6 Jun 2024

Inside Auckland's lava caves

From Our Changing World, 5:00 am on 6 June 2024
A wide shot of the interior of a cave lit by warm lights, with moss and a fern hanging from the ceiling.

Inside the lava cave in Sean Jacob's Mt Eden backyard. Photo: RNZ / Ellen Rykers

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Known for its iconic maunga like Rangitoto and Maungawhau Mt Eden, Tāmaki Makaurau Auckland is a city built on an active volcanic field that has erupted at least 53 times. 

But beneath streets, houses and parks, there are other – hidden – remnants of the city’s fiery past: hundreds of lava caves. 

The backyard cave 

Lava caves form when hot flowing lava meets air and crusts over, creating a tunnel. Eventually the lava drains away, leaving behind a cavity.  

Formed as far back as 200,000 years ago and as recently as 550 years ago (when Rangitoto erupted), lava caves in Auckland range from small cracks to lengthy tunnels. The longest, located in Wiri, stretches to 290 metres. 

The lava cave in Sean Jacob’s Mt Eden backyard is about 100 metres long. “For something that's so quiet and so peaceful when you're down here, it was sort of created by so much violence,” he says.  

The Jacob family bought the property in 2008 – in part so the cave would be protected, unlike many others across the city which have been destroyed or infilled with concrete in years gone by. 

A wide shot of a man standing in a cave at the bottom of stairs that head up towards a light and leafy backyard.

Sean Jacob in the entrance to the lava cave in his Mt Eden backyard. Photo: RNZ / Ellen Rykers

The speleologist 

Peter Crossley is perhaps the only person who went inside some of those caves that no longer exist. A speleologist, Peter has spent 50 years documenting Auckland’s lava caves.  

“Some people would say that they’re muddy, grotty, dark, infested with rats and all the rest of it. But when you look at it, you realise: it's a tunnel which has been filled with lava, almost yellow in heat, that could frazzle you in a microsecond," he says. 

Over the decades, Peter has seen surveying methods advance from compasses to state-of-the-art 3D scanning, giving scientists unprecedented detail and valuable insights into past eruptions. 

Now he’s passed on his knowledge of 180 lava caves to a new research effort. 

An older man with glasses, standing outside, holds up a 3D-printed model of a house sitting atop a large lava tunnel.

Peter Crossley holds up a 3D-printed model of a house sitting atop a lava tunnel. Photo: RNZ / Ellen Rykers

A new lava cave every month 

Jaxon Ingold, a master’s student at the University of Auckland, is collating everything we know about Auckland’s lava caves – drawing on Peter’s records, historical sources, and mātauranga Māori – so this geological heritage can be better protected and respected. 

“What I'm currently working on is: is it possible to predict where as-yet undiscovered lava caves may be located? So that we can be more careful in those areas,” says Jaxon. 

Plus, the database is growing: over the past two years, previously unknown lava caves have been discovered at an average rate of one per month – often on construction sites, says Kate Lewis, natural features specialist at Auckland Council. 

When a new lava cave is located, Kate and Myfanwy Eaves, Auckland Council archaeologist, will assess the cave for its significance – both geological and cultural. 

Three people wearing hard hats stand on a wooden platform in a cave, next to a ladder that ascends to a bright hole, where light enters and illuminates the cave ceiling.

Jaxon Ingold, Myfanwy Eaves and Kate Lewis in the cave in Ambury Regional Park, Auckland. Photo: RNZ / Ellen Rykers

What’s inside the caves? 

The most spectacular caves might have drip features, which are a bit like mini-stalactites: solidified lava dripping from the ceiling. The cave floor might be rippled like scrunched up fabric, in a pattern called ropy pahoehoe. The cave walls might have ridges indicating the rising and falling level of the lava river that once flowed, a bit like tea staining the inside of a mug. 

And on the cultural side: many caves contain artefacts from both Pākehā and Māori. Caves have been used as mushroom farms, air raid shelters and rubbish dumps.  

Māori used lava caves as rua kūmara (kumara storage pits), according to Kelvin Tapuke from Massey University. Some caves became the final resting place for the remains of ariki (chiefs) or other tūpuna (ancestors), but were later plundered. 

Listen to the episode to visit two lava caves, and to learn more about their history and what the new research project is uncovering. 

A man in a hard hat standing on a ladder in a hole in a grassy field. The sky is blue but cloudy and there is a metal grate on the ground in front of the hole. The man is smiling.

University of Auckland Master's student Jaxon Ingold descends into the lava cave in Ambury Regional Park, Auckland. Photo: RNZ / Ellen Rykers

Jaxon’s research is part of Determining Volcanic Risk in Auckland (DEVORA), a project funded by Toka Tū Ake EQC and Auckland Council. 

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