Diatremes beneath the phreatomagmatic volcanoes of the AVF have not been geophysically imaged due to weak gravity anomalies (e.g. Cassidy and Locke, 2010), indicating that if they are present, they must have similar density to the Waitemata
country rock (2000–2200 kg/m3). This is consistent with typical diatremes where 50%
of the deposit made of juvenile particles with density of ~2200 kg/m3, equivalent to a 20
vol% vesicle content (e.g. Ross and White, 2012) (Table 4.1). The lack of direct diatreme observations and weak geophysical signatures in the AVF result in a significant uncertainty in eruptive volume calculations, because diatremes are usually an order of magnitude larger than the surface volcanic edifices (Lorenz, 1986; White and Ross, 2011). Analogies of similar volcanoes are required. Hence, a shallow inverted cone frustum (e.g. phreatomagmatic crater infill, Fig. 4.2) with a 45° wall angle was assumed for application of Eq. 4.3. For phreatomagmatic crater infills, the depth was
estimated as a constant value of 15 m (for small, ≤500 m, eruptive centres), or 30 m (for
large, ≥500 m, eruptive centres). This is the same as the average measured ejecta ring
thicknesses, and consistent with the depth of the excavated lithic populations in the Maungataketake volcano (Agustín-Flores et al., 2014), as well as gravity and magnetic imaging of phreatomagmatic craters in Auckland (Cassidy et al., 2007). Magmatic crater infills (e.g. lava lakes) were recognised from aeromagnetic survey data (Rout et al., 1993; Cassidy et al., 1999; Affleck et al., 2001; Cassidy et al., 2007). Cassidy and Locke (2010) found that craters with residual magnetic anomalies of >50 nT were filled with lava. In this case, the eruptive volumes were calculated as the area of magnetic anomaly multiplied by the estimated thickness from Cassidy and Locke (2010).
For underlying diatremes beneath the crater infills, wall rock angles of 50°, 60° and 70° were assumed (Fig. 4.2). Their volumes were calculated using a simple inverted
cone geometry by Eq. 4.2. Current crater geometries were used to define rtop and rbottom.
The minimum crater radius was used, determined by the shortest distance between the digitised crater rim, based on slope angle and slope aspect maps, and the calculated centre point of the crater.
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In the bulk eruptive volume calculations feeder dykes and plumbing systems of scoria and spatter cones were not estimated due to the uncertain dyke and sill geometries.
4.3.2. Bulk proximal tephra accumulation and lava flows volumes for the
AVF
For the bulk volume estimates of the positive landforms of the AVF, a Light Detection And Ranging (LiDAR) survey-based DSM was used. The survey was carried out with a Leica Airborne Laser Scanner 50 (ALS50) and an Optech Airborne Laser Terrain Mapper 3100-EA (ALTM3100) in 2005–2006 and 2008. Details of LiDAR processes and survey are reported in Kereszturi et al. (2012b). The mean survey density
varied from 0.04 to 1 point per m2, corresponding to resolutions of between 5 to 1 m,
respectively. The original data have been pre-processed, including height correction, vegetation filtering and artificial infrastructure removal by Fugro Spatial Solutions (www.fugrospatial.com.au). The original DSM was interpolated using a Triangulated Irregular Network (TIN) and converted into a gridded model at 2 m resolution. This bare surface DSM was resampled by the nearest neighbour method into a medium resolution DSM (10 m) in order to enhance calculation time and reduce error due to filtering and post-processing. The pre-eruptive terrain beneath volcanoes and lava flows was either: (1) modelled as a flat surface in the southern and northern, low-lying parts of the field, or (2) interpolated from spot heights by the natural neighbour method (Sibson, 1981) in the central elevated areas. The flat base surfaces were created with a constant height (Appendix B), based on the elevation of lowermost outcropping pyroclastic or lava rocks. The sub-surface spot heights were derived from drill core descriptions (n = 488) and field observations (n = 26). Using these two surfaces, the volumes were obtained by Eq. 4.6.
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Figure 4.5 (A) Scan of a scoria hand specimen from the Rangitoto scoria cone. (B) Binary image of the same sample, showing the distribution of vesicles in white. The red box is the area considered in the 2D vesicularity calculations. (C) Field photo of a moderately vesiculated lava flow texture from the lava flow field of Rangitoto. (D) Thresholded binary image showing the distribution of largest vesicularity population. (E) Graph showing the results of density measurements on scoria (n = 48) and lava rock (n = 42) samples from Rangitoto and Browns Island volcanoes. The densities were measured as envelope density by Micrometrics Geo PyC1360 density analyser. Due to the small diameter (i.e. 2 cm in diameter) of the samples measured in the density analyser, these density and vesicularity values are considered as minimum values. The vesicularity is calculated proportional to 2.8 g/cm3.
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The limits of volcanic edifices and lava flows were based on a combination of geological maps (Kermode, 1992; Hayward et al., 2011), high-resolution orthophotos (0.5 m), drill core datasets (e.g. PETLAB database, www.pet.gns.cri.nz) and new field mapping. Where DSM-based bulk volume estimates could not be performed accurately (e.g. smaller volcanic edifices, with ill-defined boundaries), bulk volumes from Allen and Smith (1994) were adopted. Additional details for each volcano examined are contained within Appendix B.
4.3.3. Bulk medial to distal pyroclastic volume for the AVF
Erosion and reworking of basaltic tephras, along with the intensive urban modification over the AVF make it impossible to map fall distributions and provide reliable estimates of distal tephra volumes. The youngest volcano, Rangitoto (553–504 yrs BP) has a local tephra blanket that is mapped onshore (Needham et al., 2011), but other tephra falls are preserved only at isolated locations within lacustrine successions (Augustinus et al., 2011; Shane and Zawalna-Geer, 2011). The distal tephra layers associated confidently with their potential source (n = 24) have recently been revealed by drilling (Molloy et al., 2009; Augustinus et al., 2011; Shane et al., 2013) and matched via statistical likelihoods (Bebbington and Cronin, 2011).
Due to these sparse data, the range of erupted tephra must be approximated using historical eruption analogues. The typical tephra blankets of various comparable
eruption styles are: 10.3×106 m3 [fire fountaining eruptions of Kilauea Iki, Hawaii
(Parfitt, 1998)], 38×106 m3 [violent Strombolian eruptions of Jorullo scoria cone,
Mexico (Rowland et al., 2009)], or 30×106 m3 [phreatomagmatic eruption of the 1886
AD Rotomahana eruption New Zealand (White and Ross, 2011)]. In order to obtain a systematic bulk volume range for the tephra blankets around the AVF volcanoes, Eqs 7 and 8 were applied, which include proximal deposits, such as scoria cones and ejecta rings. However, these are likely overestimates (Appendix B) and cannot be reliably used for field evolution interpretations.
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