Australia: The Land Where Time Began
The Australian continent is a stable landmass that has been mostly flat for many millions of years. Over long periods of time the soils have continued to weather, the small amount of volcanic activity doing little to add fresh nutrients to them. The high ranges of the distant past have been eroded to stumps, in a number of cases the substance of the soils have been through several cycles of soil-to-sediment-to-rock-to soil, being weathered, the land becoming flatter overall as time passed.
The soils of Australia are mainly composed of weathered rocks, forming on sorted sediments in sand sheets, dunefields and riverine plains. The soils have not been replenished by glaciers for 300 million years. The small amount of volcanism that has occurred on the continent has resulted in some patches of fertile soils surrounded by the usual impoverished soils.
As with just about everything else in Australia, the classification of soils is different from the rest of the world. A group of soil scientists from Macquarie University in Sydney has produced a book in which a soil classification scheme for Australia soils has been developed that allows for the difference from soils of other continents (Soils: A New Global View), the Australian soils not resulting from the glaciation of the Pleistocene, as they did in the Northern Hemisphere.
The entire Australian continent is situated in the centre of a tectonic plate, far from the active regions on the margins of the plate. This is good from the point of view that there are very few earthquakes and no volcanic eruptions to worry the population, but has done nothing to improve the fertility of the soils. The bedrock is mostly granite, with some granite-derived sedimentary rocks such as sandstones, shales and low-grade metamorphics. The continent has been so stable for so long that it has become a continent of low relief. In the distant past it has has had mountain ranges that rivalled the Himalayas in size, but all that is left of the once mighty mountain ranges are worn down stubs.
Full development of epimorphic and near surfaces processes has resulted from the age and stability of the continent, leading mainly to the widespread texture-contrast soils. They are usually associated with valley margins, and middle and lower hill slopes. They are sometimes found in valley-fill deposits.
Siliceous and earthy sands cover vast areas of Australia, being the endpoint of granite pedogensis (soil formation) that has been accumulating since the time when Australia was part of Pangaea. These soils drain rapidly to the watertable, having little water-holding capacity. There is very little runoff, reduced even further by the dry air over central and northern Australia, and in summer, very high temperatures.
There are 3 kinds of clay soils, forming a class of epimorphic product, related to the type of bedrock.
These occur along the Great Divide. The degree of breakdown of the lava flows, there are 2 main types of clay soil that form on this bedrock. In one type, the result is mainly of kaolin, with red, yellow and brown oxides and hydroxides of iron and aluminium produced by strong weathering and leaching. The other type, smectite-rich clay, that is highly plastic and has coarse aggregates, that expands when wet and contracts when dry. These clays can form gilgais. This type results from less weathering.
Fine-grained Cretaceous sediments
These clays formed at the time of a marine incursion of the Great Artesian Basin. They are coarse-structured grey and brown clays rich in smectite. They are often associated with carbonates. Their characteristics are determined by the parent rock. It is these clays that lead to the nature of the Channel Country flood plains, anabranching rivers, and dust storms.
Deposition of fines
The fine sediment that forms these clays are deposited from suspended load in rainwash and floodwater. It is carried from areas that have developed texture-contrast soils along rivers to be deposited on floodplains. These clays are found widely across the Riverina, the riverine floodplain of the Darling and on the floodplains surrounding the Gulf of Carpentaria.
The unquestionability of agricultural practices and land use practices in Australia has been found to result from basing them on practices that work in the relatively new Pleistocene soils found in the Northern Hemisphere. These management problems have been made much worse by the erratic Australian climate that often goes from extreme drought to extreme flood in many parts of the continent, and even these extreme events are erratic and unpredictable.
The lack of understanding of the true carrying capacity of most of the Australian continent has led to the misguided belief, by many Australians, as well as the multitudes wishing to start a new life here, that a land the size of Australia has plenty of room for many more people. Then there is the problem of little and diminishing amounts of fresh water. Flows from the Great Artesian Basin have decreased because of many years of overuse, when more was being extracted than was being added at the source. The problem of diminishing water supplies could potentially be exacerbated by the increasing danger of contamination of underground water supplies by industrial processes, such as those associated with coal seam gas extraction. In many parts of the country agricultural land is being taken over to accommodate the expanding cities and their industrial activities. The comparatively small area of productive agricultural land available in Australia is being continually reduced to accommodate the continually increasing population.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|