Volcanic Eruption in Hunga Islands, Tonga

Q What is the context  ?

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

Q What are some details about Hunga Volcano ?

A 

• 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.

Q What has been the impact of the eruption ?

A 

• 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.

Q Why is it so explosive even after being underwater?

A 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.

Q How has it emerged out to be so big?

 A

• 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.