The ancient volcanic chain, reported in the journal Nature, runs from Cape Hillsborough on the central Queensland coast, south-west through central New South Wales to Cosgrove in Victoria.
"This volcanic chain was created over the past 33 million years, as Australia moved north-northeast over a mantle plume hotspot which we believe is now located in Bass Strait," said the study's lead author, Dr Rhodri Davies of the Australian National University.
"This track, which we've named the Cosgrove hotspot track [after an extinct Victorian volcano in the chain], is nearly three times as long as the famous Yellowstone hotspot tracks on the North American continent."
This kind of volcanic activity is surprising because it occurs away from tectonic plate boundaries where most volcanoes are found.
These hotspots are thought to form above mantle plumes, narrow upwellings of hot rock that originate at Earth's core-mantle boundary almost 1,800 miles below the surface. A volcano chain is created as the tectonic plate moves over the hotspot.
The newly identified volcanic chain is the most westerly of three major volcanic chains running along eastern Australia.
The authors examined 15 extinct volcanoes in eastern Australia that had been known about for quite some time and appeared to follow a generally similar track.
"The volcanoes in central Queensland showed an age progression, so they got younger towards the south, and so too did those in New South Wales and Victoria," Davies said.
The researchers looked at the movement of the Australian tectonic plate.
"Australia is actually the fastest moving continent on Earth, moving towards Indonesia at around seven centimetres per year," Davies said.
The researchers found the chain of now-extinct volcanoes in Queensland, New South Wales and Victoria had all passed over the same fixed mantle plume hotspot as the Australian continental plate tracked north-northeast.
"We showed that these volcanoes are surface manifestations of the same mantle plume," Davies said.
"However, the two groups of volcanoes were geochemically very distinct from each other and were separated by a gap of 700 kilometres, so no-one ever put these two volcanic chains together."
Chain helps give better understanding of volcanism
Davies and colleagues used seismology to map the thickness of Earth's crust and mantle -- known as the lithosphere -- that lies under eastern Australia.
They found volcanoes in central Queensland erupted through lithospheres about 50 miles thick while those in New South Wales and Victoria had melted through lithospheres about 62 miles thick.
But the gap between the Queensland volcanoes and those in New South Wales and Victoria occurred because the lithosphere in this region was at least 93 miles thick.
"So the mantle plume can't melt through in those regions, so there's no volcanoes on the surface," Davies said.
The thickness of the lithosphere also explained differences in the chemical composition of the volcanic rocks at different locations.
"If you take a mantle plume of a specific temperature and raise that to a depth of say 130 kilometres below the surface, specific minerals from the surrounding rock will enter that melt," Dr Davies said.
"And if the plume reaches shallower depths of say 100 kilometres, additional elements will enter the melt, changing the chemical composition.
"By looking at this chain of volcanoes in Australia and understanding the composition of the volcanic rock and how they evolved with time, we will understand volcanism on other continents and through earlier periods in Earth's history which is still poorly understood."
Read more at Discovery News
No comments:
Post a Comment