Australia: The Land Where Time Began

A biography of the Australian continent 

Physical Weathering 

The processes usually associated with physical weathering are thermal expansion, pressure release, crystal growth and the activities of the biota.

According to the authors, thermal expansion has been considered to be a cause of disintegration for many years. The unequal expansion and contraction resulting from diurnal and annul temperature changes have long been said to lead to granular disintegration as well as sloughing off of flakes, shells and massive sheets, according to the authors. Rocks consisting of different coloured crystals absorb heat at different rates, resulting in different rates of expansion. This was suggested to be the cause of the breaking of the rock into fragments by granular disintegration. Rock surfaces are expected to heat to higher temperatures than rock only a few centimetres below the rock surface, as rock is a poor heat conductor, when the rock is heated by the Sun. It was said that the outer mass expanded, detaching from the main mass of the rock, in the form of a thin shell or sheet. Exfoliation was the term originally used for this process, but the authors suggest it best to avoid this term as it has been used for so many different forms of laminae, flakes, shells, and sheets of various thicknesses.

According to the Twidale & Campbell, it is known that rocks can flake and split when exposed to the intense heat of bushfires or nuclear explosions. This observed effect has been used in Egypt and India for quarrying for many years, fires being lit on the cleared rock surface which is then broken up with hammers when a surface sheet arches. The authors point out that such heating is too local and ephemeral to be considered to be similar to heating during normal fluctuations in nature, even in very hot deserts. They claim there is clear observational, as well as strong experimental evidence, that chemical weathering predominates, even in the most extreme desert environments found in nature. An example given by the authors is from the desert near Cairo, where temperatures are very high and rainfall is very low. There are granite columns from the Pharaonic times that were shaped about 3600 years previously, that have since fallen on their sides, being partly shaded and partly buried in the sand of the desert. Based on the belief that it is the expansion and contraction occurring when exposed to the Sun in such hot, arid environments that are expected to be weathered. It has been found that the shaded surfaces, and especially the buried surfaces, are the sites of weathering. There is some moisture in the surface sand, even in this extremely arid area, and in this case there has been enough to lead to decay and disintegration. The authors say that disintegration and lamination can be found at much greater depths than the diurnal or seasonal fluctuations in temperature can be detected. They also state that experimental evidence suggests that repeated heating and cooling either leads no detectable weathering, or if it does, it is much slower than chemical weathering associated with moisture.

The long-accepted hypothesis connecting weathering with insulation appears to be so logical it has been difficult to get other proposals accepted, especially as there are features that have been difficult to explain by other mechanisms. According to the authors, a couple of such examples are blocks that have been fractured intricately and boulders that have been cleanly split. These forms have been attributed to heating by the sun. Such split boulders have been found in both the humid tropics and in colder climates. The authors suggest they may be split by gravity.

If water gains access to closely packed minerals that interlock, the bond may be weakened. It has been suggested that the small, possible physical contribution to this process consists of a water molecule or an OH radical being inserted into the crystal lattice, thereby opening the way for chemical weathering processes to begin. It has been suggested that in the long term this process may possibly be significant, because chemical changes that are very slight in some rocks, as in spheroidally weathered rocks, appearing to be unlikely to involve volume change in their disintegration. Some believe a process called slaking, the alternate wetting and drying of rocks, may lead to fatigue and eventual disintegration.

Granites, as well as many other types of rocks, are subdivided into concentric widely separated sheets or shells, 1-10 m apart, by arcuate fractures transgressing other sedimentary, metamorphic and igneous textures and structures. These fractures have been called sheet fractures. They are usually attributed to offloading, or possibly to other processes associated with pressure release following erosion of superincumbent strata. The authors claim this hypothesis, though appearing reasonable, doesn't explain evidence from the field, making it theoretically suspect. According to the authors, a recent suggestion is that strong horizontal compressive crustal stresses may be responsible for sheet fractures.

Pressures are generated by growing crystals, and it has been suggested that this pressure is sufficient to shatter some rocks, the best-known example being ice. Temperatures frequently oscillate about the freezing point of water at high elevations and in sub-Arctic regions. It is believed by most that water in cracks and crevices in the rocks alternately freezes and thaws, that is, oscillating between the solid and liquid phase. The pressures involved are sufficient to disrupt many rocks, especially fissile rocks, though the pressures are much lower than they would be is the water/ice were fully enclosed instead of partially closed. Riving, frost splitting, rapidly disintegrates such rocks. A chaotic mass of frost-shattered plates, called Felsenmeer (sea of stones), covers many shale outcross in sub-Arctic regions. This process can even affect massive crystalline rocks such as granite.

Some have doubts about the mechanism of freeze-thaw of water shattering, in part because of the problem of attaining and maintaining the confinement necessary for the operation of frost-shattering, and partly because ice doesn't continue to expand as the temperature continues to decrease. Ice contracts at and below -22o C, after which point its structure changes. There is a suggestion that the frost-shattering may in fact result from the accumulation of multiple layers of water on the rock surfaces, which causes the layers to separate leading to fragmentation. According to this suggestion the system is self-sealing because the layers trapped in the crevices freeze first, being locked in by subsequent layers, leading to a situation where the problem of confinement may be at least partially overcome, allowing pressures to be attained that would not be possible if the water in the crevices was not confined. The authors suggest this simple physical mechanism appears to be adequate to explain the shattering that is observed in the field.

Another mechanism of rock disintegration is crystallisation of salt under certain conditions, as in arid areas, and coastal areas where the climate is arid, either seasonally or permanently. Salt crystallisation is known to easily cause shattering, as in clays that are poorly consolidated, partly weathered rocks that have been weakened, and in soft materials. Even solid rock such as granite can be disrupted by the formation of salt crystals. In Western Australia there are salt lakes around which granite rocks have been weathered and locally planated. The mechanism of disintegration by salt crystal growth has been called exsudation by J. T. Juttson. The authors suggest that salt crystallising appears to contribute to the shattering of rocks in some coastal regions. The mechanism of halite crystallising has been attributed to the sculpting of rocks that are exposed on the bed and shore of Lake Gairdner in the Gawler Ranges, South Australia. On cobbles carried to the lake bed by streams, the upper parts that are exposed have been weathered away completely, indicating that the mechanism of salt weathering works rapidly. According to the authors, salt weathering is also responsible for landforms such as alveolar (honeycomb) weathering and tafoni weathering, the most commonly involved form being salt (NaCl) and gypsum (CaSO4.2H2O). Evidence has been found that often the salt involved in rock disintegration originates in sea spray, even far inland, having been carried on the wind, though salt derived from rock breakdown is also of significance, one process facilitating the other.

See Source 1 for more detailed information on Australian Landforms

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

Links

  1. Soil stratigraphy in a deeply weathered shield landscape in south-western Australia
  2. The nature and possible origins of soluble salts in deeply weathered landscapes of eastern Australia
  3. A soil catena on weathered basalt in Queensland
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 19/04/2011

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading