Most tsunamis are triggered by earthquakes; only about 5% are associated with volcanoes. So what happened in the Pacific?
Hunga Tonga-Hunga Ha’apai, and the ensuing tsunami, packed a devastating punch
OST OF THEworld’s volcanism takes place underwater. This is not just because most of the world’s surface is underwater. It is because plate tectonics has placed the seams where the Earth’s crust is created and destroyed: in the ocean’s depths. The volcanic activity associated with creation is slow and steady, and it takes place a long way beneath the waves, all of which makes it very benign. That which accompanies the destruction is a lot more troublesome.
The Economist todayHandpicked stories, in your inboxA daily newsletter with the best of our journalismSign upWitness the eruption of the Hunga Tonga-Hunga Ha’apai volcano in the Kingdom of Tonga at 04:15GMT on January 15th, which created a huge cloud of ash and a potentially devastating tsunami.Read more: The Economist »
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M OST OF THE world’s volcanism takes place underwater. This is not just because most of the world’s surface is underwater. It is because plate tectonics has placed the seams where the Earth’s crust is created and destroyed: in the ocean’s depths. The volcanic activity associated with creation is slow and steady, and it takes place a long way beneath the waves, all of which makes it very benign. That which accompanies the destruction is a lot more troublesome. The Economist today Handpicked stories, in your inbox A daily newsletter with the best of our journalism Sign up Witness the eruption of the Hunga Tonga-Hunga Ha’apai volcano in the Kingdom of Tonga at 04:15GMT on January 15th, which created a huge cloud of ash and a potentially devastating tsunami. The eruption was driven by subduction which sees one plate sink beneath the edge of another. Subduction zones are responsible for most of the world’s most violent eruptions, most famously in the “Ring of Fire” (see map)—actually more like a horseshoe—around the Pacific. The Tonga Trench sits at its south-western tip. The volcanoes associated with the trench do not tower into the sky like those of Chile, Indonesia or Japan. But they can still pack a punch. Hunga Tonga-Hunga Ha’apai, which has a base about 20km across and rises some 1,800 metres above the seafloor, takes its name from the two parts of the rim of its crater, or caldera, that rise above the sea: the small uninhabited islands of Hunga Tonga and Hunga Ha’apai. An eruption that started in December 2014 filled in the gap between them with lava and ash, making the two islands one. A series of eruptions which began late last year saw the now unified island grow further. In the eruption of January 15th a far larger amount of molten rock escaped from the magma chamber below the caldera. The sudden depressurisation of the gases dissolved within the lava, along with the vaporisation of vast amounts of seawater, made the eruption spectacularly explosive. A cloud of ash reached more than 20km into the sky; a global system of sensors operated by Vaisala, a Finnish company, revealed the column of ash spewing forth to be roiling with lightning—some 100 bolts a second at the peak. The explosion was audible many thousands of kilometres away, its shock wave measured all around the world. Satellites showed the dust it threw up spreading out into a disc more than 250km across once it reached the stratosphere; it looked as if the planet had developed a pimple half as big as England. Closely linked to this explosion above the surface seems to have been an implosion below. When an eruption empties out a magma chamber the rock above it is undermined. This is the process which creates calderas. The eruption on January 15th seems to have seen at least a partial collapse; satellite data show that only a small part of one of the old islands remains above the waves. The collapse probably played a role in the tsunami that followed. Most tsunamis are triggered by earthquakes; only about 5% are associated with volcanoes. But the shock wave which comes from a caldera collapse is an effective way of starting one. How much damage the eruption did is as yet unclear. On January 17th Australia and New Zealand sent surveillance flights to assess the damage. Communications with Tonga have been sparse. Satellite pictures show tsunami damage to various structures near the coast of Tongatapu, the main island in the kingdom, which is 65km south of the volcano. Heavy falls of volcanic dust—centimetres or even tens of centimetres—will have come down on the island, too, imperilling fresh-water supplies and other vital infrastructure. Effects beyond Tonga, though, are likely to be minimal. Some large volcanic eruptions affect the climate by injecting large amounts of sulphur dioxide into the stratosphere, where it produces a long-lasting smog of tiny particles which reflect sunlight and thus change weather patterns around the world. The amount of sulphur that came out of Hunga Tonga-Hunga Ha’apai seems much too low to have any large-scale cooling effect, though its influence on the temperature and dynamics of the southern hemisphere’s stratosphere may be large enough to be of interest to scientists. Geologists have found evidence of similar spasms at Hunga Tonga-Hunga Ha’apai, one about 1,000 years ago, one about 2,000 years ago. In both of those cases there seem to have been multiple eruptions. That does not necessarily mean the same will happen this time. The people living close to the volcano will hope that there is no more damage to come. More from