Auckland Volcanic Field

   New Zealand's North Island is underrated as a geological wonder. Yes the South Island has mountains and glaciers and faults and, and, and... But the North Island has volcanoes (Figure 1) including the ‘supervolcano’ under Lake Taupo. Having spent all of my life living in geologically stable places I am rather excited that Auckland, where I'll be for part of December, is only a few hours drive from the Taupo Volcanic Zone. This week I discovered that I won’t actually have to drive very far to reach one of the North Islands many volcanoes. Auckland, it turns out, is sitting in a volcanic field (Figure 2). I shall be able to roll out of bed into the ‘6km ring of death’. How exciting! Scaremongering aside what is the Auckland Volcanic Field all about? 

Figure 1: Locations of the many volcanoes on
New Zealand's North Island

   The Auckland Volcanic Field is related to a hotspot located beneath the North Island. Excess heat from the hot spot caused small volumes (<0.4 km³) of magma to form which rose to the surface resulting in phreatomagmatic and magmatic eruptions [1-3, 5-7]. The field has an areal extent of approximately 360 km² [1-3, 5-8] and to date between 3 to 4 km³ of material, mostly trachybasalt and basanite, has been erupted. Approximately 60% of this material is associated with Rangitoto, the youngest volcano (600-700yrs). It is also one of the few to have erupted more than once [1-8]. Onset of the volcanism is unclear. Radiocarbon and thermoluminescence give maximum ages of 141 ka whereas K-Ar ages are variable, affected by excess Ar, with a maximum age of 250 ka [2]. However, it has been noted that the frequency and magnitude of the Auckland eruptions have increased over time [2, 3, 6, 8 & 9].

Figure 2: Volcanic centres of the Auckland
Volcanic Field. From Ruaumoko website

   There are about 50 eruptive centres in the Auckland Volcanic Field (Figure 2), most of which are monogenetic, one eruption cycle [1-3, 5-8]. Thirty-five of the volcanoes show evidence of phreatomagmatic eruptions, 11 of these are only tuff rings expressed as maars. Twenty-four of the 35 have three phases, an early tuff ring surrounding a later stage scoria cone which is finally breached by lava flows [1 & 5]. There are also small shields (Rangitoto), 'frozen' lava lakes and plugs in present. The volcanic structure is dependent on magma-water interaction and the duration of the eruption [5 & 6]. The majority of the Auckland volcanoes erupted on land therefore, aquifers, lakes and rivers are thought to be the dominant water source for the phreatomagmatic eruptions [1]. The country rock in the north of the field is Mesozoic greywacke and 0-1km thick Miocene flysch while in the south it is Pliocene - Pleistocene sands and silts which are a few metres thick [1]. Aquifers are mainly associated with the Pliocene - Pleistocene sediments but they do occur in the fractured parts of the Miocene flysch [1].

   The lack of exact ages and the monogenetic character of the Auckland Volcanic Field volcanoes means that it is almost impossible to determine patterns [2]. This uncertainty has resulted in various studies and simulations with the aim of mitigating casualties in a future eruption [2 - 4, 6, 8 & 9]. The article which drew my attention to the Auckland Volcanic Field is in response to Sandri, et al (2011) which assesses the risk associated with a future eruption in Auckland. In 2008 the New Zealand government ran 'Exercise Ruaumoko'. This is a similar concept to California's 'Shake Out' except Ruaumoko only involves government agencies. During the Ruaumoko simulation it was assumed that the initial base surge would have a 1-3km run-out [3, 4 & 9]. However, further research combined with modelling done by Sandri, et al (2011) suggest that the run-out is larger. The purpose of the Sandri, et al (2011) was to assess the future risk of the Auckland Volcanic Field and create a volcanic hazard map for potential new vents. The study had two aspects a long-term hazard map and a short-term simulation similar to the Ruaumoko exercise.

Mount Eden. From the Ruaumoko and GSN websites

References:

1: Allen, S.R., Bryner, V.F., Smith, I.E.M. and Ballance, P.F. 1996: Facies analysis of pyroclastic deposits within basaltic tuff-rings of the Auckland Volcanic Field, New Zealand. New Zealand Journal of Geology and Geophysics, pp 309-327

2: Cassidy, J. and Locke, C. A. 2004: Temporally linked volcanic centres in the Auckland Volcanic Field. New Zealand Journal of Geology and Geophysics, pp 287-290

3: Lindsay, J., Marzocchi, W., Jolly., G., Constantinescu, R., Selva, J and Sandri, L. 2010: Towards real-time eruption forecasting in the Auckland Volcanic Field: application of BET-EF during the New Zealand National Disaster Exercise ‘Ruaumoko’. Bull. Volcanology pp 185-204

4: Sandri, L., Jolly, G. and Lindsay, J. 2011: Combining long- and short-term probabilistic volcanic hazard assessment with cost-benefit analysis to support decision making in a volcanic crisis from the Auckland Volcanic Field, New Zealand. Bull. Volcanology pp 1-19

5: Shane, P. and Smith, I., 2000: Geochemical fingerprinting of basaltic tephra deposits in the Auckland Volcanic Field. New Zealand Journal of Geology and Geophysics, pp 569-577

6: Auckland Council webpage

7: Auckland Volcanic Field page and Rangitoto fact sheet, GSN website

8: Wikipedia

9: Exercise Ruaumoko webpage

The papers form the New Zealand Journal of Geology and Geophysics are open access