Underwater Explosion that Reached Space: Volcanologist Comments on the Details of the Eruption of Hunga-Tonga

Шлейф дыма от вулкана, сфотографированный российским спутником. Фото: twitter.com/roscosmos

The eruption of the underwater volcano Hunga-Tonga-Hunga-Ha’apai, which occurred in Polynesia on January 14, became, according to geologists, one of the largest. The ash column rose to a height of 20 kilometers and went beyond the troposphere — for the first time a shock wave could be observed from space. Sergey Samoylenko, a volcanologist, founder of the Vulcanarium Museum in Kamchatka, tells how the new island of the Kingdom of Tonga was born, how the eruption occurred and whether we should be expecting new dangers.

The volcanic island Hunga-Tonga arose relatively recently, in 1912, in the very place of the arc of the Tongan archipelago, where the Pacific and Australian plates meet. Over the hundred years of its existence, it erupted several times and, eroded by the sea, gradually broke into two islets – Hunga-Tonga and Hunga-Ha’apai. In 2014, during another long eruption, the separated parts of the land fused back into one island, which received the long name Hunga-Tonga-Hunga-Ha’apai.

"The new activity of Hunga-Tonga began on New Year's Eve, December 29: an underwater eruption began in the center of the fused island, and two weeks later it reached its peak," says Sergey Samoylenko. “On January 15, there was a really big explosion. The ash column rose to a height of 20 kilometers — for the first time we could observe the shock wave from space. We could see it reach the tropopause, change clouds, participate in the process of evaporation and condensation. The explosion has not yet been evaluated on the Volcanic Explosivity Index, but these events give us the opportunity to assume that the index will be 7 out of possible 8, when the volume of ejected materials is more than 100 cubic kilometers."

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Извержение вулкана Хунга-Тонга, вид с орбиты. Фото: NASA

In our time, over the past 2000 years, there have been about four eruptions with an index of 7, not a single catastrophe has reached the 8-point mark. The scale of activity of the underwater volcano Hunga-Tonga is comparable with one of the most destructive volcanic tragedies in history, which occurred in Indonesia in 1883, when the explosions of Krakatoa destroyed most of the island and more than a hundred cities and settlements. The eruption of Krakatoa was rated  6 on the Volcanic Explosivity Index. However, significant damage was caused not so much by the eruption itself as by the tsunami that followed it.

"An underwater explosion sets in motion a large amount of water," our expert continues. “When an earthquake occurs at the bottom of the ocean, about three kilometers of water come into motion, but with an underwater explosion, only the first hundred meters. Therefore, the tsunami that arose after the eruption of Hunga-Tonga did not bring significant destruction. It passed all over the Tongan archipelago, reached New Zealand and Australia, was recorded in California, but the wave height did not exceed three meters. Of course, this causes significant damage and erodes the coastline, but due to the isolation and remoteness of the islands of the Tongan archipelago, it cannot be compared to the disaster after the destruction of Krakatoa – then the tsunami claimed many more lives than the eruption itself."

Despite such a powerful explosion, which contributed to another geological change in the Polynesian region, new catastrophes of this scale are not expected in the near future.

"After the ashes had dispersed, we saw the wreckage of the island, which once again split in two: Hunga-Tonga and Hunga-Ha’apai again became separate islands," notes Sergey Samoylenko. “From all this we can conclude that there was a paroxysm — a powerful eruption that emptied the upper part of the magma chamber. Paroxysm precedes a long period of rest, so we will not hear about this volcano soon. A little activity is likely to continue, due to which a new volcano will build up between the islands, but there will not be large-scale disasters here for a very long time."