From the depths

A VOLCANIC eruption that started in mid-December around 65km north-west of Nuku’alofa, the capital of Tonga, has ended. It leaves behind the world’s youngest land mass and the Pacific archipelago’s 177th island. Less than 2km across and rising 100m above the water’s surface, it appears to be made of volcanic ash rather than solidified lava (an attempt on January 24th to land and study it close up was foiled by the ocean swell). It may therefore prove short-lived.

Volcanic islands that last longer can be valuable. Scientists use Surtsey, which appeared off the coast of Iceland (itself a volcanic island) in 1963, to study the colonisation of virgin land by plants and animals. Since Surtsey at least ten underwater volcanoes have spewed forth enough material to breach the ocean’s surface and create new islands (see map).

Most are small and erode away soon afterwards. Japan’s proximity to the tectonic-plate boundaries known as the “Ring of Fire” makes it earthquake-prone, but also means it has gained three new islands since Surtsey appeared, a record now matched by Tonga. (One of Japan’s and two of Tonga’s have since vanished.) Other countries to gain new islands during that period include Indonesia and Yemen.

New creations that turn out to have staying power can also expand a country’s offshore territorial rights. Under the UN Convention on the Law of the Sea, countries can claim rights over fishing, shipping and mining up to 200 nautical miles (370km) from their coasts. A country that claims a new island off its coast can use it as the basis for a new offshore territorial claim, too.

Niijima, a volcanic island that appeared about 1,000km south of Tokyo in 2013, was at first expected to erode and vanish in short order. But it continued to grow, and last year merged with an older volcanic island, Nishinoshima. The joint land mass is still increasing. Once it settles down, and if it looks likely to last, Japan may make a new claim. Coastal erosion and rising sea levels mean that the sea often takes away land. But every now and then it gives it, too.