Scientists pulling together a computer model of past eruptions under what is now Auckland city have identified a "forgotten" volcano under the suburb of Grafton, less than 2km from the city centre.
Researchers identified the little-known volcano by analysing records from boreholes drilled for foundations of buildings, roads or water pipes.
Geologist Bruce Hayward, of Geomarine Research, used the samples to trace lava flows by measuring changes in the thickness of the lava flows and volcanic ash was able to identify a buried volcanic crater.
"The crater is about 1km across and filled with solidified lava flows," Dr Hayward said.
Auckland is built on the Auckland Volcanic Field, a group of about 50 volcanoes that have erupted over the last 250,000 years. Most of the volcanoes erupted only for a few months or years and then became extinct.
Checks on the second oldest geological map of the Auckland Volcanic Field, published by early explorer and geologist Ferdinand von Hochstetter in 1864, showed four volcanic vents in the vicinity of the city's Domain.
"So although this find is exciting, it is clear that Hochstetter recognised the presence of a volcano in this locality before it was covered in houses," Dr Hayward said.
The Grafton volcano appeared to have erupted just before the neighbouring Domain volcano, more than 50,000 years ago, but traces of that eruption were buried by layer of volcanic ash from the Domain eruptions.
Lava is very magnetic, and airborne surveys of the city have revealed an area of high gravity and magnetism in the same area that Dr Hayward has identified the Grafton volcano.
The research is part of a project to prepare the city for a future eruption.
Project leader, Dr Jan Lindsay, of Auckland University, said that the new information showed that there was still a lot to learn about past volcanic activity.
The researchers from GNS Science, and Auckland and Massey universities, have been funded by Earthquake Commission, Auckland Council, the Science Ministry, and Auckland University since 2008.