What Are Inselbergs (Monadnocks) and How Do They Form

Inselberg or monadnock (pronounced /məˈnædnɒk/) in geology, geomorphology, or geography refers to an isolated, steep-sloped ridge, hill, small mountain, or knob sitting in a relatively flat to gently sloping surrounding area. It appears like an island rising from the sea.

Three hypotheses explain the possible formation, the most popular being the two-stage model, i.e., etching and removal. Examples include Mount Monadnock (USA), Uluru (Australia) Spitzkoppe (Namibia). However, these landforms are widespread in Africa and occur in North America, South America, Europe, and Australia.

Learn more about inselbergs or monadnocks, including their meaning or definition, the tree types, and the formation hypothesis. We will also look at growth and destruction occurrences and give examples.

What is an inselberg?

An inselberg or monadnock refers to isolated, steep-sided rock hills, ridges, knobs, or small mountains which rise abruptly from an extensive, nearly flat plain. It doesn’t describe the extent of isolation or how steep or high the hill is.

The term inselberg comes from two German words, ‘insel’, which means island, and ‘berg’, which means mountain. It was coined by Walter Bornhardt, a German naturalist and traveler, after observing enchanting isolated rocky hills that rose steeply from East Africa’s savanna plains as though they were islands in seas.

On the other hand, the name nonadnock came from Mount Monadnock in New Hampshire. The name originates from the Native American Abenaki words ‘menonadenak’ or ‘menadena’, meaning a smooth or isolated mountain. Thus, there is no difference between monadnock and Inselberg; the words are interchangeably usable unless you refer specifically to Mount Monadnock.

Inselbergs may vary in size and can occur in a group forming a range or alone. Also, they only apply to hills formed by lowering land. Thus, small volcanoes and tectonic hosts aren’t inselbergs.

Inselberg examples

Examples of inselbergs include Uluru (Ayers Rock) in Australia, Savandurga and Mandar hills in India, Spitzkoppe (Namibia), Nouragues French Guiana (Guyane), Mount Monadnock, New Hampshire USA, North Berwick Law in Scotland UK, Mount Chiperone in Mozambique, Waze rock in Nigeria, among many others.

Here are a few images of some of the monadnocks in the world.

Inselberg types

Some sources may classify inselbergs into dome, castellated, and nubbins. The last two types form from bornhardts; thus, the three are genetically related.

1. Bornhardts (dome inselberg)

Initially considered as a type of inselberg, a bornhardt is a dome-shaped, bald rock outcropping with steep sides measuring at least 30 meters tall and several hundreds of meters wide common on massifs. However, it can refer to any hill or mountain with a dome shape and often with convex bare slopes that may have very little talus.

It often has poorly jointed rocks and can occur within a hilly place, i.e., meaning it doesn’t have to be an island, mountain, or hill. Examples include granite domes in Yosemite National Park, USA, and Sugar Loaf ( Pão de Açúcar) in Rio de Janeiro, Brazil.

2. Castellated inselbergs (Castle koppies or kopjes)

Castellated inselbergs are well-jointed with angular outlines and a castle appearance, but a keen observation of their massive lower morphology will reveal pillars, walls, notches, or towers. They show step appearance with horizontal benches separating vertical surfaces due to horizontal and vertical fractures, respectively, and are often smaller than bornhardts.

3. Nubbins (block or strewed boulder inselbergs)

Nubbins have chaotically arranged loose boulders or blocks lying on each other. They are bedrock rooted with individual angular, rounded, or both measuring from 1-2 meters to over 10 meters. They form from the degradation of dome or castellated monadnocks.

Besides these three, another closely related landmark is a tor (castle koppie or kopje), which describes a large rock outcrop or residual landform with an abrupt rise from a gentle or smooth slope not large enough to be a hill. Tors are smaller than inselbergs, often not exceeding 10-15 meters in height, and form part of the slope, not the hill itself.

Lastly, please don’t confuse it with a butte (bute), a small, steep-sided hill made of boulders or jointed blocks, often vertical, whose is considerably flat and smaller than the height. A butte is smaller than a mesa, tableland, or plateau. Also, unlike boulders, it is bedrock rooted.

Lithology

Indeed, most of the inselbergs in the ancient shields in Africa, Australia, and South America are built of coarse-grained granitic, especially poorly jointed potassium feldspar rich, the abundant lithology in these areas.

A good example of granitic inselberg is the Spitzkoppe inselbergs between Usakos and Swakopmund, Namibia, the highest peaking at 2,200 ft above the floor of the Namib desert. In Uganda, they are commonly made of granite or gneiss and never of volcanic or amphibole rocks.

However, these landforms aren’t restricted to a certain or single lithology. They may be of other lithologies, including gabbro, syenite, rhyolite, migmatite, gneiss, quartzite, arkosic sandstone, or conglomerate. For instance, the famous Uluru inselberg in  Northern Territory, Australia, is made of felspar-rich coarse-grained sandstone known as arkose.

Lastly, only a few inselbergs show lithological boundaries at the base of the hill and a different mineral composition compared to the plain. Most have a joint lithology and are built on massive bedrock.

How are inselbergs formed?

The origin or formation of inselbergs is a hotly contested matter with three hypotheses proposed. However, there is a consensus they can form in various ways but look the same, i.e., convergent landform evolution may result from contrasting pathways.

The three hypotheses explain how inselberg or monadnock form are two-stage (most accepted), planation and scarp retreat/pediplanation. Let us look at each.

1. Two-stage theory– etch-planation

The two-stage theory focuses on differential subsurface decomposition or weathering to form regolith as the first stage, followed by erosion that removes the regolith or mantle as the second stage. John Downie Falconer talked about it as early as 1911, but Charles Rowland Twidale championed it. Other known contributors are David Leslie Linton and Michael Thomas.  

This hypothesis explains that during a rainy or wet season, etching or subsurface weathering (fracturing and chemical weathering) will occur at different rates from one area to another, forming regoliths. The difference in rates is due to the varying resistance of the rocks. For instance, highly fractured compartments (less resistant) will decompose more deeply than resistant ones (less fractured).

During the dry season, erosion and denudation will carry regolith, exposing unweathered landforms or rock compartments like Inselbergs, tors, and bornhardt. The reason for some remaining rock compartments includes wider joint spaces, quartz or potassium feldspar enrichment, and primary masses that are poorly fractured or petrological differences.

However, for this to happen, the denudation rate must exceed the subsurface decomposition rate. Otherwise, you will not see any of these landforms. Instead, the basal surface will keep dropping.

This two-stage hypothesis is the most accepted, with quarries and deep excavation along Africa’s equator providing evidence of hill-like (about 50 meters) features within mass decomposed bedrock. However, most weathering mantles are less tall than inselbergs, especially those over 200-300 meters, unless many stages or episodes of weathering or erosion cause it.  

2. Scarp recession and pediplanation

It states that inselbergs form from protracted erosion results in 1) recession of a slope next to a flat or gently sloping area, i.e., scarp recession, or 2) coalescence of several pediments, i.e., pediplanation. This theory is associated with Lester C. King’s work on Canon of Landscape Evolution.

The action of rivers, sea waves, wind, and weathering along valley sides causes scarp recession and pediplanation, resulting in reduced hill or inselberg as a final surviving remnant following scarp retreat over large areas. Thus, these landforms result from circumdenudation following a prolonged scalp retreat.

3. Planation

Some massive inselbergs in arid and semi-arid areas with flanks that have jointed compartments suggest that their exposure and growth in their overall height was not due to deep weathering. However, it may be due to long-term, successive differential lowering or planation of jointed bedrock by water, ice, or wind, creating a plain with a few resistant landforms or hills.

This hypothesis may account for inselbergs in the Namib desert or elsewhere. This same method may expose minor granite intrusion in less resistant rocks.

Inselberg growth and destruction

Inselbergs can grow or increase in height or undergo destruction. Only durable, originally large, or recently uplifted massifs remain today.

Once isolated, inselbergs are long-lived and quite durable because their steep surfaces quickly shed water, keeping them dry. This reduces chemical weathering. Also, their poorly jointed bedrock makes them resistant to physical weathering. Thus, they can survive for millions of years.

However, insolation and exfoliation (depending on jointing patterns) may reduce height as debris beaks off and falls or slides down the slope.

On the other hand, they can grow by height as water runoff from their slopes will go to the foot slopes or piedmont, adding more moisture and increasing weathering. Episodic erosion of formed regolith will make them taller if the rate exceeds summit lowering. For instance, Australia’s inselbergs have increased in height over time.

Climate and occurrence

There are two notions of occurrence: one can occur everywhere and is independent of climate, and the other is climate-dependent.

The notion that they are climate dependent brought the idea that they occur in tropical areas whose humid conditions favor deep weathering. Those in North America, Europe, or areas not in the tropics date back to the early Cenozoic Era when these regions were warmer and wetter. However, a tropical zone doesn’t necessitate their formation.

Inselbergs are widespread in Africa, especially with crystalline rocks, including in the Nigerian Savanna, East African plains (Tanzania, Kenya, and Uganda), Zimbabwe, the Namib desert, and Angola. Some occur in the southern Sahara Desert margin in Sudan, Libya, and Niger.

Besides Africa, they occur in central and western Australia, the Indian Peninsula, as well as in South America, and North America. Also, places like the Czech Republic, Poland, Scandinavia, hungry, and Germany in Europe, with some reported even in Antarctica, have these landforms.

References

• Migoń, P. (2013). Weathering Mantles and Long-Term Landform Evolution. In Shroder, J. (Ed). Treatise on geomorphology (Vol. 4 pp. 137-139 ). (2013). Elsevier.
• Ollier, C. D. (1978). Inselbergs of the Namib Desert: process and history. Zeitschrift für Geomorphologie, Supplementband 31, 161–176.
• Migoń, P. (2004). Inselberg. In Goudie, A. (Ed.) Encyclopedia of geomorphology (Vols. 1-2, pp. 564-567.). Routledge.
• Huggett, R. J. (2011). Fundamentals of geomorphology (1st ed.). Routledge.
• Migoń, P. (2006). Granite landscapes of the world (geomorphological landscapes of the world) (1st ed.). Oxford University Press.
• King, L. C. (1953). Canons of Landscape Evolution. Geological Society of America Bulletin, 64(7), 721. https://doi.org/10.1130/0016-7606(1953)64[721:cole]2.0.co;2
• Thomas, M. F. (1965). Some aspects of the geomorphology of domes and tors in Nigeria. Zeitschrift für Geomorphologie  N.F., 9: 63–81.
• Twidale, C.R. (1995). Bornhardts, Boulders and Inselbergs. Cadernos do Laboratorio Xeolóxico de Laxe (vol. 20: 347-380).
• Ollier, C.D. (1960). The Inselbergs of Uganda. Zeitschrift für Geomorphologie, 4 (1): 43–52.
• Falconer, J.D. (1911) The Geology and Geography of Northern Nigeria (p. 246). Macmillan, London.
• Inselberg. (2023, May 23). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Inselberg&oldid=1156593662