Australia: The Land Where Time Began
The mid-latitude deserts of the world are a function of wind circulation, occupying atmospheric high pressure zones. Factors also contributing to the formation and maintenance of arid conditions, other effects of circulation of oceans and atmosphere, rain shadow effects, cold ocean currents, interior position on a continent.
The rain shadow effect, when mountain ranges intercept moisture-laden air masses, forcing them higher where their moisture condenses and falls as rain or snow. The downwind side of such ranges are usually much drier than the upwind side. An example is the Great Dividing Range of Australia. Any moisture in winds blowing across the ranges from the nearby ocean to the east falls as rain on the eastern slopes of the Divide, leaving little if any rain for the western side of the Divide, which is much drier.
Ocean currents affect the adjacent parts of a continent, warm water currents have increased amounts of moisture in the air above them as a result of evaporation from the ocean surface. Coastal deserts form where there are cold water currents, the air above these currents is drier so when wind blows from the ocean to the land there is less moisture to fall as rain. This is the situation in West Australia.
Land in the centre of large continents suffers from the effects of central location, a continental climate. Any rain bearing weather systems that do move from the west move across the continent almost to the east coast before they encounter significant high ground to initiate precipitation. The desert ranges of inland Australia are like oases in a desert, collecting some of the rain available and storing it in their rocky valleys, allowing them to act as refugia for desert wildlife when the usually dry conditions get even drier in periods of drought.
Deserts are found in areas with less than 250 mm/yr of rainfall. They have long periods of drought, that can last for a number of years, with little or no rain. They receive their infrequent rain, usually in a short period of time.
Deserts experience very high temperatures, the hottest places on Earth are deserts. This means that any rainfall or flood water they receive is rapidly evaporated if it doesn't soak into the soil rapidly. The temperatures in Marble Bar average above 37.8o C (100 F) for 154 days per year, but in the summer of 1923-24, Marble Bar in Western Australia set the Australian temperature record, 37.8o C for 160 days consecutively, from 31 October 1923 to 7 April 1924. As well as being a record for Australia, it has been claimed to be a world record. During this hot spell the temperature peaked at 47.5o C on 18 January 1924. The highest temperature ever recorded at Marble Bar was 49.2o C that has been measured on 2 days, 11 January 1905 and 3 January 1922.
This high temperature has actually been exceeded, also in Western Australia, at Mardie the temperature reached 50.5o C on 19 February 1998 during a heat wave, late for a heatwave. This is the highest temperature recorded in Western Australia, and 2nd highest in Australia, the highest being recorded at Oodnadatta in South Australia on the 2 January 1960 when it reached 50.7o C, the highest temperature ever recorded on standard instruments. On the days leading up to Mardi's record temperature several other places recorded temperatures above 49o C. In the summer of 1998 Nyang endured an average temperature for the summer of above 43o C.
Over much of north western WA the summer temperatures often go above 37.8o C (100 F). Because Marble Bar is far enough inland to miss out on any coastal cloud cover and so the only time the temperatures are lowered is when the northwest monsoon trough moves far enough south to bring heavy cloud cover, and sometimes rain, to the area around Marble Bar. In the summer of 1923-1924 the monsoon stayed further north and no cyclones occurred anywhere in Australia that year, a truly unusual year. During its record-breaking heat wave only 79 mm of rain fell on Marble Bar in the form of 2 brief storms, and only 12 mm of rain fell before the next wet season began in December of 1924. The tropical section of Western Australia experienced a severe drought in 1924, with no cloud cover to relieve the seemingly endless days of extreme heat.
A characteristic of deserts that many who haven't travelled in deserts find surprising, especially in winter, is that the night time temperature can drop to freezing, even after a very hot day. Even during the hottest part of the day the temperature in soil at depths greater than 1 m change very little.
All deserts have a climate of extremes, drought-flood, hot-freezing.
The unprotected land surfaces are very susceptible to erosion. After heavy rain, often a great distance from the desert flood plains, erosion of the often ancient channels can be significant. In deserts, wind is the most important agent in the changing of the landscape, the windblown sand acting like sand blasting to change the shape of exposed rock surfaces.
The deserts covering about half of the Australian continent are very diverse, ranging from the high, rolling red sand dunes to great expanses of polished stones of the gibber plains. Few desert areas are completely barren, mostly having a covering of at least sparse spinifex clumps, and often tussocky grasses, as well as mulga, mallee and small scrub. The soils of much of the desert areas have a surface of hard red clay pans, with white tracts of the dry river beds and salt lakes in places.
The deserts of the north are often hit by spectacular storms during the summer monsoon season, nearly every afternoon, beginning with an approaching wall of dust reaching to the clouds, and as it passes over the torrential rain and spectacular lightning shows begin.
The daily summer afternoon storm approaching about 4 PM, Mt. Isa, in the 1960s.
The ranges of central Australia are surrounded by the deserts, Victoria being the only mainland state to not have a desert area. The Australian deserts are divided into 2 parts, the Western Desert, made up of the Great Sandy Desert, the Gibson Desert, and Great Victoria Desert. The Arunta Desert is composed of the Simpson Desert and Sturt's Stony Desert. 2 other deserts not included in this division are the Tanami Desert "rock holes" to the Aboriginal People, and the Channel Country, a riverine desert.
The largest dunes occur in the Simpson Desert, trending roughly north-south, up to 300 km long and 40 m high, mostly parallel, with interdune spaces, swales, from 100 m to 1 km wide. Most of the other deserts have smaller dunes and sandy undulations, often trending east-west. The Great Sandy desert extends almost to the west coast, where its red dunes meet the white sands of the 80 Mile Beach. Gibber plains occur mostly in Sturt's Stony Desert, but also occur as patches throughout the arid country.
The Channel Country, to the north of Sturt's Stony Desert, is the most unusual of Australia's deserts. As dry as the Australian deserts are they were even drier during the glacial maximum of the Pleistocene ice age,
Deserts are not completely rainless, but rainfall is low. There are long periods when no rain falls, when it does fall the desert soils are affected more than soils in wetter areas. The increased effectiveness of rain as an agent of erosion is not because it is heavier or more intense than in other areas, it is because by the very nature of deserts they tend to have low vegetation cover. In areas where vegetation is more dense the canopy and grasses reduce the power of the rain drops on the soil by acting as an umbrella, and desert soils lack the often dense mass of roots and leaf litter to slow the runoff, giving the water more time to soak into the soil. Less and slower runoff leads to reduced erosion. So desert soils are subject to the splash effect and streams can quickly form because the water tends to run off the desiccated desert soils. A big part of the reason the soils are at first impermeable is that they often have a water resistant crust that some water must soak into before they become permeable. The result is that much of the water runs off. Because of the hot conditions of the desert soil a lot of water evaporates before it can soak in.
In deserts, floods are the rule rather than an exception, the rapidly running water scouring the water channels and carrying away large amounts of soil. The intermittent nature of the river flows in arid areas means there are large amounts of unconsolidated debris deposited along the length of the streams. In the next flood these rocks and pebbles increase the effectiveness of the flowing water as erosive agents. Between flood events not much change takes place, so that the floods become a major force in the changes made to arid lands.
It is believed chemical and physical weathering due to moisture is important, though just how important is uncertain because of the possibility of the effects of past weathering events, that occurred in wetter times, being preserved in the present landscape.
Most rivers that arise or flow through deserts eventually soaks into the dunes and disappears. An Australian river that is an exception is the Murray, like other similar rivers around the world, it arises outside the arid areas and flows through the arid area before if loses all its water. This type of river is called an allogenic, or exotic, river. The normal desert rivers flow erratically. They tend to rise rapidly after rain, and fall just as quickly when the rain stops, because these rivers experience flash floods when heavy rain falls upstream, often hundreds of kilometres away. Because the rain causing a flash flood is often over the horizon, occasionally people are caught crossing a dry river bed when the water arrives so rapidly that they are washed away before they can get out of the way. In some desert rivers the water continues to flow beneath the creek bed in the loose material, sand or gravel.
In desert uplands the streams flow in steep-sided rocky valleys. Sheet wash, streams and tributaries scouring the bare hillsides of loose material. A large amount of rock debris is carried to the river courses and from there to the flood plains. Some is deposited in the scarp-foot or piedmont zone to form alluvial fans or aprons., much being transported to valley floors or to the lower parts of enclosed basins. In the piedmont zone also, rivers from the uplands erode pediments.
The water and fine sediments carried beyond the piedmont end up in the centre of the local drainage basin. In these areas deep sediments are deposited, many ephemeral lakes are formed, eventually becoming claypans or salinas. Hundreds of such playas, large and small, occur in the central Australian arid zone. The large amount of salt found in the salinas (about 400-700 million tonnes for Lake Eyre) may have been derived from 3 sources. First, it may have entered the various lake basins at a time when the sea extended far inland. Second, it may have been dissolved out of the rocks: connate salts. Third, it may have blown in from the sea, or from coastal areas: so-called cyclic salt. In Australia most of the salt accumulated in the salinas and the regolith appears to be connate, though the contribution of materials carried on the wind is significant.
Aeolian erosion, or wind erosion and transport, is a major mechanism for the erosion by sand blasting, movement of sand and finer particles, and deposition in places like dunes. The most common erosion of deserts by the wind is by moving the top layers of the sand and dust, generally lowering the surface. The eroding of rocks by the sand-blasting effect of the wind blown sand occurs up to about 1 metre above the surface. Wind erosion is generally a minor mechanism of erosion, though it can produce spectacular results.
The largest landforms formed by wind erosion are deflation hollows scoured out by the wind. These are shallow depressions of various shapes and sizes. They usually form in weak, unconsolidated sediments. They can be eroded down to the level of the water table, but can be very large, up to several hundred metres across.
Zeugen, or mushroom rocks, are the result of strata, especially soft strata, being etched away close to ground level, where most sand is moved by the wind.
Wind becomes significant as an agent of transport and deposition in desiccated areas with little vegetative cover. Wind can carry sand grains up to about 2 mm in diameter. Calcareous dust has been spread over Eyre Peninsula, Yorke Peninsula and the Adelaide Plains and Mt Lofty Ranges and the western part of the Murray Basin. This dust originated from fragmented mollusc shells exposed either in coastal dunes or during periods of lower sea level. Siliceous dust deposited over large areas of New South Wales and north-western Victoria came from the Murray River Basin and the channels of the Murray River. Salts, such as gypsum, found in arid interior soils, and halite, are carried by the wind. The wind sorts weathered rock debris into 3 main sizes, dust, sand, and coarse sand and gravel.
Most of the sand storms of desert regions are actually dust storms. In these dust storms, dust can be carried in suspension thousands of metes above the ground. The worst dust storms in Australia involve dust composed of soils from cultivated fields and overgrazed areas.
Where dust that has settled on desert areas and is concentrated in river sediments fertile soils are formed. Dust from Australian dust storms has reached as far as New Zealand..
Most dune-building sands are quartzite, mainly because it is very hard, so takes a long time to be worn down. The grain size is usually between 0.15 and 0.30 mm in diameter. It is usually well sorted. Sand is carried by saltation, by moving a bit at a time, not in suspension as are the smaller dust particles. It is rarely carried higher than 1 m above the ground, and almost never more than 2 m.
In the boundary layer close to the ground surface the sand moves slower than the air above it. Turbulent eddies near the surface can lift sand gains out of the boundary layer where they can be moved by the wind. When the eddy slows the sand grains are dropped into the boundary layer again so fall to the ground. When the particles hit the ground they can bounce if they hit a hard surface and re-enter the faster flowing air and be carried a bit further forward. The grains can also hit sand on the ground and knock it into the air to be carried further. This process continues moving the sand along the ground. This movement of the sand along the ground slowly is called creep or reptation.
Wind cannot move any particles larger than the sand grains. The removal of the finer particles eventually results in a land surface of coarser material that is resistant to movement by wind. This is the basis for the different effects of the feet of native fauna, such as kangaroos, that are padded, and the hard hooves of cattle. The feet of the native fauna have less effect on the surface crust. Cattle, on the other hand, break the crust up with their hooves, allowing the soils to be picked up by wind and blown away.
Extensive plains of western Australia are covered by dunes. They are mainly comprised of linear, longitudinal or seif dunes. Some areas have reticulate or chain dunes close to sources of large amounts of sand, usually river channels in most deserts.
In the Lake Eyre Basin, the lake is the lowest point in Australia, and its drainage system includes rivers from the monsoonal parts of northern Australia, especially the Channel Country, large volumes of sand and dissolved salt are brought to the lake. This happens when the flood waters in the north are sufficient to keep the rivers flowing all the way to Lake Eyre. In past, wetter, climates it would have happened more frequently than it does now. The great dune fields are situated downwind from the major sand deposits such as depressions, claypans, salinas, river channels and valleys. So in a very real sense, the desert dunes are based on the water that flows into them, or at least nearby. When the sand deposits dry after the floods, the wind blows the sand to the lee side of the deposits. In central Australia this is the northern side, source bordering dunes or lunettes. These objects deflect the wind generating turbulent eddies on the lee slope. Sand ribbons are deposited between the eddies or vortices.. Downwind these ribbons merged, and eventually a smaller number of larger dunes formed. In depressions like the usually dry Goyder Lagoon, sand from channels have formed similar structures. After they formed, cross winds moved the sand to the north-northwest. In central Australia these crosswinds are typically from the south-southwest to the south-southeast. These dunes are advancing at a few tens of metres per year, in a stop-go fashion, during droughts. In the 1920s and 1930s a rabbit plague depleted the vegetation to such an extent that dunes migrated as though it was a drought. Now Australian dunes are completely devoid of vegetation, as they are in Nubia and Arabia.
In the Great Sandy Desert and the Great Victoria Desert major dunefields have developed to the west of the arid interior. These dunes appear to have had the same origin. In the Great Sandy Desert the dune migration is usually from southeast to northwest. The Great Victoria Desert dunes move eastwards.
An old longitudinal dune field extends across the northern Eyre Peninsula, Spencer Gulf and northern Yorke Peninsula and to the northern Adelaide Plains. At this point the dunes trend NW-SE, but in the Murray Basin, to the east of the Mt Lofty-Flinders Ranges upland, they trend W-E, extending across the plains into northwestern Victoria. Theses dunes are relics from a past, drier climate rather than the climatic conditions at present in these areas. The evidence points to a time of origin in the last glacial, with lower sea levels, as they extend below sea level off the coasts of northeastern Eyre Peninsula and northwestern Yorke Peninsula.
Evidently, the dune fields date from the Late Pleistocene. They cover surfaces of Miocene, Pliocene, and Early Pleistocene and Early Pleistocene lake deposits. Various luminescence techniques have been used for direct dating.
Australian desert dunes are comprised mostly of quartz sand. The quartz grains contain radioactive contaminants, e.g. uranium and thorium. When the crystal is buried, shielded from light and heat, energy produced by radioactive decay is contained within the crystal. If the crystal is exposed to light or heat the energy is dissipated to the atmosphere. By exposing the sand grains in the lab the energy (glow) in the grains can be measured, the length of time since the sand grain was last buried can be established. Samples are taken from the base and the just under the crest of the dune, while excluding light. Assuming the sand crystal was buried when the dune was formed, the age of the sand grain should be the age the dune was formed.
At Waikerie in the Murray Basin, dating puts a period of dune formation about 157,000-33,000 years ago. This covers a long-term lake-full phase in Lake Eyre 65,000-60,000 years ago. A single date from northwestern Yorke Peninsula indicates about 125,000 years ago as the start of dune formation.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|