Add to collection
  • + Create new collection
  • In January 2022 the world was shocked to hear of a sudden huge volcanic eruption close to Tonga, causing a tsunami and volcanic ash to cover the Kingdom’s islands.

    This article has been republished from The Conversation under Creative Commons licence CC BY-ND 4.0 and is written by Shane Cronin, Professor of Earth Sciences, University of Auckland.

    The Kingdom of Tonga doesn’t often attract global attention, but a violent eruption of an underwater volcano on 15 January has spread shock waves, quite literally, around half the world.

    Rights: NIWA

    Satellite imagery of the Hunga Tonga–Hunga Ha’apai volcanic eruption

    This tweet from NIWA shows high-resolution Himawari satellite imagery of the Hunga Tonga–Hunga Ha’apai volcanic eruption near Tonga on 15 January 2022 as it happened.

    Imagery courtesy of Himawari satellite, Meteorological Satellite Center/JMA.

    The volcano consists of two small uninhabited islands, Hunga-Ha’apai and Hunga-Tonga, poking about 100 m above sea level 65 km north of Tonga’s capital Nuku‘alofa and hiding below the waves is a massive volcano, around 1800 m high and 20 km wide.

    Rights: Professor Shane Cronin

    Hunga Tonga–Hunga Ha’apai volcano map

    A massive underwater volcano lies next to the Hunga-Ha’apai and Hunga-Tonga islands.

    The Hunga-Tonga-Hunga-Ha'apai volcano has erupted regularly over the past few decades. 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.

    Our research into these earlier eruptions suggests this is one of the massive explosions the volcano is capable of producing roughly every thousand years.

    Why are the volcano’s eruptions so highly explosive, given that sea water should cool the magma down?

    If magma rises into sea water slowly, even at temperatures of about 1200℃, 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. 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

    Two scales of Hunga eruptions

    The 2014/15 eruption created a volcanic cone, joining the two old Hunga islands to create a combined island about 5 km long. We visited in 2016, and discovered these historical eruptions were merely curtain raisers to the main event.

    Mapping the sea floor, we discovered a hidden 'caldera' 150 m below the waves.

    Rights: Professor Shane Cronin

    Hunga Tonga–Hunga Ha’apai seafloor map

    A map of the seafloor shows the volcanic cones and massive caldera of Hunga Tonga–Hunga Ha'apai.

    The caldera is a crater-like depression around 5 km 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.

    We found evidence of two huge past eruptions from the Hunga caldera in deposits on the old islands. We matched these chemically to volcanic ash deposits on the largest inhabited island of Tongatapu, 65 km away, and then used radiocarbon dates to show that big caldera eruptions occur about every 1000 years, with the last one at AD1100.

    With this knowledge, the eruption on 15 January seems to be right on schedule for a 'big one'.

    What we can expect to happen now

    We’re still in the middle of this major eruptive sequence and many aspects remain unclear, partly because the island is currently obscured by ash clouds.

    The two earlier eruptions on 20 December 2021 and 13 January 2022 were of moderate size. They produced clouds of up to 17 km elevation and added new land to the 2014/15 combined island.

    The latest eruption has stepped up the scale in terms of violence. The ash plume is already about 20 km high. Most remarkably, it spread out almost concentrically over a distance of about 130 km from the volcano, creating a plume with a 260 km diameter, before it was distorted by the wind.

    The GIF above showing ash plume is from GOES West satellite, courtesy of NOAA.

    This demonstrates a huge explosive power – one that cannot be explained by magma-water interaction alone. It shows instead that large amounts of fresh, gas-charged magma have erupted from the caldera.

    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 2000 km away. Soon after the eruption started, the sky was blocked out on Tongatapu, with ash beginning to fall.

    All these signs suggest the large Hunga caldera has awoken. Tsunamis are generated by coupled atmospheric and ocean shock waves during an explosion, but they are also readily caused by submarine landslides and caldera collapses.

    It remains unclear if this is the climax of the eruption. It represents a major magma pressure release, which may settle the system.

    A warning, however, lies in geological deposits from the volcano’s previous eruptions. These complex sequences show each of the 1000-year major caldera eruption episodes involved many separate explosion events.

    Rights: UNOSAT

    How an eruption changed an island

    These two satellite images show the dramatic change to the Hunga Tonga-hunga Ha’apai volcano after the explosion on 15 January 2022. It clearly shows the destroyed ‘land bridge’ area around the volcano and collapse of the caldera. Due to disrupted communications systems, satellites have played an important role in gathering data about the eruption and its consequences.

    Before image from Sentinel 1 satellite. After image from Pléiades satellite, © CNES.

    Hence we could be in for several weeks or even years of major volcanic unrest from the Hunga-Tonga-Hunga-Ha'apai volcano. For the sake of the people of Tonga I hope not.

    Related content

    This article looks at what we’ve learnt a year later about this violent eruption and predicting future submarine volcanic eruptions.

    We have a wide range of related resources curated in these handy introductory articles below:

    Realistic contexts connect students to authentic scientific processes and purposes. It’s all explained in Volcanoes resources – planning pathways.

    The PLD article Children making evidence-based decisions about volcanic risk details how students were able to use science knowledge to make decisions and take action to plan for natural disaster emergencies.

    We have also created a Pinterest board to current and new curate content relevant to this eruption.

    Activity ideas


    Hunga Tonga–Hunga Ha’apai is a caldera volcano. The Rotorua geothermal area was formed by a massive caldera. Watch the Rotorua caldera formation animation then go outside and conduct the student activity Calderas in the sandpit.

    Go on a volcano hunt covering the length of Aotearoa, and discover how scientists and others help us stay safe in these shaky isles with the activities Watching Rangitoto erupt, Who's on your team? and Home disaster kit.


    Use an interactive or paper-based Venn diagram to illustrate the key similarities and differences between tsunami waves and surf waves.

    Using a shallow tray of water is a great way to demonstrate wave generation and behaviour.

    Useful links

    KidsHealth has information about helping young people cope with natural disasters.

    Previous eruptions in 2014 created a new island and research suggests this is one of the massive explosions the volcano is capable of producing roughly every thousand years.

    Did you know that the 'pulse' of a volcano can be used to help predict its next eruption, read this 2019 article from The Conversation.

    In this RNZ news article Shane Cronin says that early data from Tonga's violent volcanic eruption suggests it is the biggest since Mount Pinatubo in the Philippines 30 years ago.

    Find out more about Why the Tongan volcano triggered a worldwide tsunami in this RadioNZ Our Changing World programme from July 2022.

    Follow Professor Shane Cronin on Twitter at @scronin70.


    This article was written by Shane Cronin, Professor of Earth Sciences, University of Auckland.

    The article was originally published in The Conversation, 16 January 2022. Read the original article.

    Rights: The Conversation

    The Conversation

    The Conversation is an independent source of news and views, sourced from the academic and research community and delivered direct to the public.

    The Conversation

      Published 17 January 2022, Updated 9 March 2023 Referencing Hub articles
          Go to full glossary
          Download all