Inselbergs, or isolated rock hills or mountains, are formed by erosion and long-term weathering. A type of disintegration landform, inselbergs are steep sided and rise abruptly from a gently sloping terrain or level surrounding plain. Their formation may be initiated by a volcanic process that causes a body of relatively erosion-resistant hard rock inside a body of softer rock to rise, which eventually erodes to expose the inner hard-rock portion.
The interior portion left after the softer exterior of an inselberg has been removed by erosion often owes its strength to the tightness of its jointing. The formation may, however, eventually be destroyed or transformed by the marginal collapse of exfoliation sheets and joint blocks over time.
The term inselberg, which is German for "island mountain," was coined in 1900 by geologist Wilhelm Bornhardt. The term was originally used to describe the landscape features found by the geologist in southern Africa but now encompasses a wide range of formations such as rock domes, tors and buttes. The term monadnock, of Native American origin, is also used to describe a lone mountain or hill that rises above the surrounding area. The descriptively-named Mount Monadnock in New Hampshire, for example, stands almost 1,000 feet taller than any other landscape formation within 30 miles.