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Ancient Australia’s Super-Eruptions

Credit: Microstocker/Adobe

Credit: Microstocker/Adobe

By Milo Barham

Sediments beneath the Nullarbor Plain have revealed that super-eruptions in eastern Australia more than 100 million years ago were powerful enough to blast crystals right across the country.

The raw power of volcanic eruptions has long captured the human imagination. While their effects have been historically catastrophic – think of what Mt Vesuvius did to Pompeii and Herculaneum – they have also produced fertile soils and geothermal power.

Yet the scale of the volcanic episodes that have been documented historically are mild compared with “super-eruptive” events evident in the geological record. Recent work in Western Australia has found evidence that ancient volcanoes along the eastern margin of Australia were so violent ~106 million years ago that they blasted volcanic material more than 2000 km away to the western side of the landmass, making them some of the most powerful eruptions known.

Previous work on the eroded plumbing of these volcanoes, in particular the Whitsunday Islands region, had hinted at their explosivity, but the eruptive power of volcanoes in this large igneous province (LIP) has been underappreciated. By analysing distinctive minerals such as zircon recovered from Western Australia, a collaboration of researchers from Curtin University and the Geological Survey of Western Australia have, for the first time, been able to demonstrate the destructive magnitude of eruptions in the eastern Australian LIP.

The margins of the tectonic plates that make up the Earth’s crust are commonly defined by volcanic peaks, whether this be far beneath the waves along mid-oceanic ridges where new crust is born, or thrust high into continental spines (e.g. the Andes). Between roughly 130 and 95 million years ago, volcanic activity characterised several thousand kilometres of Australia’s eastern margin as Zealandia rifted free to form parts of present-day New Zealand, New Caledonia and the extensive submerged region of the Lord Howe Rise. This separation was all part of the final break-up of the supercontinent Gondwana, leading to the isolation of the Australian continent and a unique evolutionary path for its flora and fauna.

Typically the magnitudes of ancient volcanic eruptions are estimated by measuring the volume of material ejected from a volcano, either by mapping the thickness, or distance from the source, of specific deposits. However, the ability to resolve and quantify discrete eruptions over geological timescales becomes undermined by weathering and erosion. Thus, over time, the scale of ancient eruptions typically becomes underestimated as the extent of any remaining eruptive products shrinks and a purely eruptive projection of distal volcanic particles becomes impossible to prove. As a result of this, super-eruptions in deeper geological time are relatively poorly documented.

Recent drilling beneath the Nullarbor Plain by the Geological Survey of Western Australia has begun yielding insights into the region’s geological evolution as well as ancient Australian volcanism. Initially, this work focused on assessing the ages, environments and sources of sediments that accumulated during the separation of Australia’s southern margin from Antarctica. Samples were processed and analysed by the John de Laeter Centre’s laser system, which can blast away minute targeted portions of minerals (50 times smaller than a pin-head) in order to analyse their specific geochemical fingerprint.

Although many of the grains the team analysed revealed that they were derived from ancient crustal blocks that underlie or are adjacent to the modern Nullarbor region, the chemistry of one sub-population of crystals was unique in Western Australia. Isotopes of uranium, lead and hafnium indicated that these grains were only 106 million years old, much younger than any known igneous event in the region, and that they reflected an anomalous input of molten material from the deeper Earth.

Furthermore, when the grains were examined with an electron microscope, the internal concentric patterning and excellent crystal shape of these “young” grains strongly suggested they were from a volcanic source. However, there are no known volcanoes of this age along the central and south-western margin of Australia, which was relatively devoid of igneous activity until some 50 million years later. Instead, the grains were a perfect match to those found in the volcanic systems of eastern Australia’s LIP.

Mineral grains can be transported thousands of kilometres in rivers, glaciers or during coastal long-shore drift, so the occurrence of eastern Australian detritus in Western Australia might not seem untoward. In fact, during the late Cretaceous (~100–70 million years ago) the southern margin of the continent played host to the largest ancient delta in Australia (~125,000km2); the Ceduna Delta is thought to have scavenged material from a great swathe of the interior of eastern Australia via giant continental drainage systems. However, when it came to the volcanic grains recovered from Western Australia, such a river, or even ocean transport, could be ruled out due to a number of critical observations.

  • The volcanic crystals must have been rapidly incorporated into the sediment, since their ages are a perfect match to microfossils recovered from adjacent samples in the core.
  • There is a lack of coarse sediment in the region, as could be expected at the mouth of any large river system.
  • There are no other grains with an eastern Australian “flavour”. Such grains were incorporated into the rivers of the Ceduna Delta, and would also have been transported by any river system or coastal erosion bringing the volcanic grains across to Western Australia.
  • The volcanic crystals recovered would be expected to have more rounded shapes if they had experienced thousands of collisions during transport in a river or beach setting. Instead they are relatively pristine, like they have effectively come straight from the volcanic vent.

Considered together, the young volcanic grains must have been transported aerially in an eruption cloud directly over any intervening portions of the eastern half of Australia before “raining out” near their final resting place.

In order for dense sand-sized crystals to be transported in an eruptive cloud for thousands of kilometres we must understand the violence of the eruption itself. These events would have been incredibly explosive, thousands of times more powerful than Mt St Helens. The eruptions would have punctured the atmosphere with a deafening roar and streaming eruption column, which would be so choked with material that at times it would have collapsed under its own mass, and sent glowing clouds of searing hot gas, ash and rock across the local landscape (which is preserved today in the Whitsunday region). Parts of the more stable eruption column, as well as phoenix plumes rising with thermal buoyancy from the eruptive flows and surges, would have pushed the finer particles higher and allowed for them to interact with atmospheric currents. The concentrations of material and inherent turbulence of the volcanic clouds would have retarded the settling rate of constituent particles sufficiently to allow long residence times in the atmosphere and long-distance transport of even some larger grains.

Australia straddled polar latitudes 106 million years ago, and the global continental arrangement would have generated strong polar easterly winds during at least the Southern Hemisphere winter. Thus the perfect combination of atmospheric winds and extreme eruptive violence facilitated the distribution of volcanic material across Australia from the east.

Eruptions in more geologically recent times allow us to better understand the scale of the destruction that likely resulted from these Australian events. Roughly 75,000 years ago, Toba (Indonesia) erupted with what is thought to have been a comparable fury, depositing similar sand-sized particles at least 2670 km distant in India and causing dramatic global cooling.

More recently, in 19th century Indonesia, documented eruptions of Krakatoa (1883) and Tambora (1815) occurred on scales thought to be tens of times less powerful than those of eastern Australia, yet they still could be heard thousands of kilometres distant, affected global systems (reducing average temperatures, producing “the year without a summer” and inspiring celebrated artwork on the other side of the world, such as Turner’s sunset paintings) and caused thousands of immediate local deaths through direct tsunami and volcanic products.

With this new work we are able to connect the local volcanic edifices of eastern Australia with distant volcanic material, and for the first time paint a picture of the violence that was tearing Australia’s eastern margin apart over 100 million years ago. Were such an event to occur today it would likely be heard across the continent, cause global temperatures to temporarily drop by several degrees, devastate agriculture, and exacerbate regional and global conflict for resources.

Milo Barham is a Lecturer in the Department of Applied Geology at the Western Australian School of Mines, Curtin University.