Previously unknown lava caves are being stumbled onto at a rate of about one a month in Auckland.

Auckland is built on a large potentially active volcanic field comprising of 53 volcanoes and hundreds of lava caves — some up to 290m long and lava caves are relatively rare by world standards and can only be created under very specific circumstances.

“The type of lava and the speed of the flow will determine if the outside layer cools down fast enough to harden. This creates a tunnel for the lava to flow through, and when the lava drains away a cave is left behind,” said researcher Jaxon Ingold of Waipapa Taumata Rau/ University of Auckland.

“Many people know there are lava caves underlying the volcanic parts of Auckland, but the exact information is spread around lots of different groups — like local iwi, the council and experts who have been investigating and mapping some of the network.”

The 600 square kilometre volcanic field was stretched from Lake Pupuke and Rangitoto Island in the north to Matukutururu (Wiri Mountain) in the south, and from Mt Albert in the west to Pigeon Mountain in the east.

Ingold has joined with Auckland Council geo-heritage experts Kate Lewis and Christina Bloom, as well as speleologist Peter Crossley to attempt to pull together a complete database of historical records and new discoveries.

The project by DEVORA (Determining Volcanic Risk In Auckland) — a long-standing research programme funded by Toka Tū Ake EQC and Auckland Council — wanted to better chart the lava cave locations, so future developments could happen safely without damaging these special features.

DEVORA researchers engaged Ingold to identify and collate the existing information of the hundreds of lava caves that existed across the city and help develop a system to share that information sensitively with relevant parties.

Natural Features Specialist Lewis said the main project goal was to create a database of currently known and newly discovered lava caves that can be utilised by planners, engineers and relevant people in order to protect them and to strengthen our understanding of lava caves and lava flows in Tāmaki Makaurau.

Newly discovered lava caves are reported to Auckland Council and the team documents these caves and helps find solutions for completing any planned works while avoiding or minimising further disturbance to the cave.

“We want to encourage Aucklanders to report new caves and share any information we may not have. There are several reasons why this information could be sensitive, so we want to reassure people we will treat that information appropriately,” Lewis said.

Lava caves were also of importance to Māori. Even caves that have never been open to the surface and accessible by humans prior to their discovery may be culturally and spiritually significant.

“Because these sites are incredibly significant for local Māori, we recognise the importance of working closely with local iwi to get a better picture of these sites,” Ingold said.

Some caves were already known by iwi and their location was highly sensitive so Auckland Council worked with mana whenua on how best to protect them.

Chief resilience and research officer Jo Horrocks at Toka Tū Ake EQC said the organisation invested in DEVORA research to better understand all aspects of the Auckland volcanic field, including the lava caves.

“This project teaches us important geological and geoheritage lessons, like understanding lava flow patterns or predicting where other caves could be located, which is helpful for anyone involved in planning and building future housing or infrastructure,” Horrocks said.

Lewis said many lava caves were scheduled as “outstanding natural features”. Under the Auckland Unitary Plan all newly discovered lava caves more than one metre wide needed to be investigated and potentially protected, which was similar to archaeological sites.

“Our team has become extremely responsive to the needs of anyone reporting a new cave during works in order to find options to continue the project,” Lewis said.

“We help them develop construction methodologies to reduce vibration and work carefully around the cave, as well as working with geotechnical engineers, where required, to re-design structures over and around caves safely.”