Australia: The Land Where Time Began
The most recent volcanic eruption on the Australia mainland was about 4,000-5,000 years ago at Mt Gambier, in the Newer Volcanic Province, that covers about 15,000 km2 of Victoria and South Australia. In this area are about 400 small shield volcanoes, as well as explosive vents, that were active from the Tertiary to the Holocene.
Mt Gambier in South Australia is Australia's most recently active volcano, having last erupted about 4,500 year ago. It is considered to be dormant, rather than extinct, so there is the potential for another eruption.
Volcanic fields in Australia
It is thought that eastern Australia has been passing over a hotspot as it drifted north, leading to a chain of volcanic activity from north to south. Basaltic lava flows are prominent along the Eastern Uplands, as well as some of the offshore islands. Lord Howe is one such island that has been interpreted as a shield volcano that has been eroded down to a remnant of its former mass. The lava layers in this remnant indicate that the deposition took place in at least 2 major phases of volcanic activity in the Late Miocene, about 6-7 Ma. Together with several seamounts, it has been suggested to be part of a chain of seamounts trending north-south for about 1000 km that were formed as the continental crust moved north across a hotspot at the rate of about 6 cm/year. Based on this suggestion it is expected that if a new volcano formed at the present it would be about 400 km along the chain to the south of Lord Howe Island (Twidale & Campbell, 2005). The northernmost seamount in this chain is Nova Bank, that has been dated to about 23 Ma.
The volcanic provinces of Australia would be expected to display a similar consistent age distribution pattern to that of the seamounts, assuming the validity of the interpretation mentioned above, with the oldest in the north and the youngest in the south. The pattern has been found to be more complex than a simple continuum from north to south.
The youngest volcanic eruptions in Australia occurred in the district of Mt Gambier in southeastern South Australia, near the margin of the continent. About 5000 years ago Mt Gambier and Mt Schank erupted. Near Warrnambool in western Victoria, the Tower Hill Complex, displays evidence of very recent volcanic activity, as does the Nulla field, that was probably volcanically active about 13,000 years ago. There are also several plains and plateaux, such as the McBride Field, the Sturgeon Field and the Chudleigh Field, of Late Tertiary and Pleistocene age, where radiometric dates of about 5 Ma have been obtained.
According to Twidale & Campbell, the pattern of age distribution of volcanic activity in Australia does not indicate activity associated only with the passage over hotspots, of which there are thought to be as many as 5, 3 on land and 2 offshore, though they could be regarded as centres of volcanic activity rather than hotspots. These possible hotspot tracks tend to trend NNE-SSW. The authors suggest tectonic regimes, imposed compressive (quiescent) and extensional (eruptive), can explain the discontinuous nature of the eruptive activity.
There is also volcanic activity associated with the separation of Australia from Antarctica at the breakup of Gondwana, that is not connected to any activity associated with possible hot spots. Volcanic activity associated with separation of Mesozoic and Early Cainozoic age are present in some of the older eruptions in eastern Victoria and northern Tasmania.
Volcanic activity in modern times is associated with the Indonesian arc in such places as Papua New Guinea and the adjacent islands, where it is linked to the subduction zone along the northern margin of the Australian Plate. Another problem with the eastern Australian provinces is that the lava chemistry varies, leading to more complications. There is disagreement about a number of proposals, such as whether hotspots or developing oceanic rifts are the cause of the volcanicity, or whether hot fluids and gases have invaded the lithosphere to form the magma chambers from which the eruptions subsequently took place. The lava flows have proven very useful for dating the landforms in their vicinity.
In Australia there are also older volcanic rocks that are widely distributed in ancient depositional sequences interbedded with sedimentary rocks. They form important components of both the Yilgarn and Pilbara blocks. Several ranges in north Queensland, such as the Newcastle Range near Einasleigh is underlain by acid volcanics of Late Palaeozoic age, well-developed columnar joints indicating the volcanic nature of the country rock. In the Featherbed Range near Chillagoe, further to the north, there are sequences of bedded silicic volcanics that have been weathered differentially and eroded, resulting in ridge and valley forms. There are ancient puys or small domes in the landscape composed of viscous rhyolite. The Gawler Ranges in arid central South Australia, has one of the most extensive exposures of silicic volcanic rocks, dacite and rhyolite, dating from the Middle Proterozoic. The well-developed orthogonal and sheet fractures of the landscape are more typical of granitic than volcanic terraines.
Drainage patterns of a region can be changed by volcanic eruptions, overprinting the pre-eruption landscape when lava domes replace the pre-existing landform, the drainage system being replaced by a drainage system that radiates from the peak of the lava dome. When lava domes are so large they cover a region they can be preserved as occurs on the western slopes of the Tweed Volcano, as well as on the Comboyne Plateau and the Ebor Volcano, both of which are in New South Wales.
Lava tongues can have an effect on the local scale when a tongue flows down a valley dividing an existing stream into 2 separate streams that each run between the tongue and the bedrock slopes. When the lava is of an erosion-resistant nature it can eventually lead to a reversed landscape in which the rock surrounding the lava is eroded down below the level of the lava, which then becomes a ridge. To the northeast of Cobar in New South Wales is just such a formation, El Capitan, a basalt-capped ridge from the Miocene the stands 40 m above the surrounding plains.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|