Australia: The Land Where Time Began
Australian Landscapes - Deep Weathering
Water is the most important weathering and erosion agent on Earth. Erosion, weathering as well as transport and deposition, each of these parts involving water, is the mechanism by which the landforms are shaped. Weathering operates at or near the surface, altering or breaking down rocks in situ, at the range of ambient temperatures in these situations. Cementation and hardening of a horizon often occurs within the regolith, the weathered zone, though the term is most commonly used to refer to the breakdown and weakening of the rock, leading to its disintegration or alteration.
Erosion is the physical breaking down of rock surfaces by water, both running and by wave power, wind and glaciers and removal of the debris. The eroded debris is eventually deposited first on slopes, then in river valleys and lakes, and much of it reaches the sea before it is deposited on the continental shelf, the very fine silt being carried further, past the edge of the continental shelf.
The morphogenetic or geomorphic system is the term used to describe the combined processes of weathering, erosion and deposition, though it is often simply called denudation. In any given region, the chronology of denudation is the complex of weathering, erosional and depositional events that have occurred over time, and the role of each in producing the land surface of the present. Rock is weathered, then eroded and transported, water being the main agent involved in all these processes. The erosive abilities of running water is enhanced by the inclusion of debris being transported in the water, as well as chemicals and biota.
Igneous rocks, that crystallise out below the surface of the earth, and sedimentary rocks that are composed of rocks that have been deeply buried and have had the water they once contained squeezed out of them by the pressure generated by the weight of the thick deposits above them, and or heating, are both not in equilibrium with the environment they experience when they are exposed to the atmosphere and water. Weathering is the process involving physical and chemical changes that occur when the rock is exposed to air and/or water. As a result of weathering new minerals form and sometimes the volume changes, as the material of the rocks is brought closer to being in equilibrium with the conditions existing at or near the surface.
As a result of some types of weathering processes some minerals can be lost and others can be added. Some minerals are translocated to lower levels of the same site were they are deposited , and in open systems some can be removed from the site and even from the system. Within the profile there can be considerable movement of material. The other type of weathering, physical weathering, involves rock disintegration that does not involve mineralogical changes. One or more of the minerals forming the rocks is altered in the process of chemical weathering. Though these definitions are not precise, they indicate the possible types of processes that can be involved.
Factors involved in weathering
The climatic regime of any given area strongly influences the weathering processes occurring in that area. In subarctic lands rocks, especially closely bedded, foliated, fissile ones, are most susceptible to shattering by ice. Weathering in the humid tropics tends to be of the chemical type, under conditions that favour chemical weathering, consistently high temperature, constantly wet, and because chemical reactions take place faster when in warm, wet conditions, the weathering reactions occur 3-4 times faster in the tropics than in temperate areas. Humic acids levels are also high in the permanently damp soils of the tropics, so as the water percolates down though the soil profile the humic acid is carried along with the water. Many of the ancient regoliths of Australia were subject to chemical weathering for many millions of years prior to the aridification the continent, when it was mostly covered by humid rainforests that continued on from its time as part of Gondwana. These regoliths now mostly underlie arid areas, the deep weathering having taken place in earlier times when large volumes of humic acid and water made its way down to the water table. In silicate weathering, silicon cations from the crystal lattice are exchanged with hydrogen ions from the water, weathering occurs quite rapidly, by geological standards, under tropical conditions. As a result there are many regoliths that have been intensely altered in earlier times under the humid, warm rainforests.
The minerals the rocks are composed of also influences the processes taking place during chemical weathering. Olivine is the least stable of the common minerals found in rocks, others being calcium-plagioclase feldspar, augite, hornblende, soda-plagioclase, feldspar, biotite, orthoclase (potash feldspar), muscovite and quartz. According to the authors, some micas require only a decade or less to begin showing signs of decay once they come in contact with meteoric waters (water derived from precipitation). Feldspars need to be in contact with meteoric water for centuries before they show signs of decay, and quartz takes much longer, though it eventually succumbs, presumably by dissolution (Twidale & Campbell, 2005).
Basic rocks, such as basalt, usually weather much faster than acidic rocks, such as granite. Basalt contain high levels of olivine, augite and plagioclase feldspars, that are rich in calcium. Granite is rich in quartz and orthoclase. When a rock surface is exposed to moist soil or regolith it becomes pitted as a result of differential weathering. When granite is weathered, quartz and large feldspar crystals are left in micro-relief by the alteration of mica and small feldspars.
The structure and texture of the rock can override the determination of the susceptibility of the rock to weathering by the mineral composition. The fracture density, the number of joints per unit area, determines the number of places, and surface area, at which a rock can be attacked by weathering agents. Weathering may be slow for a massive outcrop of basic rock (normally highly susceptible to weathering) if it has few if any joints, while a similarly massive outcrop of acid rock, that is normally highly resistant to weathering, that has many joints, may not be as resistant to weathering as the basic rock with no joints. According to the authors, a number of different rock types give rise to similar landforms because of their fracture characteristics, though they have contrasting compositions and origins - granite, limestone, sandstone, conglomerate, silicic volcanics. In granite, conglomerate and limestone, rounded boulders are formed in which the spacing of fractures determines their shape and size.
The rate of weathering, especially when chemical alteration is involved, is also affected by the size of crystals or constituent fragments, the total surface area of the constituent crystals being greater in fine-grained rocks. These surfaces are especially susceptible to chemical attack, though not always to mechanical attack, because they are chemically reactive due to their unsatisfied chemical bonds.
If the erosion rate is negligible, leading to the accumulation of weathered debris because it is not removed quickly enough, the land may be protected to some degree from further weathering by a protective veneer formed by the weathered mantle. In areas with rapid erosion, the weathering debris is evacuated soon enough to keep fresh rock surfaces available to the weathering agents. It is said in such cases there is a renewal of weathering. It has also been argued that continuous weathering is ensured by the presence of water contained in a regolith. The concentrations of CO2 in the vapour space between soil particles is increased by the respiration of plant roots, bacteria, fungi and soil invertebrates leading to the formation of carbonic acid when it dissolves in water. This carbonic acid then becomes the main reagent acting on calcium silicates, such as many of the plagioclase feldspars.
Water can also gain entry to rock joints where tree roots have forced apart the sides of joints. Another biological process that allows the increased access of water is by the lichen hyphae, as they work their way along cracks and fissures and along crystal boundaries. Bacteria can work their way into crystals by use of their exudates that can dissolve various substances, such as those that live on kaolinite, all such mechanisms increasing the access of water to facilitate the weathering of rocks. The authors suggest that as bacteria have existed for millions of years they would have been an important agent of weathering for a very long time. Even much larger organisms such as worms and rabbits increase the access of water to rock surfaces by their burrowing activities. In the tropics the burrowing activities of worms are added to by those of termites.
A regolith, between a few metres to 10s of metres thick, results from weathering. In places, such as in Brazil, west Africa and Australia, thicknesses of up to 200 m have been found. There is often a sharp boundary between the bottom of the regolith and the parent rock. The weathering front is the narrow zone, the plane, that separates the regolith from the basement of parent rock. Zones or horizons with differing physical and chemical characteristics usually subdivide the regolith. The course the weathering has taken, the sequence of events occurring over the period of time the disintegration and alteration has been occurring, is indicated by this profile. The sequence of events in the profile follow the progress of the weathering front from the top of the rock, where weathering began, to the current position of the weathering front, at the deepest point it has reached. The oldest events of the sequence are therefore at the uppermost surface of the rock, where the meteoric water has been in contact with the rock surface for longest time, with progressively younger events occurring with depth, the most recent occurring at the weathering front, the deepest point reached by the front at the time of sampling, where the earliest stage or rock breakdown is taking place, weathering extending down intro the mass of the rock.
It has been found that the processes involved in weathering varies widely depending on a number of characteristics of a particular site, such as rock type, slope, climate, among others, but selective weathering of minerals that are more vulnerable has occurred first, producing a rough, pitted surface, has occurred on a variety of rock types. According to Twidale & Campbell, the weathering front is often characterised by laminated or flaked rock, or a thin sheet of spalls of rock, wherever the weathering front is, such as the regolith base or in contact with fresh rock (corestones) within the regolith. The breakdown of the laminae results in the presence of particles or fragments above the weathering zone, the rocks above this level becoming more altered towards the surface. This has led to the suggested sequence of rock weathering - pitting, lamination, disintegration, alteration. As weathering has occurred near the top of the regolith some decay products, especially oxides of iron, silica and manganese, have migrated to the regolith base in the infiltrating water, where they precipitate at or near the weathering front, near the fresh rock that is still impermeable.
Significance of weathering
Preparation of rock for erosion and transport is the main result of weathering, most of the erosion that takes place by wind and water requiring some breakdown and weakening of solid rock before they can have much impact. Twidale & Campbell suggest that this may also apply to erosion by glaciers. The regolith, a mantle of altered and/or disintegrated rock, that varies in depth from place to place. Saprolith (saprolite) is a weathered mantle that is clay-rich. Grus is the term used for disintegrated granite and when some clay is included with the weathered granite the term used is growan. The surface layer of the regolith where some organic matter, and with the development of horizons and differentiation, is included is the soil. The mantle (not the layer of the rock beneath the crust) is usually mostly friable, though over wide areas there are zones where there are accumulations of cemented material or duricrust, with compositions of various types - ferruginous, siliceous, calcareous, gypseous.
There are places and some features where there is no duricrust. The bedrock can be seen to be essentially fresh on many hills composed of granite, though widely exposed and is still in a cohesive state, though in basins and clefts there are often some patches of detritus. The bedrock of Uluru is a green-grey colour, but it is thinly covered with a red clay with no friable material, apart from in some rock basins.
Regolith is usually easily eroded because of the usual friable nature, leading to the weathering front, that is underlain by relatively stable fresh rock, being exposed. Etch or etched surfaces, is the term used for these exposed weathering fronts, where the surface has been etched by chemical erosion. A number of well-known landforms have been initiated at the weathering front. Twidale & Campbell suggest evidence exists for the origin of polygonal cracking at the front, though it isn't believed to occur very often. Gutters result from runoff from exposed slopes extending along and scouring the weathering front. Preferential weathering at the front produces linear clefts. The exploitation of bedrock weaknesses produces saucer-shaped depressions, that develop into various basins (gnammas) following exposure, depending on the rock type and the incline where the depression was situated. Exposure of corestones leads to boulders that are etch forms, and stripping of planate weathering fronts produces extensive plains and platforms, and hills that were projecting into the regolith, that are all etched forms.
As well as being involved in the formation of soils, it is an agent for the concentration of minerals, such as aluminium in bauxite. Weathering produces a wide range of landforms, directly or indirectly. Weathering leads to the development of resistant layers in soil - resistant crusts, pans and horizons (duricrusts) - or surficial accumulations, laterite, bauxite, silcrete and calcrete. Theses resulting features form impenetrable surfaces, that are tough and resistant. They become resistant capping on the plateaux, mesas and butts when they are dissected as the landscape is inverted by erosion. They have also been used in dating by correlation and extrapolation. They are also useful as indicators of palaeoenvironments.
The main role of weathering in the environment is the breakdown of rocks to small enough particles that can be transported by wind and water. In all places and at all times, gravity is the moving force of transportation, the agents being wind and water. The means of transportation varies from place to place and from time to time according to the climate and weather, wind strength and availability of abrading particles such as sand, and amount of running water (Twidale & Campbell, 2005).
See Source 1 for more detailed information on Australian Landforms
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