By María Luisa Paúl
Volcanic eruptions are not new in Tonga.
A big chunk of this Polynesian nation – an archipelago of over 170 islands – owes its existence to volcanic activity, which created its chain of western islands starting thousands of years ago.
The Home Reef, an active underwater volcano in the area, is the mother of a handful of them. Every so often, it spews out a mix of lava, steam and ash that piles up – and boom, an island, or sea mound, is born.
That was the case earlier this month, when a baby island poked through the water 11 hours after the Home Reef erupted on September 10, Nasa announced in a recent news release.
It started out about half a hectare in extent, but in a matter of days has grown to about 3.5ha because of the volcano's recent flurry of eruptions, according to Tonga Geological Services, a government agency.
While the word island might evoke images of sandy beaches and lush vegetation, that's not quite the case with these volcanic islets.
"It's more like a large layer of ash, steam and pumice over the ocean," Rennie Vaiomounga, a geologist at Tonga Geological Services, told The Washington Post. That means the new island isn't even sturdy enough to walk on, though that could change if it sticks around long enough to solidify.
Eruptions in the Home Reef – located in the Tonga-Kermadec subduction zone, one of the world's most active volcanic arcs – have often produced new landmasses. But the sporadic emergence of resulting islands is somewhat of a "geological puzzle", Vaiomounga said.
"We never know when the island will appear or when it will disappear," he said.
It can take centuries, decades or sometimes just a couple of years for a volcano to erupt and form an island. The first recorded Home Reef blast was in 1852.
Five years later, it erupted once more. In both cases, small islands were produced, but they were temporary. The same thing happened again in 1984 and 2006, according to the Smithsonian Institution's Global Volcanism Program.
Each time, the little islands take on a different shape. The one from 1984 – which measured a little over 74ha – looked like a rectangular cheese board, with handles and all. In 2006, the island that formed was more akin to a thumb sticking out of the water, with the rounded mound developing cliffs that stood at least 50 metres tall.
This time around, the new island looks like an almost perfectly circular mole poking out some 15m above the ocean's surface. Its surface is big enough to fit 6 1/2 standard football fields.
When volcanoes erupt, magma gets fragmented into teeny-tiny glass shards that get shot into the air – otherwise known as ash, Vaiomounga said. If that mix of shattered minerals, glass and rocks comes out of an underwater volcano, like the Home Reef, it drifts into the ocean.
A telltale sign of an underwater eruption is a pool of volcanic pebbles – or pumice – that leaves the surface of the water looking like a rocky beach, Vaiomounga added.
Sometimes, those minerals travel hundreds of miles away and eventually wash ashore. But they can also build up and form an island.
How long the nascent island will survive is another question. The one from 2006, for example, sank by 2008, when the volcano's summit plunged about 10m below the water, according to Smithsonian Institution records.
The Late'iki volcano, also in Tonga, created an island that disappeared after two months back in 2020. That same volcano had previously produced an island that remained for 25 years, according to Nasa.
The ephemeral islands often don't live long because erosion chips away at them. The minerals that turn them into islands slowly return to the sea beds of their makers, which will then spew new islands in the future – a geological circle of life, Vaiomounga said.
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