Explained: Why volcanic eruption in Tonga was so violent, what to expect next

The Kingdom of Tonga doesnrsquot often attract global attention but a violent eruption of an underwater volcano on January 15 has spread shock waves quite literally around half the worldThe volcano is usually not much to look at It consists of two small uninhabited islands HungaHarsquoapai and HungaTonga poking about 100m above sea level 65km north of Tongarsquos capital Nukulsquoalofa But hiding below the waves is a massive volcano around 1800m high and 20 kilometres wideThe HungaTongaHungaHaapai volcano has erupted regularly over the past few decades During events in 2009 and 201415 hot jets of magma and steam exploded through the waves But these eruptions were small dwarfed in scale by the January 2022 eventsOur research into these earlier eruptions suggests this is one of the massive explosions the volcano is capable of producing roughly every thousand yearsWhy are the volcanorsquos 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 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 coolBut this process doesnrsquot 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 waterVolcano researchers call this ldquofuelcoolant interactionrdquo and it is akin to weaponsgrade 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 speedsTwo scales of Hunga eruptions The 201415 eruption created a volcanic cone joining the two old Hunga islands to create a combined island about 5km long We visited in 2016 and discovered these historical eruptions were merely curtain raisers to the main eventMapping the sea floor we discovered a hidden ldquocalderardquo 150m below the wavesThe caldera is a craterlike depression around 5km across Small eruptions such as in 2009 and 201415 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 calderaLooking at the chemistry of past eruptions we now think the small eruptions represent the magma system slowly recharging itself to prepare for a big eventWe 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 65km away and then used radiocarbon dates to show that big caldera eruptions occur about ever 1000 years with the last one at AD1100With this knowledge the eruption on January 15 seems to be right on schedule for a ldquobig onerdquoWhat we can expect to happen now Wersquore still in the middle of this major eruptive sequence and many aspects remain unclear partly because the island is currently obscured by ash cloudsThe two earlier eruptions on December 20 2021 and January 13 2022 were of moderate size They produced clouds of up to 17km elevation and added new land to the 201415 combined islandThe latest eruption has stepped up the scale in terms of violence 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 windThis demonstrates a huge explosive power ndash one that cannot be explained by magmawater interaction alone It shows instead that large amounts of fresh gascharged magma have erupted from the calderaThe 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 Soon after the eruption started the sky was blocked out on Tongatapu with ash beginning to fallAll these signs suggest the large Hunga caldera has awoken Tsunami are generated by coupled atmospheric and ocean shock waves during an explosions but they are also readily caused by submarine landslides and caldera collapsesIt remains unclear if this is the climax of the eruption It represents a major magma pressure release which may settle the systemA warning however lies in geological deposits from the volcanorsquos previous eruptions These complex sequences show each of the 1000year major caldera eruption episodes involved many separate explosion eventsHence we could be in for several weeks or even years of major volcanic unrest from the HungaTongaHungaHaapai volcano For the sake of the people of Tonga I hope notThis story has been sourced from a third party syndicated feed agencies Midday accepts no responsibility or liability for its dependability trustworthiness reliability and data of the text Midday managementmiddaycom reserves the sole right to alter delete or remove without notice the content in its absolute discretion for any reason whatsoever