Scientists have revealed the explosive history behind a spectacular eruption that created one of the planet's newest islands.
In an article published today in the American Geophysical Union's journal EoS, University of Auckland volcanologist Professor Shane Cronin and Kiwi and Tongan colleagues suggest the late 2014 event was the latest chapter in a long and explosive history in the area, which lies about 65km north of Tonga's capital Nuku'alofa.
It followed a 2009 blow that also spewed steam, smoke, pumice, and ash hundreds of metres into the sky.
Months after the most recent eruption, which built a new island between two others nearby, the scientists visited the fresh structure to build a clear picture of how it formed, and how past volcanism had shaped that part of the Tonga volcanic arc.
The plumes that spewed from the ocean were produced by an "explosive interaction" between seawater and magma rising from a plateau about 150m below the ocean surface - itself part of a massive, submerged volcanic edifice named Hunga and rising more than 2km from the seafloor.
In less than three weeks, the eruption had built up a circular area of land spanning 2km across, reaching a height of 120m and nicknamed Hunga Island.
During their expedition, the scientists found the lower beds of the new island's cone were formed by jets of pyroclastic rock fragments during the eruption, while the steeper, upper part of the cone suggested a gradual "drying" as the ocean interacted with the magma less and less.
In just a matter of days after the eruption, the island's cone had reached its maximum diameter - but the formation still continued to increase in height over the next two weeks.
"Once the vent was completely surrounded by pyroclastic deposits, much higher eruption columns began," the scientists wrote.
"Such Surtseyan eruptions - from a shallow sea or lake water - have only rarely been witnessed since the phenomenon was first seen during the formation of Surtsey, Iceland, in 1963."
Soon after its birth, the island was further shaped by wave erosion at its base: on one side, it shrank by 500m and left 40m-high collapsing cliffs.
In their work, the scientists collected samples to chemically characterise the new volcanic material and compare it with deposits of the broader volcano.
The volcano that created it was one of many formed along the Tonga-Kermadec volcanic arc as a result of subduction of the Pacific Plate beneath the Indo-Australian Plate.
And since its formation, Hunga volcano and its surrounding structure was thought to have been responsible for many other big eruptions.
Charcoal in one pyroclastic flow deposit was dated back to 1040 to 1180, corresponding with other ashfall deposits found on another island 65km away, and possibly connected to an unknown tropical eruption in 1108 CE believed to have produced more than 1C of global cooling.
Using a multibeam sounder, the scientists also mapped the sea floor to reveal a 150m-deep depression that included a shallow area associated with earlier eruptions south of the new island.
The scientists believed this caldera likely formed when an older Hunga edifice collapsed violently into the sea.
"This collapse may be the source of the unknown South Pacific eruption about 1000 years ago."
They concluded that these observations highlight how rapidly new volcanic forms are eroded in the area - and imply that the volcanic record in the Tonga region was "extremely fragmentary".
"In future visits, we will continue investigating past eruptions while extending submarine surveys and sampling around the new island to monitor the ongoing changes in response to storms and other events."