English Article

Why the Volcanic Eruption in Tonga was so Violent

The Hunga-Tonga-Hunga-Ha'apai volcano has erupted regularly over the past few decades. During events in 2009 and 2014/15 hot jets of magma and steam exploded through the waves. But these eruptions were small, dwarfed in scale by the January 2022 events. The Kingdom of Tonga doesn’t often attract global attention, but a violent eruption of an underwater volcano on January 15 has spread shock waves, quite literally, around half the world.The volcano is usually not much to look at. It consists of two small uninhabited islands, Hunga-Ha’apai and Hunga-Tonga, poking about 100m above sea level 65km north of Tonga’s capital Nuku’alofa. But hiding below the waves is a massive volcano, around 1800m high and 20 kilometres wide. The Hunga-Tonga-Hunga-Ha’apai volcano has erupted regularly over the past few decades. During events in 2009 and 2014/15 hot jets of magma and steam exploded through the waves. But these eruptions were small, dwarfed in scale by the January 2022 events.Our research into these earlier eruptions suggests this is one of the massive explosions the volcano is capable of producing roughly every thousand years. 

Why are the volcano’s eruptions so highly explosive, given that sea water should cool the magma down?

If magma rises into sea water slowly, even at temperatures of about 1200 degrees Celsius, a thin film of steam forms between the magma and water. This provides a layer of insulation to allow the outer surface of the magma to cool. Also Read Concerns mount for Tonga after tsunami triggered by underwater volcano But this process doesn’t work when magma is blasted out of the ground full of volcanic gas. When magma enters the water rapidly, any steam layers are quickly disrupted, bringing hot magma in direct contact with cold water.