
Volcanic Eruption in Hunga Islands, Tonga
Feb 20, 2022
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.