Scientists unearth remarkable volcanic origins of Lord Howe Island

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.

It is also the eroded remnant of an extinct shield volcano, its mountains formed from almost 7-million-year-old lava 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.

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.

A research paper recently published in the journal Gondwana Research suggests the two chains formed from the same deep Earth 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.

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. They can bend, and even split in two.

Dr Mather said that meant the Lord Howe and Tasmantid chains were part of the same family of 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.

"So this research was essentially trying to unpick how that could possibly be."

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.

When the hotspot melts and punches through a weaker section of the plate, erupting lava accumulates, and builds a 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.

"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.

It takes a long time for a new seamount to form. In the case of the Lord Howe seamount, the Australian plate has moved north over a hotspot for more than 40 million years.

A twin Tasman volcanic chain

So, how did two chains form close together?

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.

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.

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.

"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.

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.

Dr Mather said two unrelated plumes, coming from separate patches of deep mantle, would generally not produce the same 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.

But she said more data was still needed to refine the model and its accuracy.

"It's a bit forensic; it's a bit like a detective story," Dr Orth said.

"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.

"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."

Dr Mather said further studies would be done, including high-resolution imaging of the ocean floor and more rock sample collection.

Why does this matter?

Dr Mather said their plume branching model might also explain other closely spaced plume 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.

He said it would also help map Earth's interior and tectonic plate 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.

"Here we have found that plumes are actually dynamic creatures that evolve as they encounter obstacles within the 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."

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.

"Based on our [simulation] model, dual chains can't be sustained forever," he said.

"Based on some evidence from eruption volumes for the Lord Howe and Tasmantid seamount chains, it seems the Lord Howe chain is running out of juice … whereas the Tasmantid chain is increasing its volumes.

"So this is a sign that perhaps some time in the distant future, millions of years into the future, the Lord Howe seamount chain will stop."

Dr Mather said it would be millions of years before a new volcano formed along the Tasmantid or Lord Howe chain.

"They don't have much significance in terms of danger, because the formation of each island in a chain is usually separated by about 5 million years," he said.

Dr Orth said, more broadly, understanding more about where future undersea volcanoes might form across the world could be helpful for shipping, tsunami predictions and understanding interactions between the atmosphere and the Earth.

"One of the things about the ends of volcanic hotspot tracks is they are usually under water … at the end of the Hawaii chain there is a volcano … which is just starting to form now on the sea floor," she said.

"The Earth is a complex thing and understanding some of it better, and knowing where we maybe need to go and look for some of these features could be really helpful long-term."