Australia: The Land Where Time Began
Lake Eyre, the lowest part of the Australian continent, is situated in the deserts of South Australia, has an area of 9,300 square kilometres. It is between 12 and 17 m below sea level at its lowest point. It is made up of 2 parts, Lake Eyre North and Lake Eyre South, the smaller of the 2, joined by the Goyder Channel, usually a channel of salt. About 1/3 of the lake, mostly in the southern part, is covered by a hard salt crust. To the north of this crust is the slush zone, where a thin layer of salt covers an area of mud that never dries.
The crust of salt on the lake bed is 50 cm thick in the south and reduces to almost nothing in the northern part. Beneath the crust there is a zone of silty gypsum-rich mud up to 6 m deep. The full lake bed lies below sea level, but the lowest point is difficult to define, because its precise location varies with time, depending, at least partly, on salt crystallisation. It is always near the southern margin of Lake Eyre North, averaging about 17 m below sea level. It is always in one of the bays, Belt Bay, Jackboot Bay or Madigan Gulf.
To the west, the basin is bordered by the Peake and Denison and Dalhousie Dome provinces, to the south by the Willouran Range, Flinders Ranges and Olary Range, by the Barrier Ranges in the east. To the north, the basin extends into the Northern Territory, Queensland and New South Wales and is bordered by Mesozoic rock domes. Late Cainozoic uplift has resulted in southern margin being deeply defined and faulted.
A low, precipitous cliff marks the western edge of the lake. This cliff is capped by a layer of gypsum and gypcrete up to 2 m thick. This is an odd formation because gypsum is highly soluble and very soft. This layer is compact and crystalline, and in the arid conditions prevailing in this area, is much more resistant than the underlying silts, so in this place it forms an effective caprock. The western scarp, in detail crenulated by desiccation, is overall straight. The strata sequence exposed in the cliff is found several metres beneath the lake bed, a short distance to the east. This indicates vertical displacement. Mound springs are aligned with this fault so they probably result from the artesian water being forced to the surface along fractures, Earthquake activity is recorded in the area. All this suggests faulting.
Also, where Warburton and Kalaweerina Creeks enter the lake bed, they have cut shallow straight grooves, one of which is 84 km long and 54 cm deep, the other is 40 km and 13 cm deep. They flow parallel to the western cliff line, so they may also be fault-controlled. Maximum salt precipitation occurs in the southern part of Lake Eyre North. This was thought to be as a result of southerly tilting of the bed, but it could also result from sand and silt deposition in the northern areas where major rivers enter the lake. The sloping floor of Lake Eyre North intercepts the almost horizontal water table at the southern end.
Floodwater enters the northern part of Lake Eyre North, forming shallow grooves across the sloping playa floor on their way to the deeper bays in the south. An example of these grooves is the Warburton Groove, running about 85 km in a straight line from the mouth of Warburton Creek in the north to Belt Bay in the south.
When dry, the lake bed looks completely flat, but there is a drop of 4 m from north to south over a distance of 120 km. There are depressions on both eastern and western sides, south of the Cooper and Neales estuaries respectively, as well as Warburton and Kalaweerina grooves. Salt crystallisation causes expansion and upturning of the salt crust, so that at certain times, large areas are patterned by low salt ridges that define polygonal plates. On a large scale the bed of Lake Eyre is as flat as a table.
At 1.3 million square kilometres, the catchment of Lake Eyre is one of the largest in the world, and is the largest internal basin in Australia. Lake Eyre has been full only twice in the 20th century, in 1950 and 1974. In 1974 the amount of water held by Lake Eyre is believed to have been the largest amount in 500 years, reaching an average depth of 4 m and some parts reaching depths of greater than 5 m.
The Cooper, downstream of the Barcoo-Thompson confluence, the Diamantina-Warburton, and the Georgina, that arise in the wet subtropics, are the main river systems on the eastern side of the basin that flow about 1000 km to Lake Eyre. Streams arising in the Stuart, Everard and Musgrave Ranges, in the west, join the Neales and Macumber Rivers to flow into the lake. To the south streams from the Flinders Ranges flow into From Creek, then into into the lake. The Margaret River drains hilly country to the south-west into the lake. Screech Owl Creek flows from the north, south to the lake. To the north, streams draining the Macdonnell Ranges, such as the Finke River and some other ephemeral streams, flow towards the lake but are lost in the dunefields of the Simpson Desert. Some of the flow of the Finke passes through the edge of the Simpson into the Macumber River system. It contributes little if any runoff to the lake. The flow of all these ephemeral streams is very erratic, both from season to season, and from year to year.
In the basin the summers are hot and dry, and the short winters are cool to cold. The annual rainfall ranges from less than 150 mm/yr in the large area around the lake to almost 500 mm/yr in the far north-east of the basin. Evaporation over the whole basin greatly exceeds rainfall throughout the year. The average evaporation also varies greatly, from 3600 mm/yr at the lake to 2400 mm/yr in the north-east.
The processes that led to the formation of Lake Eyre began about 200 million years ago when a large band of land between the Gulf of Carpentaria and the area of the South Australian salt lakes began to sink, becoming a depocentre for river and lake sediments. This subsidence has continued to the present. Then about 100 million years ago the whole area was inundated by the sea. 20-30 million years later the sea receded leaving only several large rivers that are thought to have flowed to the southern coast. About 1 million years ago the land to the south of the salt lakes was tilted up, the faulting blocking the flow of the rivers to the coast. The result was a huge lake, Lake Dieri, named after one of the Aboriginal tribes living east of Lake Eyre. The existence of Lake Dieri that was up to 10 times the size of the present lake has been disputed on the basis of the lack of geological evidence (White, 1994).
Major climatic oscillations connected with the glacial-interglacial phases through the Quaternary Period have resulted in the size of Lake Eyre varying between a perennial lake much larger than the present playa, with a depth of 25 m, to completely drying up, when it underwent extensive deflation of its sediments, as the winds of the glacial peaks scoured the surface, carrying away the sediment. Groundwater controlled this deflation as evaporation lowered the water table, producing the morphology of the present day playa. It is now in a relatively stable ephemeral lake stage, the deflation of the dry playa being roughly balanced by the inflow of sediments brought by the floods in its catchment.
Study of the sediments of Madigan Bay, the largest of the Lake Eyre North bays, has demonstrated the lake's changes during the last full glacial cycle of 130,000 years.
35,000 years ago Lake Dieri (or the ancestral Lake Eyre) was 3 times the size of the present Lake Eyre and had a depth of at least 17 m. At that time, lush vegetation surrounded the lake. From 20,000 years on the climate changed so much that the rivers that fed the lakes diminished and then stopped flowing, apart from the occasional flood, and the area became as arid as it is today, the lakes shrinking until only salt lakes remained. Most of the salt delivered to the lakes was leached from the ancient marine sediments that underlie the catchment.
Lake Dieri, the ancestral Lake Eyre, was at least 3 times the size of the present lake, at least 28,000 km2 in area, compared to the 9,330 km2, at about 600,000, 60,000, and finally at 40,000-20,000 years ago. At these times it was 17 m deep. These were secular or long-term stands of the lakes. There are ancient shingle beach ridges around Lake Eyre, at 280, 160, and 70 cm higher than the 1974 lake bed. The 1970s filling of Lake Eyre showed that the beach ridges may not represent long-term lake levels. In the 1970s it was found that beaches, spits and bars could form in a few weeks. The arid zone of Australia reflects the present conditions as well as past climates.
It has been estimated that about 125,000 BP, when lake Eyre was a deep, permanent megalake of close to 35,000 km2, the combined Lake Eyre-Lake Frome system held 430 km3 of water, compared to about 30 km3 when full in historical times (Vogel et al., 2004).
The evaporation rate at Lake Eyre, and all the other dry salt lakes of arid Australia, is much higher than the precipitation rate and the inflow from flooded channels combined. Under such conditions it would be expected that the bed would have long ago dried out to bedrock, but there is permanent mud under the crust, as there is also at Lake Frome, and probably other such lakes. The explanation is probably that, as has been found at Lake Frome, evaporation from the surface draws up water from the saline aquifers underlying the area through the salt-rich marine sediments left by the huge sea that covered the area in the past. The crust would be constantly added to from this mineral-rich water.
These are the stages believed to have occurred in the formation of Lake Eyre:
The Aboriginals of the area surrounding Lake Eyre feared and avoided it. White explorers also seem to have been affected by Lake Eyre in a similar way to the Aborigines. The explorer Eyre said of it that it was "glittering and brilliant beyond conception" and he had an uneasy feeling about it, and called it a "region of magic" where all was "uncertainty and conjecture". The explorer Madigan, who had been a member of Mawson's expedition to the South Pole, likened the pinky-white crust to the pack ice of Antarctica. He found the lake fascinating, returning many times, describing the effect it had on him as the "spell of Lake Eyre".
There are some odd things about the lake. One is the foam that exudes from the salt in some places. Where this foam rises the salt crystals are fashioned into a great variety of shapes, from coin-shaped to flower or shell-like shapes. The extraordinary thing about these salt crystal structures is that there are often many, sometimes hundreds, that are apparently of identical symmetry and detail. The mechanism of their formation is still unknown.
During the day the brilliant white of the saltpan contrasts sharply with the metallic blue sky. The heat of the day is much more oppressive than in the surrounding deserts because it is greatly increased by the heat and glare reflected from the salt, and mirages are common. The best times to see the lake are at sunrise when it glows pink and at sunset when it appears opalescent.
At night it can be positively eerie, the combination of howling winds and particularly on hot nights, midnight mirages.
Among the things about the lake that have been commented on are the sandy beaches, headlands and spits, that appear strangely out of place with no water. The appearance of the lake has been described as arctic, but strangely incongruous with the oppressive heat (Grasswill & Morrison, 1981). As Madigan noted, the saltpan has the appearance of pack ice and snow, smooth in places, while in other places foam exudes from the crust, giving the impression water is moving below the surface.
It can also be a hazardous place, the blisteringly hot summer days can quickly cause heat stroke and one could die of thirst, and the floods can also be dangerous in flood times, when water rushes into the lake. The risk of becoming lost if travelling on the lake at any time without a compass is also a threat.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|