Auckland and the AVF
In a thick brain fog, crusty eyed and yawning, I sat up in bed at 4:30 am. I was in Auckland, New Zealand. It was still dark outside when I drove to Mount Eden (Maungawhau), where I hiked up a narrow dirt trail lined by tall grass stippled with dew. Coming out of the verdure, my shoes, socks and shorts were soaked through. On top of the hill, a shadow-black grouping of trees blocked the creeping morning light from behind the Hanua Ranges. The burnt orange sunrise, obstructed by cumulous, lit up like a distant mountain wildfire. Auckland city centre was under puffy, lavender-white cirrus clouds, reflecting pastel colours across the harbour. Alone in the cool and crisp pre-dawn air, I viewed the various scoria cones in the Auckland Volcanic Field (AVF) bursting through the city neighbourhoods.
Scoria is basaltic lava ejected as fragments from a volcano, typically with a frothy texture. Many cones made up of this rock have been turned into nature reserves and city parks across the city. With the exception of Rangitoto Island and Motukorea, all the volcanoes in the AVF have been modified for mining, quarrying and fortifications. In fact, Mount Eden has been adulterated numerous times for agriculture (terraced gardens), fortification, and basalt and scoria quarries.
The dormant monogenetic (that is, from single eruptions) AVF is fuelled entirely by basaltic magma (alkali basalt to nephelinite) and tephra (rock fragments and particles ejected by a volcanic eruption) from relatively shallow levels of the upper mantle. Past activity, including magmatic to phreatomagmatic strombolian and Hawaiian type eruptions, resulted in pyroclastic fall and base surges (Allen and Smith, 1994). Eruptions began at Onepoto volcano nearly 248 thousand years ago, with over half of the volcanic centres forming 40 to 20 thousand years ago (Bebbington and Cronin, 2010).
Forty-nine eruptive centres have been mapped in the AFV. The largest and youngest – Rangitoto – is the only polygenetic volcano in the system. Superb geological, stratigraphic and botanical evidence has constrained age dates for this volcano (McDougal et al., 1969). Pyroclastic deposits preserved in swamps on Motutapu Island and mainland Lake Pupuke indicate it is indeed polygenetic. Radiocarbon dating reveals eruption ages of 553 ± 7 and 504 ± 5 years BCE (Needham et al., 2011). However, according to Lindsay et al. (2011), only three volcanic centres are considered to be reliably and accurately dated (Rangitoto, 0.6 thousand years ago; Mt Wellington, 10 thousand years ago; and Three Kings, 28.5 thousand years ago).
Future large eruptions are expected in the AFV, with potentially devastating effects for Auckland and surrounding areas. A nightmarish, widespread, contemporaneous eruption would be catastrophic in its capacity for destruction. Long- and short-term eruptions could cripple the city. Numerous geohazards including lava flows, poisonous gas and ashfall would result in substantial damage to infrastructure, and cause significant business interruption and potential loss of life (Magill et al., 2005).
Five widely-separated, structurally unrelated volcanoes in the AFV are contemporaneous, highlighting both spatial and temporal differences. This is significant when considering the likelihood of the recurrence of eruptions and eruptions that are linked over time, which are key to accurate hazard assessments (Cassidy et al., 1999; Cassidy and Locke, 2010). Variations in surface expression of magmatism over time complicates long-term volcanic hazard forecasting and therefore it is unknown when or where the next eruptions will occur (Molloy and Shane, 2009).