Australia: The Land Where Time Began
The Australian Environment
The Australian continent6
Australia is an extensive, low continent with an area of about 7.66 million km2, and a mean height above the present sea level of 330 m, and 200 km2 less than 200 m above sea level. The highest point on the continent is Mt Kosciuszko that rises to 2,227 m, not an impressive height on a world scale. According to Twidale6 Australia is a compact continent that has few major islands, inlets or embayments and a coastline of a bit less than 20,000 km. The ratio of shore length to land area of about 1 km of coast for every 390 km2 (1:390), while the ratio of coast to land area of peninsula and insular Europe is 1:75. The Gulf of Carpentaria, Bonaparte Gulf and King Sound on the north coast, Exmouth Gulf and Shark Bay on the west and Port Phillip Bay and gulf St. Vincent and Spencer Gulf on the south. Tasmania and Kangaroo Island in the south Fraser Island on the east, and Groote EyIandt, Melville Island and Bathurst Island off the north coast are the only major islands. Endorheic streams that flow to basins of internal drainage serve more than half of the continental area. The Lake Eyre catchment accounts for 1.3 million km2 of central and northeastern Australia. Exoreic streams (rivers that flow to the sea) serve less than half the continent, with the River Murray being of notable length but has a low discharge rate as for much of its length it passes through arid areas.
From sources 1-5
The Australian continent is unique in a number of ways. It is the driest, flattest, oldest, it has the most erratic climate of any continent, and much of it experiences great extremes of rainfall both during a year and from year to year. As a result of this, poor soil, arid and unpredictable climate over most of the continent, the plants, and therefore the animals that depend on them, have evolved into a unique flora and fauna.
Lack of volcanic or seismic activity leading to mountain building over much of the continent for many millions of years means that what mountain ranges there were have been worn down by erosion to mere stubs of their former glory. The soil over much of Australia, especially the dry central parts, have not been renewed by volcanic activity for a very long time and are impoverished compared to soils from most other parts of the world. Another factor in the formation of fertile soils that was absent in Australia was glaciers. During the ice ages of the last 2.6 million years, glaciers covered much of the Northern Hemisphere, grinding unweathered rock into fine particles that became soil enriched with new minerals when the glaciers retreated. During this time period Australia had cold windy periods at times of Northern Hemisphere glaciations, but no, or very limited, glaciers. Any that formed were on the high country that is such a small part of the Australian landscape.
After being recycled many times over much of the history of the Earth, with no new rock added by volcanism, the soils of arid Australia have been weathered and leached more than any others making them the most nutrient-deficient soils in the world. And most of Australia spent many millions of years covered by seas. In many places these seas were eventually cut off from the ocean and evaporation formed huge salt deposits which were later buried. So added to this impoverished state of the soils there are vast areas where there are salt deposits beneath the surface which means when land is cleared for agriculture the removal of deep-rooted trees and shrubs mean the water table rises. By the time the water table gets close to the surface it has passed through these salt layers and so causes salinisation in the root zone and eventually at the surface, which makes the land useless for agriculture.
A place where salt is a serious problem is the West Australian wheat belt.
The geologic history and its climatic contrasts are reflected in the landforms. There are 3 major structural components, the stable Western Shield, the gently warped Central Basin, and the ancient orogony of the Eastern Uplands, which have been rejuvenated by differential uplift in Tertiary and later times. The result is the vast plains and plateaux of the Australian landscape. There are not many areas above about 1700 m, and even at Mt Kosciusko, the highest point on the continent, it is less then 3000 m above sea level. Not high compared with the mountains in other parts of the world.
According to Twidale & Campbell (Source 5), the lack of recent earth movement is a main cause of the widespread prevalence of relatively low-lying plains. The Late Palaeozoic was the time when the most recent episode of mountain building occurred in Australia, with the deformation and uplift of the Eastern Highlands. There have been many episodes of warping and faulting, that were widespread but minor, since the orogenesis that produced the Eastern Highlands. The authors suggest that about 30 million years would be required to base-level a continent of the size of Australia, after allowing for isostatic compensation that would result from erosional unloading, allowing plenty of time since the last major orogenesis for the formation of extensive plains by weathering and erosion. According to Twidale & Campbell, the compact shape of the Australian continent has contributed to the tendency for widespread plantation, with few major indentations or embayments along the extensive coastline. This meant that as sea level changes occurred the resulting changes of river behaviour were limited to the margins of the continent, with the coastal rivers being the only rivers affected, the inland rivers being shielded from such impacts. On the east coast the short, steep rivers have not regressed as much as would be expected, some having eroded headward by about 100 km in about 60 My. In the later Tertiary the Lake Eyre Basin began to subside, resulting in the internal (endogenetic) drainage system that drains much of inland Australia. Depositional plains are also a prominent feature of inland Australia (Twidale & Campbell, Source 5).
These components have determined the outline of the continent and the overall pattern of drainage and relief. In the east, the peripheral uplift in the Eastern Uplands has resulted in the highest ground being near the coast.
In the west of the continent, there is a narrow plain between the faulted edge of the shield and the coast. In the north-west the highest ground is in the marginal line of the Hamersley, Kimberley and Arnhem Land Plateaux. The external drainage on 3 sides of the continent is restricted to a narrow strip around the edge of the continent on 3 sides totaling about 1/3 of the continental area.
The Central Basin has 2 major inward-draining drainage systems, the interior drainage towards lake Eyre, and the Murray-Darling system. The Murray-Darling system has maintained a connection to the sea, mostly because of the extra water draining from the south-eastern part of the Great Dividing Range.
The structure of the drainage systems, where the coastal areas, where most of the rain falls, is drained to the sea, and the inward flowing drainage of the rest of the continent, that receives very little rain, conspires in such a way that vast tracts of central Australia depend for their water on the intermittent floods that are channeled from the monsoonal areas of northern Australia along normally dry channels. While the monsoon rarely fails completely, the amount of rain delivered to the north varies. It is only in very wet years that the water reaches all the way to Lake Eyre, especially as a lot of the water is absorbed into the dry channel beds and evaporates in the hot dry air of the interior long before it reaches Lake Eyre. Away from the ameliorating influence of the coast the temperatures can get very high in central Australia during summer.
Nearly all of the area inland of the peripheral drainage systems generates less the about 3 cm per year of run-off. As aridity has increased any rivers that had existed on the low gradient sandy surfaces of the shield areas have disintegrated or disappeared entirely. A similar condition, no rivers, occurs also on the Nullarbor Plain and the in the Simpson Desert.
These 3 major structural components are convenient divisions for describing the landform assemblages of Australia.
Some sites connected with the geology and environment of Australia
These are some of the sites with information on Climate.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|