Cracked rocks on Whakaari/White Island allow gas to escape, easing the build-up of pressure that triggers volcanic eruptions, new research has found.
The eruption of the volcano on 9 December last year killed 21 people and left others seriously injured, some of whom remain in hospital facing a long road to recovery.
University of Canterbury scientists, as well as GNS Science and international researchers, studied rocks ejected from the volcano during an eruption in April 2016.
They tested to see how easily the rocks cracked and how gas traveled through them and found the rocks can act like valves - opening and closing - to allow gas to escape.
Lead researcher Dr Ben Kennedy said under a microscope small minerals could be seen in and around the cracks in the rocks.
"The research team found evidence that these valve-like cracks can become clogged by minerals. If these valves become clogged, the gas cannot escape. If gas cannot escape, pressure builds, leading toward an explosive eruption."
The rocks from Whakaari / White Island behaved in a unique way, Dr Kennedy said.
"Cracks formed easily, allowing gas to escape, but because the minerals were smushy the cracks sealed again when pressed together by the weight of the volcano, forming a valve."
The research means scientists can link certain earthquakes to the movement of fluid through valves within the volcano.
The findings are "remarkable", Dr Kennedy said, and will allow scientists to track and monitor a build-up of activity that signals gas is moving through the volcano.