I have lived in Auckland, on and off, most of my life. Like most, I recognise its turbulent volcanic history only in passing. The cones and craters of Auckland’s 53 volcanic centres are landmarks, dramatic in scale but quiet in bearing—there are no plumes of steam or cauldrons of boiling mud as in Rotorua, no dramatic eruption events like on the Central Plateau. Yet the geological record makes clear that this is no benign place to establish a city—an eruption can occur at any time and almost any scale, with little or no warning.
If there was a pattern of activity in the Auckland Volcanic Field, it was blown to smithereens by the eruption of Rangitoto, which burst from the harbour and disgorged as much material as most of the previous eruptions put together.
The lack of knowledge concerning predicting and managing the volcanic hazard in Auckland was made clear during a Civil Defence exercise in 2008. Since then, scientists have invested thousands of hours in the field, taking rock and gas samples, measuring the noise of the city to image the subsurface, and trying to piece together evidence from the previous 53 eruptions in order to better understand the 54th.
The harder they look, the more they find.
While many answers have surfaced, some new and astonishing enigmas remain buried—and none buried deeper than a seismic anomaly found in the mantle beneath Auckland.
It was stumbled upon by chance just 10 years ago, as Geoff Chapple reveals in the feature in this issue.
Using reflected seismic waves from earthquakes thousands of kilometres away, geologist Nick Horspool and his team found what appeared to be an isolated hotspot some 80 kilometres beneath Auckland, like a knot of pressure beneath the city. Nobody knows how it came into being, but it’s quite likely that this is the source of magma for the Auckland Volcanic Field’s eruptions, though scientists can’t be sure. Nor do they know why the magma starts rising, or the conduits through which it may bolt—at some 20 kilometres per hour—to erupt in Auckland’s suburbs, city or sea. The prospect that so little is known of the foundations of the city might seem alarming, but this potential civil unrest may be a part of who we are.
In July 2014, 14 of Auckland’s volcanic cones returned to Māori management, to a new regional authority that will oversee the management of these maunga as icons of the landscape, and our collective culture.
Documentation presented by the authority refers to them as ‘tūpuna maunga’, acknowledging that these cones are as much a part of ancestry, perhaps, as forebears. For Auckland they are both a symbol of the volcanic threat, and to a great extent, a facet of our identity too—just as the earthquakes have become an inseparable part of the story of Christchurch and its inhabitants, for better or for worse.