Paris-sized Pumice Stone Heading for Great Barrier Reef Traced Back to 'Severe' Volcano Eruption

The source of an enormous pumice stone spotted floating in the Pacific Ocean has been discovered. The raft, which measures about 52 square miles—twice the size of Manhattan and bigger than Paris—was created during a "severe" underwater volcanic eruption about 30 miles from the island nation of Tonga.

The pumice raft was first spotted in the waters of Tonga in August. Since then, it has been floating towards Australia and is due to arrive at the Great Barrier Reef at the end of January or start of February.

Where this enormous piece of pumice came from was unclear, although scientists said it likely formed during an underwater volcanic eruption.

Researchers led by Philipp Brandl, from Germany's GEOMAR Helmholtz Centre for Ocean Research Kiel, have now traced back the source of the pumice stone to a little-known volcano —named Volcano F—about 30 miles from the coast of the Tongan island Vava'u.

Publishing their findings in the Journal of Volcanology and Geothermal Research, the team reported how they used satellite data, seafloor maps and sonar to find out exactly where the pumice came from.

The satellite images, from the European Space Agency's Copernicus Sentinel-2, found traces of the eruption at the surface of the water. Because the images could be pinpointed to a specific spot in the ocean, they were then able to find the volcano responsible. The team then used seismic data to show how at the time of the eruption, there was a series of earthquakes close to the volcano.

pumice raft
The pumice raft captured by NASA on August 13, 2019. It is expected to reach the Great Barrier Reef early next year. NASA Earth Observatory

Their analysis showed the volcano produced an eruption measuring between 2 and 3 on the Volcanic Explosivity Index (VEI). Eruptions with a VEI 2 are considered "explosive" while those with VEI 3 are "severe." The team said the eruption at Volcano F was similar to a recent event at Mount Stromboli, which erupted earlier this year.

Volcano F was found to be about 31 miles in diameter and sits just 115 feet below sea level. It is an active volcano, having last erupted in 2001. The researchers hope to carry out more detailed research on it. The 2001 eruption also led to the formation of a pumice raft, but there was no detailed research on it or its size.

Unlike other rock, pumice is extremely porous, meaning it floats. The researchers say the pumice would have formed when the lava from the eruption was foamed with volcanic gasses such as carbon dioxide and water vapor. When the rock cools, the pores remain, making it less dense than water. "During an underwater eruption, the probability to generate pumice is particularly high," Brandl said in a statement.

Satellite images were then used to show how the pumice raft slowly started to drift west until it reached a size of 52 square miles. Eventually, it started floating away. It has since fragmented and parts were stranded on reefs and islands of Fiji.

The remainder will reach the Great Barrier Reef next year, where biologists are hoping it will disperse some of the fauna it collected in its journey across the ocean.