Science

Volcanic rock sheet came from underwater volcano near Tongan island, say geologists


Mystery of where a giant pumice raft twice the size of MANHATTAN drifting towards Australia came from is finally solved – as geologists claim it was produced by an underwater VOLCANO in Tonga

  • Satellite imagery helped the geologists identify the source of the massive sheet
  • Images of ash plumes from European Space Agency match ‘Volcano F’s location
  • Pumice is a porous volcanic rock with a density so low that it can float on water 
  • The incredible sheet was captured on video by an Australian couple in August

Geologists have traced the origin of a huge ‘raft’ of floating rock in the Pacific Ocean that is three times the size of Sydney Harbour and twice as big as Manhattan. 

They say the 150-square kilometre mass of pumice was produced by the eruption of an underwater volcano 50 kilometres northwest of the Tongan island of Vava’u.

The geologists matched the location of the volcano, simply known as ‘Volcano F’, with images of huge eruption plumes captured by satellite imagery. 

The giant raft of rock was initially spotted near the volcano’s location by an Australian couple cruising in the Pacific back in August.

This pumice sheet twice the size of Manhattan was filmed by a couple on a cruise back in August

This pumice sheet twice the size of Manhattan was filmed by a couple on a cruise back in August

WHAT IS PUMICE? 

Pumice is a very porous volcanic glass.

Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano.

Its unusual foamy consistency comes from simultaneous rapid cooling and rapid depressurization.

Pumice can form during volcanic eruptions when viscous lava is foamed by volcanic gases

This creates so many pores in the cooling rock, making its density lower than that of water. 

‘When I then saw the reports on the pumice raft in the media in the summer, I became curious and started researching with my colleagues,’ said Dr Philipp Brandl, first author of the study.

Dr Brandl and his team studied imagery from a European Space Agency satellite, Copernicus Sentinel-2.

Imagery captured by the satellite from August 6 this year showed traces of an active underwater eruption on the water’s surface, including plumes of vapour and ash.

The eruption traces correspond exactly with the maps of the seafloor, which the team had previously surveyed. 

‘The eruption traces fit exactly to Volcano F’, said Dr Brandl.

Sentinel-2 satellite image of the eruption on 6 August 2019, with eruption plumes visible on the sea surface and possible gas/vapor released from eruptive drifting to the northwest

Sentinel-2 satellite image of the eruption on 6 August 2019, with eruption plumes visible on the sea surface and possible gas/vapor released from eruptive drifting to the northwest

The team had previously used sonar scanning between December 2018 and January 2019 to survey the seafloor around the volcano. 

The floor of volcano F’s large central caldera — the hollow that forms shortly after the emptying of a magma chamber — is 2,290 feet below the water surface.

The team traced the drift and dispersal of the pumice raft until mid-August.

It slowly drifted west and reached an area of up to 167 square kilometres – about twice the size of Manhattan.

This pumice will eventually break up and continue drifting towards Australia's east coast, and could impact as soon as January or February

This pumice will eventually break up and continue drifting towards Australia’s east coast, and could impact as soon as January or February

With the current direction and speed, the pumice raft is expected to hit Australia’s Great Barrier Reef at the end of January or beginning of February.

But this is good news for underwater life, say researchers, as the pumice may act as a travel raft for marine life that will restock the Great Barrier Reef. 

‘When it gets to Australia, it will be spread from Townsville in Queensland’s north to as far south as northern NSW,’ said Queensland University of Technology geologist Associate Professor Scott Bryan. 

‘It will be able to pick up corals and other reef building organisms, and then bring them into the Great Barrier Reef.

‘Each piece of pumice is a rafting vehicle. 

It’s a home and a vehicle for marine organisms to attach and hitch a ride across the deep ocean to get to Australia.

‘Then they’re going to grow and diversify, to ultimately wash up here in Australia.’

Some of the pumice Michael Hoult and his partner collected were the size of footballs

Some of the pumice Michael Hoult and his partner collected were the size of footballs

In August, an Australian couple came across the mass of rock in the waters of the island state of Tonga.

The couple collected samples of pumice, some the size of footballs, and described the conditions as dangerous to vessels.

The German, Canadian and Australian researchers published their evidence of the massive rock’s origin in the Journal of Volcanology and Geothermal Research.



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