Mount impermanent: An ancient slippery soil may underlie the Mauao landslide

Andrea Graves

New Zealand Listener·

2 Mar, 2026 05:02 PM4 mins to read

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Mt Maunganui, or Mauao, is pockmarked with old landslides while the region’s extremely sensitive soils are unknown elsewhere in New Zealand. Photo / Getty Images

Buddhists seek always to be aware that everything is impermanent. It’s hard enough to truly grasp that babies become adults and our parents will die but harder to believe something as seemingly solid as a mountain can fail. I grew up in Tauranga with a view that featured Mauao
– aka the Mount, Mt Maunganui – and January’s exposures of its brown innards are confronting.

The volcano had 42 slips in January, including the one that cast a pall of grief over the mountain by taking six lives. Official inquiries are underway, and questions are being asked about the geological causes of the fatal slip. Soil samples are being analysed.

What’s already known is that the Tauranga region – which includes Welcome Bay, where two more people died under a landslide – has a particularly problematic ancient soil that can act like greasy liquid.

Mauao arose more than two million years ago when molten rhyolite, a rock, welled up under pressure from sliding tectonic plates. From the summit on a clear day, you see other rhyolitic domes that also popped up in the tectonically active Bay of Plenty. The rhyolite cooled and hardened into rocky mountains.

Being downwind from the Taupō volcanic zone, Mauao then received ash coatings from eruptions of other volcanoes, says Martin Brook, a professor of geology at the University of Auckland. Eventually the overlying rhyolite and ash weathered into soil. “The chemistry of volcanic minerals changes as they weather. That’s due to time, rainfall and also reactions with different gases.”

Exactly what fell on Mauao and what it weathered into is yet to be determined. But in studied areas nearby, there’s a layer known as the Pahoia Tephra Sequence (tephra being anything thrown from a volcano that’s not liquid lava). It fell from eruptions up to 2.18 million years ago. It rests there still, 10-15m or more deep buried under younger layers. The volcanic glass it contained has weathered into a clay mineral called halloysite.

Halloysite’s properties mean the layer is almost always saturated, or nearly so. Yet it’s quite stable until the pressures from above and saturation become too much due to heavy rain – especially if prolonged – or an earthquake, says Kim de Graaf, a senior lecturer in civil engineering at the University of Waikato. “It converts from appearing solid when it’s dry, and relatively stable, to becoming a kind of liquid, like a greasy kind of layer, and the material above can become unstable,” she says. “The Pahoia Tephra becomes really sensitive, and the slope above will slide over it.”

It’s like a rug being pulled out from underneath. That rug has caused severe landslides, including the massive 1981 slip of the canal conveying water to the Ruahihi hydropower station, 19 km from the Mount.

Brook says when halloysite slides, it weakens. “There’s a massive difference between the peak strength and the remoulded strength. The remoulded strength is much, much lower.”

That makes it more likely to fail again, which matters when assessing how stable old landslide sites are.

Mauao is pockmarked with old landslides. “The hill slopes are draped by a veneer of failed material which has come down in landslides in the past,” says Brook. “Much of that material is at its disturbed, remoulded strength.”

The difference between a soil’s peak strength and its remoulded strength is referred to as its sensitivity, with more sensitive soils being more likely to slip. Soils graded between 4 and 8 are considered sensitive, 8-16 are highly sensitive, and above 16 are “quick”. Depending on the shape of the halloysite particles, some Tauranga soils are graded at up to 140.

Such extremely sensitive soils are unknown elsewhere in New Zealand. But remember that landslides happen in other areas too. Everything is impermanent.

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