Australia: The Land Where Time Began

A biography of the Australian continent 

Chemical Weathering

Unlike physical weathering, that breaks rocks down into small fragments or particles, in the chemical reactions of chemical weathering the bonds between atoms constituting the minerals of the rock are broken. There are a number of chemical reactions involved in chemical weathering. The minerals comprising the various rock types are susceptible in varying degrees to chemical weathering, some being more resistant than others, but none being completely immune to chemical attack.

Weakly acidified water is a common agent of chemical weathering, dissolving many minerals. Some minerals, such as rock salt, are directly soluble in water. Other, much more resistant minerals are significantly less soluble than salt, such as quartz (SiO2), but even it is slightly soluble in water, solubility in water is an important step in chemical weathering mechanisms of many types. Apart from solubility in water, there are a number of other mechanisms that take place, such as hydration (the taking up of water), hydrolysis, in which a substance combines with OH, hydroxyl radicals.  Example of such reactions are limonite and goethite, that are formed by the combination of a substance, in this case iron, with water and oxygen, forming a variety of hydrated iron oxides. These substances are the cause of yellowish and brownish colours in many rocks that have been weathered. Many rock-forming minerals are included in the group of calcium silicates, and these react with carbonic acid.

The dissolved products of chemical weathering are transported away from their original situation in both groundwater and rivers in the form of carbonates, sulphates or chlorides. Minerals are completely or partially altered when they react with water, carbon dioxide, iron or oxygen. In rocks involved in these processes their volume can change or their structure can be weakened by weathering. Whatever the result, the reactions involved can be complex. Feldspar orthoclase, commonly found in igneous rocks, is susceptible to hydrolysis and carbonation. It produces kaolinite (a clay), alkali and bicarbonate ions in solution, as well as hydrated silica. In the humid tropics chemical weathering tends to produce compounds rich in aluminium, iron oxide and water, as sand and clay tend to be unstable. In non-tropical areas, products of weathering are sand and clay. The nodular, iron- and aluminium-rich layer near the surface, may form a thick carapace that may be laterite or bauxite, the relative predominance of iron or aluminium in the crust being the determining factor. In many parts of Australia there are laterite deposits that are believed to have formed under an earlier period when the climate was different from the present. As with many weathering concentrates, laterite is hard once it dries, making it an erosion resistant caprock. Carapaces of this and other weathering products are called duricrusts.

Silica-rich horizons form in both laterite and as thick crusts, silcrete, which is very common in central Australia, covering large areas of the southern Northern Territory, southwestern Queensland, northwestern New South Wales and northern parts of South Australia. Because it is hard and chemically stable it forms resistant caprock on many plateaux, as well as in its folded form, ridges in central Australia. It often occurs in the form of elongated outcrops containing exotic rounded pebbles. The formation of the silcrete in river valleys is suggested by it being in the form of basins on some plateaux, the plateaux being part of an inverted landscape. Thin layers along joints are formed of silcrete.

Calcrete (kunkar and caliche), pedogenic lime, that occurs widely in arid and semi-arid regions, is an accumulation in soil of  calcium carbonate. It forms massive beds and lenses, though it can also be nodular. In stream beds near springs travertine (tufa) forms. Gypcrete is a massive layer of gypsum crystals that has precipitated out of either soil water or in the beds of lakes. On the surface of the land to the west and southwest of Lake Eyre, gypcrete forms a capping.

Also important components of both chemical and physical weathering are organisms such as bacteria and algae, that are widespread in Australia and have been over long time periods. Organic acids, including humic acid, effect solution and extract soluble nutrients. On the Eyre Peninsula of South Australia, lichens on granite use their hyphae to mechanically disintegrate the rocks, and also concentrate iron they extract from constituent minerals at, and immediately beneath the surface of the rock. The dissolution of iron oxides is assisted by the production of polyphenols during the decay of eucalypt leaf litter. According to Twidale and Campbell (2005), there is also a suggestion that plants such as ferns and grasses may contribute significantly to the development of silcrete, as they accumulate silica in their tissues.

Evidence indicating an important role for organisms in the promotion of weathering has been found in areas with a variety of climatic regimes. It has been noted that lichen-covered rocks in Iceland weathered 3-5 times faster than bare rock, though as with all surfaces in nature, bacteria would have been present on the apparently barren rocks. Lichen-covered lava in Hawaii was found to weather even faster, up to 100 times faster than lavas lacking any visible organic growth.

When discussing the various processes involved in weathering, the various types of weathering, physical, chemical and biological, are separated out for convenience, but Twidale and Campbell say in the field the weathering of rock is achieved by a wide range of processes combining, processes associated all 3 categories working together. The rate of weathering of the rock is usually accumulative, increasing as the various processes work on it. The porosity of fresh granite is about 0.1 %, but after it has been weathered for some time the porosity rises to 60+ %, by which point it can be considered to be permeable. At this point water containing bacteria and chemicals can penetrate the rock, increasing still further the rate of weathering, altering the rock still further.

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