Volcanologists hope that seismic networks in French Polynesia will help identify which underwater volcano produced a sea of pumice on the surface of the Pacific Ocean north of New Zealand.
Crew from HMNZS Canterbury, with GNS scientists onboard, sighted the pumice about 85 nautical miles west-south-west of Raoul Island late on Thursday. It covers 25,000 square km.
Professor Colin Wilson of Victoria University says seismic networks in French Polynesia may help show which volcano was involved.
GNS Science says the pumice was spotted by an airline pilot on 1 August, who did not report it until later.
That has ruled out the Monowai underwater volcano, which began erupting on 3 August. Monowai is also 500km north, which is thought to be too far for the pumice to have floated.
GNS will examine the pumice collected by the Canterbury when it returns.
Professor Wilson hopes to analyse what sort of eruption it came from.