Matriz de Evaluación de Factores Externos FACTORES DETERMINANTES

Table 8.1 shows a summary of the different characteristics for the three studied volcanoes, compiled from the results of chapters 5, 6, and 7. Some of the most relevant aspects of these results are next described in a comparative manner between the three scenarios.

8.2.2.1 Eruptive centres and types of deposits

Maungataketake shows a very broad crater surrounded by a roughly irregular ejecta ring with a low rim and gentle slopes (Table 8.1). The entire shape and extent of the Motukorea ejecta ring and crater is not possible to distinguish. Although it is not directly observable that Maungataketake and Motukorea phreatomagmatic craters are cut beneath the pre-eruption ground surface, the evidence gathered during this study (moderate size crater diameter, lithic-rich ejecta ring deposits, and geophysical inference of magmatic bodies infilling a deep crater) point to the fact that those are maar craters underlain by a diatreme. North Head’s tuff cone has relatively steeper slopes and a narrow crater typical of tuff cones. Fall deposits dominate North Head, whereas base surge units characterize Maungataketake and Motukorea ejecta ring deposits (Table 8.1). The wettest deposits were emplaced at Maungataketake where the most abundant accretionary lapilli and soft-sediment deformation features occur. Unlike Motukorea base surge deposits, erosive boundaries are not pervasive at Maungataketake ejecta ring exposures. “Wetness” of deposits diminishes upwards the Motukorea phreatomagmatic sequence, with increasing evidence of pyroclastic fall. Conversely, “wetness” is relatively pervasive within the entire

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Maungataketake phreatomagmatic units. North Head deposits exhibit progressively drier sedimentary characteristics upwards.

Table 8.1 Summary of the general morphometric, stratigraphic, sedimentary, and pyroclast characteristics of the studied volcanoes

Maungataketake Motukorea North Head

Morphological

type Maar Maar Tuff cone

Volume of tephra

ring/cone (m3₎

4.3×106 _3.1×106 _1.1×106

Crater diameter (m)

1300×1100 Partial tephra ring,

~<1000

~220-280 m

Crater depth Unknown Unknown Unknown

Maximum rim

height (m) 25 40 ~50-60

Dip of outer beds Nearly horizontal to less than

20° Few degrees to less than 20° Lower beds: up to 10° Upper beds: up to ~22°

Dominant stratification

Plane parallel bedded, cross- lamination

Cross- to plane- parallel- bedded Massive to crudely stratified

Sedimentary features

Overall accretionary lapilli and soft sediment deformation

Dominant soft sediment deformation and erosive surfaces in the lower deposits

Common open framework, non erosive surfaces

Vesiculated tuff Yes No Yes

Main transport mode of pyroclasts

Wet base surges, minor fallout Wet/dry base surges,

fallout

Fallout, minor base surges Dominant grain

size

Fine lapilli to fine ash Medium lapilli to medium ash Coarse lapilli to coarse ash

Dominant juvenile grain size

Coarse ash to medium lapilli Coarse ash to medium lapilli Coarse ash to medium lapilli

Juvenile content (vol.%)

35 40-45 >90

Juvenile vesicularity

Poor (<30 %) Poor (<30%) Variable (5-65%)

General juvenile

morphology Sub-angular, sub-rounded, blocky Sub-angular, angular, blocky, fluidal Angular, blocky, fluidal

Adhering

particles Yes Yes No

Dominant lithic type

Plio-Pleistocene sediments Waitemata rock fragments N/A

Substrate type Waitemata rocks + 60-m Plio-

Pleistocene sediments Waipapa rocks + 200/300 m of Waitemata rocks Waitemata rocks

Water involved Groundwater Groundwater Superficial water

Underlain by

diatreme Yes possible, inferred Yes possible, inferred No, inferred

Followed by dry activity

Yes Yes Yes

Length of phreatomagmatic eruption

Few days Few days Hours to very few days

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8.2.2.2 Characteristics, percentage contents and distribution of pyroclasts

North Head tuff cone deposits differ strongly from the other centres in being dominated by juvenile fragments (>90 vol.%) compared to ~45 vol.% at Motukorea and ~35 vol.% for Maungataketake ejecta ring deposits (Table 8.1), which is typical of maar-diatreme volcanoes. The bulk of juvenile pyroclasts are in the range size of medium lapilli-to-coarse ash in all of the studied deposits.

North Head juvenile pyroclasts are the most angular: The Motukorea juvenile fragments are angular-to- subangular with some having fluidal shapes, while Maungataketake ones are blocky and sub-angular to sub-rounded (Table 8.1). Adhering, fine lithic-ash on the surfaces of juveniles is widespread on Maungataketake and Motukorea fragments, but not on North Head clasts. The general range in vesicularity of North Head juveniles is broader (~5 to 65%) than at the other two sites (usually <30%) (Table 8.1). Both sideromelane and tachylite occur in all volcanoes, with Motukorea containing the highest proportions of sideromelane (>50 vol.%). Palagonitization is not pervasive, but most evident in North Head and Motukorea.

North Head tuff cone deposits contain very few substrate-derived lithics (<10 vol.%), compared to Maungataketake (~65 vol.%) and Motukorea (~55 vol.%) ejecta ring deposits (Table 8.1). The lithics are concentrated in the medium-to-fine ash-size range in Maungataketake (~50-60 vol.%) and Motukorea (~30-40 vol.%). The Maungataketake lithic ash is mainly made up of particle aggregates of individual crystals of quartz and feldspar (>80 vol.%) sourced from the Plio-Pleistocene sediments (Table 8.1). By contrast, Waitemata fragments dominate Motukorea lithic ash (> 80 vol.%).

8.2.2.3 Local eruptive settings and environmental conditions

Three distinct general eruption settings are recognized (Fig. 8.1): a) >60 m-thick soft Plio-Pleistocene sediments onto hard Waitemata Group rocks (Scenario 1); b) Waitemata rocks onto basement (Scenario 2); and c) a shallow submarine setting (Scenario 3). Waitemata rocks are the most common lithology in the AVF area and these host low yield, anisotropic, heterogeneous, confined or semi-confined aquifers. Thickness of the Waitemata rock sequence is not known at Maungataketake, but beneath Motukorea and North Head it reaches at least 200-300 m.

The Auckland area has remained tectonically more stable than many parts of New Zealand since the onset of the AVF activity (250 ka) (Alloway et al., 2004; Beavan and Litchfield, 2012; Kenny et al., 2012). A paleo-environmental reconstruction at Maungataketake (Marra et al., 2006) suggests that general environmental conditions were similar to the present during the Maungataketake phreatomagmatic eruption (in the Manukau Lowlands). Cover-bed stratigraphy and dating imply that the eruption is older than 125 ka.. At Motukorea (in the northern AVF), an Ar-Ar age determination on juvenile material of 14.3 ±5.5 ka was recently obtained (Leonard Graham, written communication 2014). This is consistent with a

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mid-Holocene beach-level on the island and indicates the eruption occurred when sea level was possibly up to 30 metres lower than today. The eruption site lay above sea level near the banks of the ancestral Tamaki River. When sea level was lower during both Maungataketake and Motukorea eruptions, the now submerged area of the AVF was irrigated by ancestral hydrological networks. It is very likely that the hydrogeolgical conditions by the time of such eruptions where similar to the ones prevailing nowadays. Sea level reconstructions indicate that the most likely age for North Head eruption was between 128 and 116 ka when the sea level was 2-4 m above current sea level. The presence of surface water set the conditions for the formation of a tuff cone.

8.2.2.4 The inferences on eruptive styles and the ending of the phreatomagmatic phase

Considering the general morphometric, morphological, and sedimentary characteristics of small basaltic volcanoes (which includes tuff rings, tuff cones, and maars) (chapter 2), the studied phreatomagmatic volcanoes (Table 8.1) are within the range of medium to small size phreatomagmatic volcanoes, similarly to other phreatomagmatic vents in the AVF. Although all the cases are relative small eruptions in volume

[for example individual AVF ejecta rings usually comprise much less than 0.02 km3 _{in volume and are}

related to similar volumes of magma involved (for the exact figures see Kereszturi et al., 2013)], the stratigraphic sequences, the sedimentary structures, and the pyroclast distribution are varied within each case. These observations imply varied modes of transport of pyroclasts and changes in their rates of sedimentation and deposition

Changes in phreatomagmatic dynamics can be inferred from the bedforms, bedding transitions, lamination characteristics and the presence and absence of accretionary lapilli. It has been mentioned that the phreatomagmatic sequence of each volcano shows changes in eruptive styles from subtle changes in the Maungataketake ejecta ring formation to more distinct shifts characterized by drier upward sequences (Motukorea, Scenario 2) or transient magmatic phases (North Head, Scenario 3). However, juvenile pyroclast morphology and vesicularity suggest that in the construction of the entire ejecta rings and tuff cone water played a role to a greater or lesser extent.

The study of lithics within the sequences added important information about the type of substrate involved in the phreatomagmatic eruptions. Maungataketake and Motukorea ejecta rings are dominated by lithics sourced from shallow depths. These observations, paired with the inferred substrate conditions at the time of eruption provided information on the role of the substrate in the changing eruption dynamics. Shallow- seated explosions dominated in the construction of the Maungataketake and Motukorea ejecta rings (sections 8.2.1.1, 8.2.1.2; Fig. 8.1), while a Surtseyan eruption characterized the formation of North Head (section 8.2.1.3; Fig. 8.1).

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All eruptions eventually shifted to a magmatic phase with the accumulation of spatter, scoria, and the emplacement of small lava flows. The relative homogeneity in vesicularity and textures of the juvenile pyroclasts during the Motukorea ejecta ring construction do not indicate a change in magma flux. In this case, it is however likely that water became progressively scarcer due to the low permeability and water content of Waitemata Group rocks. At North Head, the transition from phreatomagmatic to magmatic phase is coupled with an increase in vesicularity of juvenile lapilli. This could indicate a change in magma flux or sealing of the vent area from water ingress, or both, as in the Maungataketake scenario.

8.3 Discussion

Maar and tuff cone eruptions in historical times are poorly documented. Even in the case where the chronology of a maar eruption exists (e.g. Ukinrek maars; Kienle et al., 1980) it does not record the step- by-step tephra ring construction. Despite this hindrance, it was still possible here to infer the distinct eruptive styles and the mechanisms of transport and deposition of pyroclasts in the formation of the ejecta ring (or cone) of each studied volcano (section 8.2) by using typical stratigraphic and sedimentary methodology.

These findings are in agreement with most of previous eruptive reconstructions of phreatomagmatic eruptions in the AVF (Allen et al., 1996; Houghton et al., 1999; Németh et al., 2012a) and worldwide (e.g. Verwoerd and Chevallier, 1987; Chough and Sohn, 1989; Sohn and Chough, 1989, 1992, 1993; White, 1990, 1991b; 2001; Büchel and Lorenz, 1993; Németh et al., 2001, 2008, 2012a; Auer et al., 2007; Ross et al., 2011; Lefebvre et al., 2013; van Oterloo et al., 2013).