Global Geological And Climatic Events

Undersea Volcanic Eruption in Hunga Islands, Tonga


From UPSC perspective, the following things are important :

Prelims level: Hunga Volcano

Mains level: Not Much

A distant undersea volcano has erupted in spectacular fashion near the Pacific nation of Tonga sending large tsunami waves reaching the shore.

Hunga Volcano

  • The Hunga-Tonga-Hunga-Ha’apai volcano has erupted regularly over the past few decades.
  • 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.
  • 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.
  • Researchers suggest this is one of the massive explosions the volcano is capable of producing roughly every thousand years.

Impact of the eruption

  • The ash plume is already about 20km high.
  • Most remarkably, it spread out almost concentrically over a distance of about 130km from the volcano, creating a plume with a 260km diameter, before it was distorted by the wind.
  • The eruption also produced a tsunami throughout Tonga and neighbouring Fiji and Samoa.
  • Shock waves traversed many thousands of kilometres, were seen from space, and recorded in New Zealand some 2000km away.
  • All these signs suggest the large Hunga caldera has awoken.

Why is it so explosive even after being underwater?

Answer: Fuel-coolant interaction

  • 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.
  • 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.
  • Volcano researchers call this ‘fuel-coolant interaction’ and it is akin to weapons-grade chemical explosions.

A chain reaction

  • Extremely violent blasts tear the magma apart.
  • A chain reaction begins, with new magma fragments exposing fresh hot interior surfaces to water, and the explosions repeat, ultimately jetting out volcanic particles and causing blasts with supersonic speeds.

How has it emerged out to be so big?

  • The caldera is a crater-like depression around 5km across.
  • Small eruptions (such as in 2009 and 2014/15) occur mainly at the edge of the caldera, but very big ones come from the caldera itself.
  • These big eruptions are so large the top of the erupting magma collapses inward, deepening the caldera.
  • Looking at the chemistry of past eruptions, we now think the small eruptions represent the magma system slowly recharging itself to prepare for a big event.

What next?

  • This latest eruption has stepped up the scale in terms of violence.
  • Researchers are still in the middle of this major eruptive sequence and many aspects remain unclear, partly because the island is currently obscured by ash clouds.


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