An underwater volcano off Tonga erupted on Jan. 15, triggering tsunami warnings and evacuation orders in Japan and causing large waves in several South Pacific islands, where footage on social media showed waves crashing into coastal homes.

Hunga Tonga-Hunga Ha’apai lies about 65 km (40 miles) north of Nuku’alofa, the capital of Tonga. Scientists said the volcano – which last erupted in 2014 – had been puffing away for about a month before rising magma, superheated to around 1,000 degrees Celsius (1,832 Fahrenheit), met with 20 degree (68 Fahrenheit) seawater on Saturday, causing an instantaneous and massive explosion.

Under the Pacific

The island archipelago of Tonga lies on the Pacific Ring of Fire, a region known for volcanic eruptions and regular earthquakes. The area forms a horseshoe bend that stretches from New Zealand to the Philippines, Indonesia and Chinese coasts, extending to the western coasts of North and South America until the tip of Chile.

Tonga sits at the junction of two major tectonic plates, the Pacific Plate and Indo-Australian Plate. The Pacific Plate is pushed west and sinks back into the mantle along a “subduction zone” as it reaches the Indo-Australian plate and the smaller Tonga Plate. Magma is formed above the subducting plate and slowly rises through the crust above, forming a chain of volcanoes nearby.

“We believe the base of the volcanic edifice failed and collapsed into a shallow magma reservoir, allowing the Pacific ocean waters (perhaps 100m deep) to explosively interact with the liquid rock, causing a partial caldera collapse and explosive eruption,” said James Garvin, chief scientist at NASA’s Goddard Space Flight Center.

Garvin and a team of researchers from NASA, Columbia University, Tonga and Canada have been mapping the area, above and below the water line since 2015. They have used a combination of satellite data and land-based surveys as well as sonar-based bathymetry data collected by the Schmidt Ocean Institute, in partnership with NASA and Columbia University.

“Studying this eruption cycle thanks to 21st century datasets from space, ship, drones and field studies will advance understanding and ultimately predictions of how such hydrovolcanic eruptions operate,” Garvin said.

The volcanic island has changed dramatically. Satellite images from Planet Labs on January 7 showed volcanic gas escaping through the vent. On January 15, about two hours before it erupted, images show the vent was predominantly underwater. The island was mostly obliterated during the recent eruptions, leaving two small land masses once again separated by the ocean.

Tsunami alert

The eruption caused a 1.2-metre (four-foot) tsunami wave, Australia’s Bureau of Meteorology said. The waves travelled as far as Japan, nearly 8,000 km north west of the archipelago. The government’s meteorological agency issued a warning that waves reached up to 3 metres in at least two locations on its eastern coast.

Two people reportedly drowned in Peru – over 10,000 km west of the eruption – in unusually high waves.

News from Tonga is sparse because communications are severely hampered. In its first statement the country’s government confirmed three casualties, including a British national who died while saving her dogs.

The eruption was so powerful that space satellites captured not only huge clouds of ash but also an atmospheric shockwave that radiated out from the volcano at close to the speed of sound.

Cut off from the world

Australia and New Zealand sent surveillance flights to assess the damage in Tonga’s outer islands, where communication is completely cut off.

The eruption has also severed the Tonga Cable, a 827 km submarine fibre-optic cable that connects the island country to Fiji and international networks. The CS Reliance, a cable laying vessel owned by U.S. subsea cable repair company Subcom is expected to repair the cable. The vessel is travelling from Port Moresby, nearly 4,000 km away.

“This disaster has shaken the people of Tonga like nothing we have seen in our lifetime,” Sione Taumoefolau, secretary general of Tonga Red Cross, said in a statement. “The tsunami has wiped out homes and villages, but we are already rebuilding amid the ashes.”