Monadnock or inselberg is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain.
In southern and southern-central Africa, a similar formation of granite is known as a kopje, a Dutch word (“little head”) from which the Afrikaans word koppie was derived.
If the monadnock is dome-shaped and formed from granite-gneiss, it can also be called a bornhardt.
The name originally from Mount Monadnock in southwestern New Hampshire. It is thought to derive from the Abenaki language, from either menonadenak (“smooth mountain”) or menadena (“isolated mountain”). 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 2,000 feet (610 m) above its surrounding terrain and stands, at 3,165 feet (965 m), nearly 1,000 feet (300 m) higher than any mountain peak within 48 km.
The word inselberg is German for “island mountain”; the name was originally coined by geologist Wilhelm Bornhardt (1864–1946) in 1900 to describe the abundance of such features found in southern Africa. At this point, the term applied only to arid landscape features. However, the term inselberg 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 for humid-tropical and Mediterranean climates. As recently as 1972, the term has been defined 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 term monadnock and inselberg may not perfectly match.
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 because of the tightness of its jointing. If the monadnock is dome-shaped and formed from granite-gneiss, it can also be called a bornhardt.
The presence of a monadnock or 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 kopje appears.
The kopjes 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 kopjes 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 kopjes, to include like lion, hyrax, and an abundance of birds and reptiles.