The Kingdom of Tonga exploded into global news on January 15 last year with one of the most spectacular and violent volcanic eruptions ever seen.

Remarkably, it was caused by a volcano that lies under hundreds of metres of seawater. The event shocked the public and volcano scientists alike.

Was this a new type of eruption we’ve never seen before? Was it a wake-up call to pay more attention to threats from submarine volcanoes around the world?

• READ MORE: Why the volcanic eruption in Tonga was so violent, and what to expect next
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The answer is yes to both questions.

The Hunga Tonga-Hunga Ha’apai volcano was a little-known seamount along a chain of 20 similar volcanoes that make up the Tongan part of the Pacific “Ring of Fire”.

We know a lot about surface volcanoes along this ring, including Mount St Helens in the US, Mount Fuji in Japan and Gunung Merapi of Indonesia. But we know very little about the hundreds of submarine volcanoes around it.

It is difficult, expensive and time-consuming to study submarine volcanoes, but out of sight is no longer out of mind.

Tongan eruption breaks records
The Hunga Tonga-Hunga Ha’apai eruption has firmly established itself in the record books with the highest ash plume ever measured and a 58km aerosol cloud “overshoot” that touched space beyond the mesosphere. It also triggered the largest number of lightning bolts recorded for any type of natural event.

The injection of large amounts of water vapour into the outer atmosphere, along with “sonic booms” (atmospheric pressure waves) and tsunami that travelled the entire world, set new benchmarks for volcanic phenomena.

The Hunga Tonga-Hunga Ha’apai eruption has firmly established itself in the record books with the highest ash plume ever measured.

Covid hampered access to Tonga during the eruption and its aftermath, but local scientists and an international scientific collaborative effort helped us discover what drove its extreme violence.

Eruption creates a giant hole
A team from the Tongan Geological Services and the University of Auckland used a multi-beam sonar mapping system to precisely measure the shape of the volcano, just three months after the January blast.

We were astonished to find the rim of the vast submarine volcano was intact, but the formerly 6km diameter flat top of the submarine cone was rent by a hole 4km wide and almost 1km deep.

This is known as a “caldera” and happens when the central part of the volcano collapses in on itself after magma is rapidly “pumped out”. We calculate over 7.1 cubic kilometres of magma was ejected. It is almost impossible to envisage, but if we wanted to refill the caldera, it would take one billion truck loads.

It is hard to explain the physics of the Hunga eruption, even with the large magma volume and its interaction with seawater. We need other driving forces to explain especially the climactic first hour of the eruption.

Mixed magmas lead to chain reaction
Only when we examined the texture and chemistry of the erupted particles (volcanic ash) did we see clues about the event’s violence. Different magmas were intimately mixed and mingled before the eruption, with contrasts visible at a micron to centimetre scale.

Isotopic “fingerprinting” using lead, neodymium, uranium and strontium shows at least three different magma sources were involved. Radium isotope analysis shows two magma bodies were older and resident in the middle of the Earth’s crust, before being joined by a new, younger one shortly before the eruption.

The mingling of magmas caused a strong reaction, driving water and other so-called “volatile elements” out of solution and into gas. This creates bubbles and an expanding magma foam, pushing the magma out vigorously at the onset of eruption.

This intermediate or “andesite” composition has low viscosity. It means magma can be rapidly forced out through narrow cracks in the rock. Hence, there was an extremely rapid tapping of magma from 5-10km below the volcano, leading to sudden step-wise collapses of the caldera.

The caldera collapse led to a chain reaction because seawater suddenly drained through cracks and faults and encountered magma rising from depth in the volcano. The resulting high-pressure direct contact of water with magma at more than 1150℃ caused two high-intensity explosions around 30 and 45 minutes into the eruption. Each explosion further decompressed the magma below, continuing the chain reaction by amplifying bubble growth and magma rise.

After about an hour, the central eruption plume lost energy and the eruption moved to a lower-elevation ejection of particles in a concentric curtain-like pattern around the volcano.

This less focused phase of eruption led to widespread pyroclastic flows – hot and fast-flowing clouds of gas, ash and fragments of rock – that collapsed into the ocean and caused submarine density currents. These damaged vast lengths of the international and domestic data cables, cutting Tonga off from the rest of the world.

Unanswered questions and challenges
Even after long analysis of a growing body of eyewitness accounts, there are still major unanswered questions about this eruption.

The most important is what led to the largest local tsunami — an 18-20m-high wave that struck most of the central Tongan islands around an hour into the eruption. Earlier tsunami are well linked to the two large explosions at around 30 and 45 minutes into the eruption. Currently, the best candidate for the largest tsunami is the collapse of the caldera itself, which caused seawater to rush back into the new cavity.

This event has parallels only to the great 1883 eruption of Krakatoa in Indonesia and has changed our perspective of the potential hazards from shallow submarine volcanoes. Work has begun on improving volcanic monitoring in Tonga using onshore and offshore seismic sensors along with infrasound sensors and a range of satellite observation tools.

All of these monitoring methods are expensive and difficult compared to land-based volcanoes. Despite the enormous expense of submarine research vessels, intensive efforts are underway to identify other volcanoes around the world that pose Hunga-like threats.

Dr Shane Cronin is professor of earth sciences, University of Auckland.This article is republished from The Conversation under a Creative Commons licence. Read the original article.

Vanuatu sits on the Pacific Ring of Fire and though it is accustomed to being hit by cyclones and volcanoes, it does not have clear guidelines or policies ready to respond to major emergencies.

Prime Minister Charlot Salwai stressed these concerns when he was briefing journalists at the weekend on the government order’s for a mandatory evacuation of the entire Ambae Island following the Lombenben volcano crisis.

READ MORE: RNZI reporter Koroi Hawkins in Vanuatu on strained resources for the evacuees

He said there needs to be clear policies to address disaster situations.

While Salwai said the government – through the Ministry of Climate Change and Disaster Management – was responsible to protect and save lives in emergencies, the prime minister admitted the country did not have clear plans in place to respond to specific disasters.

“We are not ready for disasters,” Salwai said.

“We have evacuation centers standby for cyclones that serves no purpose during volcano eruptions and tsunamis.

“In Vanuatu, many settlements are exposed to the coast. This is why we need a separate disaster plan for tsunamis,” he said

“With seven active volcanoes, and earthquakes occurring frequently, Vanuatu is at risk.”

‘Drowning’ islands fears
The prime minister has stressed fears about the possibility of islands in Vanuatu “drowning” due to the looming effects of climate change in the future.

“We must have our own national policies in place before we call for international help or while we continue to address climate change abroad, both regionally and internationally,” he said.

Prime Minister Salwai said the government did not want to get blamed for a large scale disaster, therefore it had issued the order for the Ambae evacuation so that it could provide mass care.

“It is better to evacuate than leave lives at risk,” he said.

“The government will extend the state of emergency period if the volcano activity remains in this stage.

“The affected families from Ambae rescued to neighboring islands will remain in temporary shelters until such time the state of emergency be lifted.”

Remain calm appeal
The President, Obed Moses, said the Lombenben volcano crisis must be considered a priority by the government, and every effort must be taken to move people to safety immediately.

The Head of State called on all citizens to remain calm and not to interfere with authorities handling the situation.

Director of the National Disaster Management Office (NDMO), Shadrack Welegtabit, said efforts were underway to evacuate people quickly to safety from the volcano affected island before next Friday, October 6.

An order was issued for all ships to standby on nearby shores for any request of assistance on evacuating 11,600 people to safety. Evacuation from Ambae officially started on Saturday but people were already voluntarily moving out.

According to the Director, 400 people had alread moved at their own cost.

Schools, community halls and churches on neighboring islands of Pentecost, Maewo and Santo were opening up to accommodate evacuees until the government found proper safe places and set up shelters and facilities, said Director Welegtabit.

A reported 283 final year students in schools on Ambae were given priority to be evacuated before mock examinations this week.

The Ministry of Education will waive school fees for third term this year and first term of 2018 for parents living on Ambae, according to an agency order.

Discounted airfares
The order, which was signed by the Minister of Climate Change and Disaster Management, Ham Lini, on the advice of the National Disaster Committee, provided for government shareholders to allow a discount of 50 percent on air fares on all Air Vanuatu flights from Ambae Island.

“Also, all government vehicles under the possession of government on Ambae be used to evacuate people from villages.”

Meanwhile, conditions remained difficult in evacuation centers with overcrowding on Ambae.

In Port Vila, business people, communities, churches,non-government organisations, international partners, including students offered humanitarian assistance.

The patrol boats LC Urata and LC Mahalia departed for Ambae with relief items, including water, hygiene kits, kitchen kits, tarpaulins, fuel and two lorries to assist with the transportation of people to ports.

Anita Roberts is a Vanuatu Daily Post reporter. Asia Pacific Report has permission to republish articles.

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ANALYSIS: By Chris Firth

If you turned on the television this week, you may have seen coverage of the potentially imminent eruption of Mount Agung volcano in Bali.

However, Mt Agung is not the only volcano in the region behaving badly. An evacuation of 11,000 residents in Vanuatu has been announced thanks to increasing levels of activity at Ambae volcano.

READ MORE: Bali’s Mount Agung threatens to erupt for the first time in more than 50 years

While both Ambae and Agung pose significant threats to local populations, they represent very different types of volcanoes.

In fact, the unique features of the Ambae volcano mean it presents immediate danger.

What’s special about the Ambae volcano?
Ambae does not fit the stereotypical image of a volcano. Rather than being a steep-sided cone, it forms a low-angled mountain, reminiscent of shield lying flat on the earth.

Instead of having a vertiginous vent filled by a lava lake (like its southern neighbour Ambrym), the summit contains a shallow depression featuring several water-filled lakes.

The largest of these, Lake Voui, is the current focus of volcanic activity, and looks unlike any lake you have seen before.

Volcanic gasses, including sulfur, chlorine and carbon dioxide, are discharged into the base of the lake. Not only do these make the lake highly acidic, but they typically give it a vibrant turquoise colour.

When the volcano last erupted in 2005, ash and lava built a cone in the centre of the lake, which eventually reached a height of around 50 metres above the lake surface.

As this happened, changing degrees of interaction between the lava, volcanic gases and the lake water caused fluctuations in its chemistry. This in turn changed the colour, which went from turquoise to battleship grey and then finally to a deep mahogany shade of red.

Since then, the volcano has continued to emit huge volumes of gas, which have caused issues for local inhabitants over recent years, as they can lead to acid rain.

Acid rain can kill plants. This is a major issue on Ambae, as much of the population lives on staple crops such as banana and taro. These plants have large leaves that are particularly susceptible to acid rain.

Over the past few weeks, gas emissions from Ambae have increased. Ash began to accompany the gas emissions around mid-September, suggesting that magma had reached the surface.

These changes in volcanic activity have repeatedly led the Vanuatu Meteorology and Geohazards Department to increase the alert level for the volcano.

Satellite monitoring indicates that volcanic activity is continuing to escalate. Recent observations by New Zealand Air Force pilots noted lava blasting out of a crater in the centre of Lake Voui.

Is this part of the Ring of Fire?
Both Bali’s Agung and Ambae sit on the Pacific’s “ring of fire”, and the same tectonic forces are responsible for both volcanoes. However, closer links between the two volcanoes are very unlikely.

On any given day, there are generally 20-30 volcanoes erupting around the world (although normally these eruptions are on a smaller scale and are away from large populations, so they do not make the news).

So how might the eruption at Ambae differ from Agung? The crater lake on Ambae offers particular hazards that might not be encountered elsewhere.

The first of these involves interaction between erupting lava and the lake water itself. The heat of the lava, which is likely to be 1000-1100℃, will rapidly turn lake water into steam, like dipping a hot frying pan into a sink of dishwater.

READ MORE: Vanuatu not ready for disasters, says PM

This scaled-up kitchen scenario can increase how explosive the eruption is, giving blasts from the volcano additional power. This may cause projectiles like lava bombs to go further, while also increasing the amount of ash produced.

A potentially more serious hazard may involve overflowing of the crater lake itself. If the eruption begins to displace water from the lake, it might trigger volcanic mudslides known as “lahars”, which would race down the volcano’s flanks, with the potential to inundate villages and gardens.

Local stories suggest villages on the island’s south coast were affected by lahars during the late 19th century, with significant loss of life.

Finally, there is a threat that activity may not be restricted to the volcano’s summit. The geological record indicates that magma has moved through fissures in the volcano’s flanks during previous eruptions, travelling laterally up to 20km from the centre of the volcano before erupting.

This means that rather than emerging on the sparsely inhabited summit of the volcano, lava may well erupt along the more densely populated coast. Such a scenario occurred in 1913 on the neighbouring volcano, Ambrym, where 21 people died.

The evacuation of the Ambae’s population will prevent such loss of life if this were to occur again.

Dr Chris Firth is a lecturer in geology at Macquarie University in Sydney. This article is republished from The Conversation under a Creative Commons licence.

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