Arguably never before has an undersea volcanic eruption been so intensely filmed as the eruption of Hunga-Tonga-Hunga-Ha'apai in the South Pacific on Friday evening. In the images from two weather satellites hovering 36,000 kilometers above the earth's surface, the American Goes-17 and the Japanese Himawari-8, a huge cloud can be seen suddenly emerging from the sea shortly before sunset. Like a mushroom cloud, it spreads kilometers high over the entire island kingdom of Tonga within a few minutes.

Lightning flashed continuously in the cloud, and ash rained down in the Tongan capital of Nuku'alofa, 40 miles from the eruption site. The eruption was so strong that a dull rumbling could be heard in New Zealand, more than 2000 kilometers away. At the same time there was a tsunami warning for the entire Pacific region. The coast of the main Tongan island of Tongatapu was flooded. Hours later, even in Japan and on the Californian coast, wave heights of up to one meter were measured.

The first reports of the submarine volcano were already in 1912. At that time, fishermen saw the sea "bubbling" between two small elongated islands. The uninhabited islands named Hunga-Tonga and Hunga-Ha'apai, each a little more than a kilometer long, are the remains of an old crater rim of a previously unknown submarine volcano. It then remained largely quiet for almost 100 years, until in the spring of 2009, a few hundred meters west of Hunga-Ha'apai, a new island suddenly began to grow with a great deal of noise. Further eruptions in 2015 filled the space between the two original islands with ash, causing them to coalesce. Since then, the volcano has borne the name of both islands. Over the past year, more and more volcanic ash accumulated until the gap was completely filled.

Plate shifts cause volcanism

Shortly before Christmas the volcano started to erupt again.

More powerful explosions and the erosive force of the sea waves caused much of the ash to be swept away between the two islands.

To study these effects in more detail, a group of Tongan geologists led by the Deputy Minister for Natural Resources left on a Navy ship on Friday morning in the direction of the volcano.

Throughout the day they saw smaller explosions in the sea area.

Shortly after 5 p.m., the massive eruption that could later be seen on the satellite images took place.

Ash was thrown more than ten kilometers high.

The geologists were still able to get to safety.

The volcanism in the southwest Pacific occurs because the Pacific crustal plate pushes under the Australian plate between New Zealand and Samoa.

This creates the more than 2000 km long Kermadec-Tonga Trench.

The collision of the plates does not only cause earthquakes.

Because the subducting Pacific Plate is gradually melting on its way into the Earth's mantle, volcanism occurs to the west of the Deep Sea Trench.

Most dangerous land-based fire mountains, such as Mount Fujiyama in Japan, Cotopaxi in Ecuador, or Popocatepetl near Mexico City, form over similar subduction zones.

In the South Pacific, volcanoes remain hidden beneath the surface of the sea until they explode in spectacular eruptions, as happened in Tonga over the weekend.

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