Volcanic eruptions are not new to Tonga. A large part of the Polynesian nation, an archipelago of more than 170 islands, owes its existence to volcanic activity, which created its western island chain thousands of years ago.
The Home Reef, an active underwater volcano in the area, is the mother of a handful of them. From time to time, it emits a mixture of lava, steam and ash that piles up, and boom, an island or a sea mound is born.
That was the case earlier this month, when a baby island entered the water 11 hours after the Home Reef eruption on Sept. 10, NASA announced in a recent press release. It started at about an acre, but in a matter of days it has grown to about 8.6 acres due to the volcano’s recent flurry of eruptions, according to Tonga Geological Services, a government agency.
While the word island might conjure up images of sandy beaches and lush vegetation, that’s not exactly the case with these volcanic islets.
“It’s more like a big layer of ash, steam and pumice over the ocean,” Rennie Vaiomounga, a geologist with Tonga Geological Services, told The Washington Post. This means the new island isn’t even tough enough to walk on, although that could change if it stays long enough to solidify.
Eruptions on Home Reef, located in the Tonga-Kermadec subduction zone, one of the world’s most active volcanic arcs, have often produced new land masses. But the sporadic appearance of the resulting islands is a bit of a “geological puzzle,” Vaiomounga said.
“We never know when the island will appear or when it will disappear,” he said.
The Tonga volcano released an unprecedented amount of water into the atmosphere
It can take centuries, decades, or sometimes just a couple of years for a volcano to erupt and form an island. Home Reef’s first recorded eruption was in 1852. Five years later, it erupted again. In both cases small islands occurred, 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 small islands take a different shape. The 1984 one—which measured a little over 185 hectares—looked like a rectangular cheese board, handles and all. By 2006, the island that formed was more like a thumb sticking out of the water, with the rounded mound developing cliffs that were at least 164 feet high.
This time, the new island looks like an almost perfectly circular mole that rises about 49 feet above the surface of the ocean. Its surface area is large enough to fit standard 6½ football fields.
When volcanoes erupt, the magma fragments into small glass fragments that shoot into the air. — also known as ash, Vaiomounga said. If this mixture of shattered minerals, glass and rocks comes out of an undersea 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 these minerals travel hundreds of kilometers away and eventually reach the coast. But they can also build and form an island.
How long the nascent island will survive is another matter. The 2006 one, for example, collapsed in 2008, when the top of the volcano sank about 33 feet underwater, according to Smithsonian Institution records. The Late’iki volcano, also in Tonga, created an island that disappeared after two months in 2020. This same volcano had previously produced an island that remained for 25 years, according to NASA.
Ephemeral islands often do not live long because erosion wipes them out. The minerals that make them islands slowly return to the seabeds of their creators, which will then wash up new islands in the future, a geological circle of life, Vaiomounga said.