In short:
Two chains of mainly submerged volcanoes run along Australia's east coast, with Lord Howe one of the only remnant volcanoes above the sea.
asserted
Howe → submerge → sea
The chains were formed as the Australian tectonic plate has moved over a deep Earth hotspot for millions of years.
asserted
plate → form → years
A new research paper suggests the chains formed from a single hotspot, and that the upwelling of hot rock, known as the mantle plume, split in two.
Lord Howe Island rises dramatically from the Tasman Sea, its towering basalt peaks overlooking coral reefs, about 600 kilometres from the Australian mainland.
It is a World Heritage-listed island, home to many plants and animals found nowhere else on Earth.
uncertain
It → suggest → Earth
It is also the eroded remnant of an extinct shield volcano, its mountains formed from almost 7-million-year-old lava flows.
asserted
mountains → erode → flows
Many don't realise Lord Howe Island is at the southernmost end of a chain of mostly submerged, extinct volcanoes, known as seamounts, that stretches for about 1,500km along Australia's east coast.
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that → realise → coast
It is known as the Lord Howe seamount chain, and runs parallel to another line of undersea volcanoes, the Tasmantid chain, which has sat less than 1,000km away for millions of years.
Lord Howe Island and the nearby Ball's Pyramid volcanic sea stack are the exposed parts of the much larger, underwater volcanic complex.
The long-running coexistence of the Lord Howe and Tasmantid seamount chains, side by side, and relatively close to each other, has long intrigued scientists.
asserted
coexistence → know → scientists
A research paper recently published in the journal Gondwana Research suggests the two chains formed from the same deep Earth hotspot.
uncertain
chains → publish → hotspot
"With Lord Howe Island, enough material had erupted on the sea floor so that Lord Howe Island could actually penetrate the surface of the water and become a subaerial island that people can live on," said the paper's lead author, Ben Mather, from the University of Melbourne's School of Geography, Earth and Atmospheric Sciences.
uncertain
author → erupt → Geography
The paper suggests mantle plumes, upwellings of super-heated rock from Earth's interior that build volcanoes, don't always rise in a straight line.
uncertain
that → suggest → line
They can bend, and even split in two.
asserted
They → bend → two
Dr Mather said that meant the Lord Howe and Tasmantid chains were part of the same family of volcanoes.
asserted
chains → say → volcanoes
"It's very unusual that the two volcanic chains should be so close together, and that they should sustained over such a long geological time period, over millennia, in the middle of a tectonic plate," Dr Mather said.
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Mather → sustain → plate
"So this research was essentially trying to unpick how that could possibly be."
uncertain
that → try → ?
How volcanic chains are formed
Dr Mather said the hotspot mantle plume acted like a blowtorch under the drifting Australian tectonic plate, one of the massive slabs of rock that glides over the Earth's mantle.
asserted
that → form → mantle
When the hotspot melts and punches through a weaker section of the plate, erupting lava accumulates, and builds a seamount.
asserted
lava → melt → seamount
Eventually the plate, drifting very slowly to the north, carries each new seamount away from the hotspot, forming a chain of undersea mountains over millennia.
asserted
plate → drift → millennia
"Hotspots are tools we use to tell us how continents have moved through time … they form reference points because we know that mantle plumes stay roughly fixed, while tectonic plates slide over the top of them," Dr Mather said.
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Mather → use → them
It takes a long time for a new seamount to form.
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seamount → take → time
In the case of the Lord Howe seamount, the Australian plate has moved north over a hotspot for more than 40 million years.
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plate → move → years
A twin Tasman volcanic chain
So, how did two chains form close together?
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chains → form → chain
Dr Mather said in the case of the Lord Howe and Tasmantid chains, his team developed a 3D numerical modelling tool, which showed how the volcanic plume had split into two long-lived branches to move around an obstacle deep beneath the Tasman Sea.
asserted
plume → say → Sea
The research paper argues that on the way up from the Earth's molten interior, the plume hit a slab of old seafloor that had sunk about 500km into the Earth's mantle at a '"subduction zone" — a place where two tectonic plates collided.
asserted
plates → argue → zone
Dr Mather explained the plume could not go up through the slab, so it split into two, and those branches then reached the surface and the two chains of volcanoes eventually formed.
uncertain
chains → explain → volcanoes
"Reconstructing the past positions of these sunken slabs beneath the Tasman Sea reveals exactly the geometry our model predicts: a strip of slab material with gaps on either side, where each plume branch can rise to the surface," Dr Mather said.
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Mather → reconstruct → surface
He said testing was also done on rock samples collected from the ocean floor which revealed lava from both the Lord Howe and Tasmantid seamount chains were chemically near identical for any given age.
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lava → say → age
Dr Mather said two unrelated plumes, coming from separate patches of deep mantle, would generally not produce the same rock.
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plumes → say → rock
Karin Orth, an earth scientist at University of Tasmania, said the new modelling was a "very elegant" way of explaining the formation of two closely spaced parallel volcanic tracks.
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modelling → say → tracks
But she said more data was still needed to refine the model and its accuracy.
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data → say → model
"It's a bit forensic; it's a bit like a detective story," Dr Orth said.
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Orth → say → story
"The other aspect of this we need is the information … in this case, there's a lot going on in the Coral Sea and to the north in New Guinea that we don't fully understand, because we don't necessarily have good data on that.
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we → need → that
"So, there are problems with the model, because the model just doesn't have enough information at particular ages … like any theory and model, it will get refined."
asserted
it → be → theory
Dr Mather said further studies would be done, including high-resolution imaging of the ocean floor and more rock sample collection.
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studies → say → floor
Why does this matter?
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this → matter → ?
Dr Mather said their plume branching model might also explain other closely spaced plume tracks.
uncertain
model → say → tracks
"The same process may explain other puzzling pairs of volcanic chains worldwide, including the closely spaced volcanic tracks at Yellowstone [in the United States]," he said.
uncertain
he → explain → States
He said it would also help map Earth's interior and tectonic plate movements.
"
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it → say → movements
Plumes are usually drawn in textbooks as a straight vertical pipe from the core mantle boundary deep within the Earth to the surface," he said.
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he → draw → surface
"Here we have found that plumes are actually dynamic creatures that evolve as they encounter obstacles within the mantle.
asserted
they → find → mantle
"So if one branch of the plume can shift, then counting chains is no longer a good way to construct the configuration of past tectonic plates, we have to account for these anomalies."
asserted
we → shift → anomalies
Dr Mather said the research also offered potential insights into where future undersea volcanoes might emerge along the Tasmantid and Lord Howe chains millions of years in the future, including signs the Lord Howe chain was essentially running out of puff.
uncertain
chain → say → puff
…and 7 more, not listed.
The Aporia