Auckland’s forgotten volcano

Scientists researching and mapping Auckland’s volcanic field thought they had discovered a hitherto unknown crater lying beneath the suburb of Grafton. However, it turned out early explorer and geologist Ferdinand von Hochstetter had beaten them to it by some 150 years.

Rights: Dr Bruce Hayward, Geomarine Research, courtesy of GNS Science.

Grafton volcanic crater

The location and size of a buried and long-forgotten volcanic crater in the central Auckland suburb of Grafton.

Auckland city is built on the Auckland Volcanic Field, a group of about 50 volcanoes that have erupted over the last 250,000 years. Scientists believe that most of the volcanoes erupted only for a few months or years and then became inactive.

7-year study of Auckland’s volcanic history

The researchers have been analysing records from boreholes drilled for foundations of buildings and roads and for water supply as part of a 7-year study of the volcanic history of Auckland to gain a better understanding of when the volcanoes actually erupted. The borehole data from across the city will eventually be compiled into a central database where it can be used to model past eruptions. The work, called the DEVORA project (DEtermining VOlcanic Risk in Auckland), will help predict likely scenarios for when the next volcano forms in Auckland and help prepare the city for a future eruption. Found about more about this project in the article, Determining Auckland’s volcanic risk.

Geologists studied the boreholes by looking at thicknesses of lava flows and volcanic ash in the layers. They were then able to piece together the volcanic puzzle by linking lava flows between boreholes and were surprised to find a volcano hidden beneath the suburb of Grafton, close to the Auckland University Medical School.

Geologist Dr Bruce Hayward, of Geomarine Research, describes the crater as about 1 km across and filled with solidified lava flows.

Grafton volcano shown on 1864 map

The crater was absent on modern geological maps but close inspection of one of the country’s oldest geological maps of the Auckland Volcanic Field, published by von Hochstetter in 1864, shows 4 volcanic vents in the vicinity of the Auckland Domain.

Rights: Bruce Hayward

Auckland volcanic field

Auckland is built of an active volcanic field. This map shows the volcanoes from past eruptions.

In 2014, 14 Tūpuna Maunga were returned to Ngā Mana Whenua o Tāmaki Makaurau (the mana whenua tribes of Auckland). The Tūpuna Maunga o Tāmaki Makaurau Authority (Maunga Authority) was established to co-govern these sacred sites.

Map from Volcanoes of Auckland: A field guide, by Bruce Hayward, 2019, Auckland University Press.

“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,” says Dr Hayward.

“It would appear that this Grafton volcano erupted just before the neighbouring Domain volcano, more than 50,000 years ago.”

A thick layer of volcanic ash from the Domain eruptions buried and hid the Grafton volcano until recent boreholes have shown its full extent and nature.

Over the last few years, geophysicists at The University of Auckland have been studying the rocks under Auckland using gravity and magnetic measurements. Because lava is very magnetic, airborne surveys of the city have revealed where lava is present, even where it is not visible at the surface.

One area of high gravity and magnetism is in the same area that Dr Hayward has identified as the ‘new’ Grafton volcano, confirming Dr Hayward’s (and von Hochstetter’s!) findings.

The DEVORA project is led jointly by GNS Science and The University of Auckland in collaboration with Massey University. The project is funded by the Earthquake Commission, the Auckland Council, the Ministry of Science and Innovation and The University of Auckland. Project DEVORA started in late 2008.