Australia: The Land Where Time Began

A biography of the Australian continent 

Inselbergs see Flared Slopes See Passive Fractures weathering  see Sheet Fractures

An inselberg is a steep-sided rock that rises abruptly from a plain.

There are 3 types of inselberg, domical or bornhardts, knolls or block-strewn nubbins and small castle koppies. The domical forms are found over a wide area, the Eyre Peninsula and the Everard Range in the northwest of South Australia, the New England Plateau (Bald Rock), New South Wales, Wilson Promontory in Victoria and the Porongurup Range in Western Australia.

There are many nubbins in northern Australia at places such as the Pilbara, the Mt Bundey area near Darwin, in the Northern Territory, and in northwestern Queensland, near Naraku and Dajarra, and Cathedral Rocks on the New England Plateau.

Bornhardts are always found in massive bedrock where there are very few open joints, most being in granite or gneiss, though they also occur in silicic rocks of volcanic origin such as dacite, as well as in sandstone and conglomerate, and they may have their congeners in limestone. In most bornhardts the rock type is the same as is present beneath the adjacent plains. Unusually orthogonal systems of fractures define them, with convex-upward sheet fractures being well developed. They meet the adjacent plains in a sharp slope break, called a piedmont angle, their flanks being steep-sided. Bornhardts are most easily and dramatically found in arid and semiarid areas, over a wide range of climatic environments, though they are not restricted to those areas. They are found in areas where there are planate remnant surfaces at more than 1 topographic level, multicyclic landscapes, suggesting that the landscape formed over more than 1 phase or cycle.

There are a number of explanations that have been proposed for bornhardts, some not being applied generally, rather to particular structures. The Pic Parana in the southeastern part of Brazil, together with some others reported from elsewhere in the world, that appear to be blocks that have been thrust up along faults, though most bornhardts are obviously not fault scarp bordered. Some are small intrusions that have been exposed by preferential erosion of the weaker host rock, though most bornhardts are not of this type, having been shaped within large granite emplacements or batholiths. According to the authors, it is not possible that such an explanation could apply to the many instances where inselbergs have developed in sedimentary rocks.

There are 2 general explanations that have been widely accepted. Inselbergs have been interpreted by many as the last surviving remnants of scarp retreat over long distances. In caprock situations, scarp retreat is a reality, in areas with a primary resistant capping such as quartzite, or there is a duricrust, though it is more difficult to accept this explanation in granite terrains. It has been suggested that this explanation could possibly apply because of the different behaviour of granite in environments that are wet or dry, dry granite being stable, though it decays rapidly when it is in contact with water. The evidence from the field does not support this, being much more consistent with the second major general theory. According to this second general theory, as with many boulders, many bornhardts form in 2 stages, being formed beneath the land surface then being exposed by removal of the regolith.

The granite massive is subdivided in compartments with contrasting fracture density. Water in contact with the well-fractured compartment leads to weathering resulting in the formation of a regolith that is easily eroded. The massive compartment is much less susceptible to weathering, and so erodes much more slowly. The massive rocks remain upstanding, forming projections at the weathering front. At this point water is shed much more rapidly, slowing weathering, as they remain dry for most of the time. The authors suggesting this can be seen as a positive feedback mechanism. The massive rock rising above the land surface becomes rounded, either by differential weathering (cf. corestones) or because sheet structure develops. Evidence supporting this 2-stage development of bornhardts has been found in several places such as southern and West Africa and Australia, indicating that bornhardts are etch forms. Excavations have found incipient bornhardts, fresh granite masses that are convex-upward, having been prepared by differential weathering beneath he surface of the land. In some places there are domes that appear to have only recently been exposed. The fracture spacing in the hills contrasts with that of the bedrock of the surrounding plains. It is believed the fracture densities observed at the present surface are typical of those at depth as well as in the compartments that were eroded to form the plains. As a plantation surface of low relief suggests there was sufficient time for subsurface weathering to occur, it makes understandable the association with multicyclic landscapes. As bornhardts, being etch forms, form at the base of the regolith, whatever the atmospheric climate, the lack of zonality is explained. The authors say there is much evidence that favours a 2-stage development.

The authors suggest bornhardts are the basic form leading to the development of the other 2 types. Different patterns of subsurface weathering has been suggested to explain the development of nubbins and castle koppies. Especially in warm, humid climates, such as occurs in northern Australia, nubbins result from the disintegration of the outer 1 or 2 shells of sheet structure to form blocks and boulders. Disintegrated sheets have been observed in some quarries, as well as on many nubbins, suggesting the weathering probably occurs beneath the land surface, the compact bornhardt being seen in gaps between the heaps of block or boulders. The authors suggest koppies probably result from the subsurface weathering of domes where only the dome crest is exposed at the surface as a platform or low dome. A steep-sided residual is formed by the rotting of the dome margins.

 

Uluru-Ayre's Rock

The Olgas-Kata Tjuta

Sources & Further reading

1. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd
2. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994

Ranges