{"id":311351,"date":"2026-03-03T18:53:12","date_gmt":"2026-03-03T18:53:12","guid":{"rendered":"https:\/\/www.newsbeep.com\/nz\/311351\/"},"modified":"2026-03-03T18:53:12","modified_gmt":"2026-03-03T18:53:12","slug":"mount-impermanent-an-ancient-slippery-soil-may-underlie-the-mauao-landslide","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/nz\/311351\/","title":{"rendered":"Mount impermanent: An ancient slippery soil may underlie the Mauao landslide"},"content":{"rendered":"

Andrea Graves<\/p>\n

New Zealand Listener\u00b7<\/p>\n

2 Mar, 2026 05:02 PM4 mins to read<\/p>\n

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Mt Maunganui, or Mauao, is pockmarked with\u00a0old landslides while the region’s extremely sensitive soils are unknown elsewhere in New Zealand. Photo \/ Getty Images<\/p>\n

\n Buddhists seek always to be aware that everything is impermanent. It\u2019s 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
\n \u2013 aka the Mount, Mt Maunganui \u2013 and January\u2019s exposures of its brown innards are confronting.\n <\/p>\n

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. <\/p>\n

What\u2019s already known is that the Tauranga region \u2013 which includes Welcome Bay, where two more people died under a landslide \u2013 has a particularly problematic ancient soil that can act like greasy liquid. <\/p>\n

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. <\/p>\n

Being downwind from the Taup\u014d 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. \u201cThe chemistry of volcanic minerals changes as they weather. That\u2019s due to time, rainfall and also reactions with different gases.\u201d<\/p>\n

Exactly what fell on Mauao and what it weathered into is yet to be determined. But in studied areas nearby, there\u2019s a layer known as the Pahoia Tephra Sequence (tephra being anything thrown from a volcano that\u2019s 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.<\/p>\n

Halloysite\u2019s properties mean the layer is almost always saturated, or nearly so. Yet it\u2019s quite stable until the pressures from above and saturation become too much due to heavy rain \u2013 especially if prolonged \u2013 or an earthquake, says Kim de Graaf, a senior lecturer in civil engineering at the University of Waikato. \u201cIt converts from appearing solid when it\u2019s dry, and relatively stable, to becoming a kind of liquid, like a greasy kind of layer, and the material above can become unstable,\u201d she says. \u201cThe Pahoia Tephra becomes really sensitive, and the slope above will slide over it.\u201d <\/p>\n

It\u2019s 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.<\/p>\n

Brook says when halloysite slides, it weakens. \u201cThere\u2019s a massive difference between the peak strength and the remoulded strength. The remoulded strength is much, much lower.\u201d <\/p>\n

That makes it more likely to fail again, which matters when assessing how stable old landslide sites are.<\/p>\n

Mauao is pockmarked with old landslides. \u201cThe hill slopes are draped by a veneer of failed material which has come down in landslides in the past,\u201d says Brook. \u201cMuch of that material is at its disturbed, remoulded strength.\u201d <\/p>\n

The difference between a soil\u2019s 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 \u201cquick\u201d. Depending on the shape of the halloysite particles, some Tauranga soils are graded at up to 140. <\/p>\n

Such extremely sensitive soils are unknown elsewhere in New Zealand. But remember that landslides happen in other areas too. Everything is impermanent. <\/p>\n

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Copy LinkEmailFacebookTwitter\/XLinkedInReddit<\/p>\n","protected":false},"excerpt":{"rendered":"Andrea Graves New Zealand Listener\u00b7 2 Mar, 2026 05:02 PM4 mins to read Subscribe to listenAccess to Herald…\n","protected":false},"author":2,"featured_media":311352,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[27709,36671,8831,2196,8666,22059,6726,1970,6256,169446,42037,23966,5821,169448,121,373,169447,83475,6098,47963,169444,169449,302,140711,56662,80560,17707,7810,13233,21524,111,43,139,69,3766,11925,59342,42472,29191,5339,4525,1227,1361,223,86662,169445,9880,1533,102],"class_list":{"0":"post-311351","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-new-zealand","8":"tag-adults","9":"tag-always","10":"tag-an","11":"tag-ancient","12":"tag-aware","13":"tag-babies","14":"tag-become","15":"tag-believe","16":"tag-brown","17":"tag-buddhists","18":"tag-confronting","19":"tag-enough","20":"tag-everything","21":"tag-exposures","22":"tag-fail","23":"tag-featured","24":"tag-grasp","25":"tag-grew","26":"tag-hard","27":"tag-harder","28":"tag-impermanent","29":"tag-innards","30":"tag-its","31":"tag-januarys","32":"tag-landslide","33":"tag-mauao","34":"tag-maunganui","35":"tag-may","36":"tag-mount","37":"tag-mountain","38":"tag-new-zealand","39":"tag-news","40":"tag-newzealand","41":"tag-nz","42":"tag-parents","43":"tag-seek","44":"tag-seemingly","45":"tag-slippery","46":"tag-soil","47":"tag-solid","48":"tag-something","49":"tag-tauranga","50":"tag-that","51":"tag-the","52":"tag-truly","53":"tag-underlie","54":"tag-view","55":"tag-will","56":"tag-with"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/311351","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=311351"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/311351\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/311352"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=311351"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=311351"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=311351"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}