Australian Volcanoes

Australia's only active volcano is rumbling fiercely, with new NASA photos revealing its lava lake has overflowed its crater. The volcano in question, Big Ben, is happily located on Mawson Peak in the remote southern reaches of the Indian Ocean on Heard Island, an Australian territory. People only bother to visit Heard and its neighbour McDonald Island every couple of years, because there's little there but chilly wastelands and the territory is a nature reserve people aren't allowed to visit without a permit. Even fisherfolk chasing the apparently tasty patagonian toothfish, aka Chilean Sea Bass, don't often bother landing. 

No permanent human presence exists on the islands, beyond an automated weather station. NASA keeps an eye on the islands, though, because of the volcano atop Heard Island's Big Ben occasionally fires up. Last October we reported things have started to look interesting on the island. 

Heard Island's remoteness and Mawson Peak's altitude - 9006 feet or 2745 metres - mean there's nothing to fear from the eruption. Even local populations of seals and birds probably aren't in peril, as few live near the Peak's summit. But Heard Island does experience "strong and persistent westerly winds ", according to its official website.

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.

• 47-16 Ma Tasmania - south
• 38-13 Ma Tasmania - north
• 33-28 Ma Queensland central coast
• 26-20 Ma Victoria - eastern
• 25-22 Ma Queensland - southeast
• 7-2 Ma Victoria - west & South Australia - southeast

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.