonged desiccation and resume photosynthesis only when liquid water is present on the surface. Cyanobacteria and cyanobacterial lichens promote physical and chemical weathering (including surface alkalisation) and generate loose mineral material that accumulates with dead thalli around the base of the outcrops, and measurements on inselberg–savanna transects in tropical South America have shown higher nitrogen concentrations in topsoils adjacent to inselbergs than in the surrounding savanna, consistent with nitrogen fixation by the rock-dwelling cyanobacteria and with the relatively luxuriant growth of trees and shrubs in the immediate vicinity of some inselbergs.