granite landforms

granite landforms Granite is an ‘acid’ (silica-rich) plutonic rock, consisting of minerals that react with moisture to a greater or lesser degree. Contact with water is thus a critical factor in the weathering and erosion of the rock, and hence in the development of granite landforms. Granite consists of interlocking crystals, and is of low porosity and permeability. But it is characteristically well-jointed, with orthogonal systems and sheeting fractures well and widely developed. Fracture patterns are critical, because the interaction of granite with meteoric and shallow groundwaters, initially circulating along fracture planes, determines morphology. Alteration of the bedrock produces a regolith (soil), the base of which is known as the weathering front. Many granite forms, including representatives of such major features as plains and residual massifs (inselbergs, Fig. 1) and hills, as well as minor forms such as boulders, basins, and gutters, are initiated at the weathering front, or, in the case of corestone boulders, within the weathered mantle, as a result of the exploitation of weaknesses in the country rock by groundwater. As the regolith is stripped, irregularities developed at the weathering front are exposed as etch forms. Other minor weathering features, however, are wholly surficial in origin, and a few, both major and minor, reflect the activity of tectonic forces.

Areally, plains are the most characteristic granite landform. Long-continued weathering has produced weathering fronts of remarkable smoothness and etch plains of essentially no relief, as, for example, in the Bushmanland Surface of Namaqualand and Namibia, and the Meekatharra plain, in the interior of Western Australia. Elsewhere, weathered granite has been eroded to form plains of rolling relief, as in the northern Transvaal, the southern Yilgarn of Western Australia, and Eyre Peninsula in southern South Australia. Bordering uplands, the granite has been eroded to form remark-ably smooth, gently inclined surfaces carrying a mantle of weathered detritus and known as mantled pediments; these, however, with the stripping of the cover material, are converted to rock pediments or platforms.

Boulders are perhaps the most common positive relief form associated with granitic outcrops. Corestones are formed beneath the surface, within the regolith. Regoliths with corestones attain considerable thicknesses, showing that in some areas weathering has outpaced erosion, but elsewhere erosion has proceeded more rapidly than the alteration of the rock, and the matrix has been removed, leaving the corestones at the surface as boulders.

Massive compartments are delineated by fractures of the regional orthogonal system, though rhomboidal patterns are also common. Such massive compartments offer few avenues for water penetration but, like corestones, they evidently become rounded, either as a result of weathering being guided by upward-curving sheeting fractures, or as a result of the preferential weathering of the edges and corners of the resistant masses to form domical hills or bornhardts. The many geometrical variations of bornhardts (e.g. ‘whalebacks’, ‘elephant rocks’, ‘sugarloafs’) reflect fracture spacing and the distribution and intensity of weathering. Block- and boulder-strewn nubbins (or knolls) and castle koppies (and the tors of south-western England) apparently evolve through the further weathering, in the subsurface, of incipient bornhardts. Where vertical fractures are prominent, needle-like forms and towers are developed, as in the Organ Mountains of southern New Mexico, and the eastern Sierra Nevada of California, around Mt. Whitney.

Some workers maintain that inselberg forms are the last remnants of denudation of the surrounding area by long-continued scarp retreat, but there is much contrary evidence. It is clear that some bornhardts are exposed intrusive stocks, and that others are upfaulted blocks. In addition, the presence of domical forms, or nascent bornhardts, in artificial excavations shows beyond doubt that some bornhardts, at any rate, are initiated by differential weathering at the base of the regolith. Stepped inselbergs and the manifest subsurface origin of many of the minor forms developed on bornhardts, the occurrence of bornhardts in valley floors and valley-side slopes as well as on high plains, and the fact that they are not confined to any one climatic zone all sustain the suggestion that most bornhardts are two-stage forms. They are comparable to boulders, except that whereas the latter are commonly detached, bornhardts retain a physical connection with the solid bedrock below. Pillars a few metres high but clearly still in physical continuity with the bedrock provide miniature examples of this model.

In many shield areas inselbergs stand in isolation above the level of extensive, almost featureless, plains. Some, like the Groot Spitzkoppe of central Namibia, are huge, but many are minute in comparison with the extent of the surrounding plains. These small inselbergs reflect long-continued subsurface weathering beneath plains of low relief and the reduction of even the most resistant compartments to small dimensions. Massifs reflect their large initial size, low fracture density, and reinforcement effects, or a combination of these factors.

Although some minor features, such as rock basins, gutters, and grooves (or Rillen), undoubtedly evolve on exposed surfaces, other examples of these forms, as well as flared slopes and scarp-foot depressions are, like the residual hills, boulders, or platforms on which they are developed, initiated at the weathering front. Some become diversified after exposure, so that basins, for example, give rise to pits, pans, armchairs, and cylinders according to local slope and structure. Others, such as rock doughnuts, rock levees, and fonts, evolve as a result of partial exposure of the bedrock. Blocks and boulders protect the rock beneath, which results in their standing on low tables or plinths. A few minor granite landforms are of tectonic origin, for crustal stress has given rise to an assemblage of minor neotectonic features, including A-tents or pop-ups, vertical and horizontal wedges, and displaced slabs and blocks. In addition, because crystals in strain are more susceptible to water attack than those in equilibrium, linear zones of strain have been preferentially weathered and eroded to produce clefts or slots.

Thus, in considerable measure, granite landforms reflect the susceptibility to attack by moisture of the minerals that constitute granite, and the access of water to the rock mass. Fractures are significant in this respect, but time, the duration of attack, is also critical, and rock stresses, remnant and contemporary, are increasingly recognized as finding expression in the landforms of granitic terrains.

C. R. Twidale

Bibliography

Godard, A. (1979) Pays et paysages du granite. Presses Universitaires de France, Paris.
Twidale, C. R. (1982) Granite landforms. Elsevier, Amsterdam.
Vidal Romani, J. R. and and Twidale, C. R. (1998) Formas y Paisajes Graníticos. University of Coruña Press, Coruña.
Wilhelmy, H. (1958) Klimamorphologie der Massengesteine. Westermanns, Brunswick.