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Ancient volcanic activity in Bunbury caused by mega-continent breakup

Media release

Curtin University-led research has shed light on why so many ancient volcanic eruptions – some now submerged in the Indian Ocean – occurred along the west coast of Australia where no volcano activity is expected to be found.

The research found the volcanic eruptions were a direct consequence of the breakup of the ancient supercontinent Gondwana nearly 140 million years ago, when India separated from Australia.

WA School of Mines, Curtin University PhD graduate, Dr Hugo Olierook, said the best of the stunning lava flows, called the Bunbury Basalt, can be found in south-western Australia, specifically at beachfronts in the City of Bunbury (for which it was named), and at Black Point on the south coast.

“The volcanic eruptions in this area were enormous, with their total volume equivalent to forty million Olympic-size swimming pools, or nearly sixty thousand Melbourne Cricket Grounds,” Dr Olierook said.

“The lava flows filled up ancient river valleys but are now mostly buried by sand and gravel.”

The team of researchers, from Curtin University, the University of Liverpool, the University of Western Australia and the Australian National University, used the argon-argon radiometric dating technique to analyse the lava flows, to understand when the eruptions occurred.

Study co-author Associate Professor Fred Jourdan, director of the Western Australia Argon Isotope Facility and a geochronologist and geochemist with WA School of Mines, Curtin University’s Department of Applied Geology, said the results were fascinating.

“We discovered that the basalts sporadically erupted out onto the Earth’s surface between 137 and 130 million years ago, which is around the time when dinosaurs roamed the Earth,” Associate Professor Jourdan said.

“This makes the Bunbury Basalt the earliest eruptions that coincide with the breakup of the Indian and Australian parts of Gondwana.”

The researchers then sought to investigate exactly what caused these enormous eruptions.

“We have two possible theories and it’s a bit like the chicken or the egg problem: which one comes first?” Associate Professor Jourdan said.

“Either the volcanic activity caused the breakup of the continent or the breakup caused the volcanic activity, and thanks to the results of this study, we now have good reason to believe it’s the latter.”
The results compliment the researchers’ earlier findings which showed that large volumes of lava erupted on the Wallaby Plateau off the northwest coast of WA about 125 million years ago.

“Our findings demonstrate that lava flows erupted at different times and places as Gondwana slowly tore apart along what is now the west coast of Australia,” Dr Olierook said.

A previous hypothesis suggested that an extra hot heat pulse from deep inside the Earth – the so-called Kerguelen hotspot – was responsible for the volcanism in WA and the supercontinent breakup. The Kerguelen hotspot produced gigantic volcanic outpourings of magma, known as flood basalts, in the middle of the Indian Ocean sometime between 120 and 100 million years ago.

“Volcanic islands like Hawaii are known to sit on top of hotspots. Our modelling, however, shows that when the Bunbury Basalt erupted, the centre of the Kerguelen hotspot was some 500 kilometres from south western Australia,” Dr Olierook said.

“There is also at least a 10 million year age gap between the Bunbury Basalt eruptions and the Kerguelen flood basalts, making it difficult to link the two to a common cause.”

Dr Olierook said it was exciting to be able to provide new evidence on how supercontinents broke up and said there was more research to come.

“Future expeditions are scheduled off the south western coast of Australia in 2017 to discover more about volcanic activity during the breakup of the Gondwana,” Dr Olierook said.

Samples used for the study were collected with permission from the City of Bunbury and the WA Department of Parks and Wildlife.

The paper, titled Bunbury Basalt: Gondwana breakup products or earliest vestiges of the Kerguelen mantle plume?, was published in the journal Earth and Planetary Science Letters and is available at: http://dx.doi.org/10.1016/j.epsl.2016.02.008