Inselberg

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Mount Monadnock in New Hampshire, United States Mount Monadnock.JPG
Mount Monadnock in New Hampshire, United States
Pena de Bernal in Bernal, Queretaro, Mexico Bernal.jpg
Peña de Bernal in Bernal, Querétaro, México
Rocca di Cavour, Piedmont, Italy Rocca di Cavour dal Monte Cucetto.jpg
Rocca di Cavour, Piedmont, Italy
Inselberg in the state of Bahia, northeastern Brazil Bahia Scenery en route to Salvador - Brazil - 01.jpg
Inselberg in the state of Bahia, northeastern Brazil
An inselberg in Western Sahara Breast-Shaped Hill.jpg
An inselberg in Western Sahara
Lion atop a koppie in the Serengeti, northern Tanzania Kopje-1001.jpg
Lion atop a koppie in the Serengeti, northern Tanzania

An inselberg or monadnock ( /məˈnædnɒk/ ) is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa a similar formation of granite is known as a koppie, an Afrikaans word ("little head") from the Dutch diminutive word kopje. [1] If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, 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 by differential erosion and lowering of the surrounding landscape.

Contents

Etymology

Inselberg

The word inselberg is a loan word from German, and means "island mountain". The term was coined in 1900 by geologist Wilhelm Bornhardt (1864–1946) to describe the abundance of such features found in eastern Africa. [2] 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 in savannah 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. 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, [3] though some authors have explicitly argued these terms are completely synonymous. [4]

View of Penon de Guatape, Antioquia Department, Colombia Penon de Guatape 02.jpg
View of Peñón de Guatapé, Antioquia Department, Colombia

Monadnock

Monadnock is derived from a Native American term for an isolated hill or a lone mountain that stands above the surrounding area, typically by surviving erosion. Geologists took the name from Mount Monadnock in southwestern New Hampshire. [5] It is thought to derive from the Abenaki language, from either menonadenak ("smooth mountain") or menadena ("isolated mountain"). [6] 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). [7]

Geology

Pietra di Bismantova in the Apennines, Italy Pietra di bismantova castelnovo monti.jpg
Pietra di Bismantova in the Apennines, Italy

Inselbergs are common in eroded and weathered shields. [8] 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. However 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. [9] [10] By this association various inselberg fields in Africa and South America are assumed to be the vestiges of eroded etchplains. [11] [12]

Clusters of inselbergs, called inselberg fields and inselberg plains, occur in various parts of the world, including Tanzania, [13] the Anti-Atlas of Morocco, [11] Northeast Brazil, [14] Namibia, [15] the interior of Angola, [16] and the northern portions of Finland [17] [18] and Sweden. [19] [upper-alpha 1]

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. [21]

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. [22] [upper-alpha 2]

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. [24] :326–327 [25]

Ecology

The inselbergs of Eastern Africa tend to be a refuge for life in the Serengeti of Tanzania and 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. [26] Many animals have adapted to the use of inselbergs, including the lion, the hyrax, and an abundance of bird and reptile life.

See also

Notes

  1. Albeit its not the usual way of describing it the strandflat of Norway was held by Julius Büdel to be an etchplain with inselbergs. [20]
  2. Twidale (1981) "Granitic Inselbergs: …" [23] 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.

Related Research Articles

Tor (rock formation) Large, free-standing rock outcrop on a gentle hill summit

A tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing rock outcrop that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest. In the South West of England, the term is commonly also used for the hills themselves – particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall.

Badlands Type of heavily eroded terrain

Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded. They are characterized by steep slopes, minimal vegetation, lack of a substantial regolith, and high drainage density. Ravines, gullies, buttes, hoodoos and other such geologic forms are common in badlands.

Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.

Butte Isolated hill with steep, often vertical sides and a small, relatively flat top

In geomorphology, a butte is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. The word butte comes from a French word meaning knoll ; its use is prevalent in the Western United States, including the southwest where mesa is used for the larger landform. Due to their distinctive shapes, buttes are frequently landmarks in plains and mountainous areas. To differentiate the two landforms, geographers use the rule of thumb that a mesa has a top that is wider than its height, while a butte has a top that is narrower than its height.

Mesa Elevated area of land with a flat top and sides that are usually steep cliffs

A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock, e.g. shales overlain by sandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust. Unlike plateau, whose usage does not imply horizontal layers of bedrock, e.g. Tibetan Plateau, the term mesa applies exclusively to the landforms built of flat-lying strata. Instead, flat-topped plateaus are specifically known as tablelands.

Cuesta Hill or ridge with a gentle slope on one side and a steep slope on the other

A cuesta is a hill or ridge with a gentle slope on one side, and a steep slope on the other. In geology the term is more specifically applied to a ridge where a harder sedimentary rock overlies a softer layer, the whole being tilted somewhat from the horizontal. This results in a long and gentle backslope called a dip slope that conforms with the dip of resistant strata, called caprock. Where erosion has exposed the frontslope of this, a steep slope or escarpment occurs. The resulting terrain may be called scarpland.

Wave Rock Rock formation in Western Australia

Wave Rock, also known as Katter Kich by the Noongar, is a natural rock formation that is shaped like a tall breaking ocean wave. The "wave" is about 15 m (49 ft) high and around 110 m (360 ft) long. It forms the north side of a solitary hill, which is known as "Hyden Rock". This hill, which is a granite inselberg, lies about 3 km (2 mi) east of the small town of Hyden and 296 km (184 mi) east-southeast of Perth, Western Australia. Wave Rock and Hyden Rock are part of a 160 ha (395-acre) nature reserve, Hyden Wildlife Park. More than 100,000 tourists visit every year.

Pediplain Extensive plain formed by the coalescence of pediments

In geology and geomorphology a pediplain is an extensive plain formed by the coalescence of pediments. The processes through which pediplains forms is known as pediplanation. The concepts of pediplain and pediplanation were first developed by geologist Lester Charles King in his 1942 book South African Scenery. The concept gained notoriety as it was juxtaposed to peneplanation.

Granite dome Rounded hills of bare granite formed by exfoliation

Granite domes are domical hills composed of granite with bare rock exposed over most of the surface. Generally, domical features such as these are known as bornhardts. Bornhardts can form in any type of plutonic rock but are typically composed of granite and granitic gneiss. As granitic plutons cool kilometers below the Earth's surface, minerals in the rock crystallize under uniform confining pressure. Erosion brings the rock closer to Earth's surface and the pressure from above the rock decreases; as a result the rock fractures. These fractures are known as exfoliation joints, or sheet fractures, and form in onionlike patterns that are parallel to the land surface. These sheets of rock peel off the exposed surface and in certain conditions develop domical structures. Additional theories on the origin of granite domes involve scarp-retreat and tectonic uplift.

Navajo section

The Navajo Section is a physiographic section of the larger Colorado Plateaus Province, which in turn is part of the larger Intermontane Plateaus physiographic Division.

Pediment (geology) Very gently sloping inclined bedrock surface

A pediment, also known as a concave slope or waning slope, is a very gently sloping (0.5°-7°) inclined bedrock surface. It is typically a concave surface sloping down from the base of a steeper retreating desert cliff, escarpment, or surrounding a monadnock or inselberg, but may persist after the higher terrain has eroded away.

Bornhardt A large dome-shaped, steep-sided, bald rock

A bornhardt is a dome-shaped, steep-sided, bald rock outcropping at least 30 metres (100 ft) in height and several hundred metres in width. They are named after Wilhelm Bornhardt (1864–1946), a German geologist and explorer of German East Africa, who first described the feature.

Saprolite Chemically weathered rock

Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds. Deeply weathered profiles are widespread on the continental landmasses between latitudes 35°N and 35°S.

Scarp retreat

Scarp retreat is a geological process through which the location of an escarpment changes over time. Typically the cliff is undermined, rocks fall and form a talus slope, the talus is chemically or mechanically weathered and then removed through water or wind erosion, and the process of undermining resumes. Scarps may retreat for tens of kilometers in this way over relatively short geological time spans, even in arid locations.

Gornaya Shoria megaliths Rock formation in Siberia

The Gornaya Shoria megaliths, meaning Mount Shoria megaliths, are rock formations forming part of Mount Shoriya in southern Siberia, Russia, lying to the east of the Altay Mountains.

A fin is a geologic formation that is a narrow, residual wall of hard sedimentary rock that remains standing after surrounding rock has been eroded away along parallel joints or fractures. Fins are formed when a narrow butte or plateau develops many vertical, parallel cracks. There are two main modes of following erosion. The first is when water flows along joints and fractures and opens them wider and wider, eventually causing erosion. The second is where the rock type (stratum) is harder and more erosion resistant than neighboring rocks, causing the weaker rock to fall away.

Flared slope Rock-wall with a smooth transition into a concavity at the foot zone

A flared slope is a landform consisting in a rock-wall with a smooth transition into a concavity at the foot zone. Flared slopes form due to various weathering patterns that are more effective at the regolith or soil-covered base of rock walls. These landforms are common in granitic rocks but can also occur in other rock types such as ignimbrite. Flared slopes are found in a variety of different lithological and climatic environments in Australia, Spain, South Africa, and the western United States.

An etchplain is a plain where the bedrock has been subject to considerable "etching" or subsurface weathering. Etchplanation is the process forming etchplains. Contrary to what the name might suggest, etchplains are seldom completely flat and usually display some relief, as weathering of the bedrock does not advance uniformly. This means that weathering is unrelated to the flatness which might be derivative of various other processes of planation including peneplanation and pediplanation. Erosion of etchplains can result in the exposure of inselbergs such as bornhardt and tors. Generally the topography exposed at a stripped etchplain, that is an etch surface, after erosion of regolith is one with many irregularities as result of structurally defined areas of rock strength.

Nubbin (landform) Small hill of bedrock with rounded residual blocks

In geomorphology a nubbin is a small and gentle hill consisting of a bedrock core dotted with rounded residual blocks. The blocks derive from disintegrated and weathered bedrock layers. In particular it is assumed that the boulders of the nubbins are the remnants of the outer one or two exfoliation shells that weathered underground, albeit some weathering can continue to occur once the boulders are exposed on surface.

References

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  26. Serengeti National Park Visitor Center, Tanzania