We think of the world as mapped. Satellite footage enables us to observe the Earth’s surface in real-time, and globally there remain numerous disputes over borders and the ownership of islands, all based on the assumption that these lines are geologically permanent.
So, naturally, it can be a shock when what appears to be a fixed scene is suddenly rocked by the emergence of new geological features, as has happened recently in Tonga.
When the underwater Hunga Tonga-Hunga Ha’apai volcano erupted, it gave birth to an entirely new island, 45km (28 miles) northwest of Nuku’alofa, Tonga’s capital. The new island has a peak of up to 250m above sea level, and reaches over 500m across.
The event caused global headlines, including prospective ideas that it could become a popular tourist destination, for people interested in setting foot on a brand new island.
However, we shouldn’t think of this as a once-in-a-lifetime event, explains David Pyle, Professor of Earth Sciences at the University of Oxford.
‘The ocean floor is dotted with volcanic islands; many are uncharted, and long-since dead,’ Pyle tells Geographical. ‘But in many tectonically-active parts of the world, for example along subduction zones, where tectonic plates are sinking in to the Earth, there are many tens of young, and potentially active, submarine volcanoes, some of which are close enough to sea level that they may build up a new island each time they erupt.’
The question in the case of the new island in Tonga – and indeed for any similar new island stories – is whether the island is geologically able to withstand the eroding action of the sea, in order to become a permanent, mapped feature. ‘Volcanic rocks that are formed by shallow submarine eruptions are often finely fragmented, and made up of very small particles of ash, and pumice,’ says Pyle. He explains that these islands often quickly disappear beneath the waves again, once the volcanic eruptions which created them cease.
‘Some classic examples of shallow submarine volcanoes include the wonderfully-named ‘Kick ’em Jenny’ in the eastern Caribbean – completely submerged, at the moment – this last broke the surface in 1939,’ he continues. ‘Graham island, in the Mediterranean, south of Sicily, which emerged for a brief period in 1831, and sparked an international territorial row; and of course, Surtsey, on the mid-Atlantic ridge, a little south of Iceland, which formed from 1963–1967, and still survives as an island.’
The picture is further complicated by the potential existence of ‘floating islands’; huge floating rafts of pumice, also caused by large submarine eruptions, which can float around on the ocean for years. ‘Perhaps something like this has led to the appearance or disappearance of ghost islands, such as the non-existent Sandy island?’ muses Pyle.