gravity and magnetic investigation of maar volcanoes, auckland volcanic field, new zealand

al Research 159 (2007) 153–163www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geotherm

Gravity and magnetic investigation of maar volcanoes,Auckland volcanic field, New Zealand

John Cassidy ⁎, Sian J. France 1, Corinne A. Locke

Department of Geology, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Received 17 December 2004; accepted 11 June 2006Available online 14 August 2006

Abstract

Detailed gravity and aeromagnetic data over maars in the Auckland volcanic field reveal contrasting anomalies, even wheresurface geology is similar. Pukaki and Pukekiwiriki, almost identical maars marked by sediment-filled craters and tuff rings, havegravity and magnetic anomalies of −6 g.u. and 20 nT, and 8 g.u. and 160 nT, respectively. The Domain and Waitomokia maars,with similar tuff rings but each with a small central scoria cone, have gravity and magnetic anomalies of 32 g.u. and 300 nT, and21 g.u. and 310 nT, respectively. These differences in geophysical expression are attributed to varying volumes of dense, magneticbasalt in the form of shallow bowl-shaped bodies up to several hundreds of metres in diameter and up to 140 m thick beneath themaar centres. These bodies are interpreted as solidified magma that ponded into early-formed phreatomagmatic explosion craters.Where magma supply was limited relative to groundwater availability, no residual subsurface basalt occurs (as at Pukaki);continued magma supply, but limited groundwater, resulted in ponding (e.g. at Pukekiwiriki) and eventually the building of a scoriacone (as at Domain and Waitomokia). There is no evidence in these geophysical data for diatreme structures below the maars or forshallow and/or extensive feeder dykes associated with these maars. If diatreme structures do occur, their lack of geophysicalsignature must be a consequence of either their small geophysical contrast with host Miocene sediments and/or masking by thestronger anomalies associated with the subsurface basalt. In addition, any magma conduits appear to be confined centrally beneaththe maars, at least to shallow depths (upper 100 m).© 2006 Elsevier B.V. All rights reserved.

Keywords: maar volcanoes; gravity; magnetic; Auckland volcanic field

1. Introduction

The Auckland volcanic field (Fig. 1) is a Quaternarymonogenetic basaltic field comprising 49 eruptive cen-tres, of which about 35 show evidence of phreatomag-matic activity and 20 of which have well preserved maarcraters and tuff rings (Kermode, 1992; Allen and Smith,1994). The volcanic field displays many similarities to

⁎ Corresponding author. Fax: +64 9 3737435.E-mail address: [email protected] (J. Cassidy).

1 Present address: Beca, P.O. Box 6345, Auckland, New Zealand.

0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2006.06.007

other Quaternary basaltic monogenetic fields such as theEifel field, Germany (Schmincke et al., 1983) and thePinacate field, Mexico (Gutmann, 2002), although it issmaller in terms of the number and size of volcanoes andhence the total erupted volume. There are few publishedgeophysical studies of the subsurface structure of maarsor cones within such fields (e.g. Wood, 1974; Buechel,1988; Rout et al., 1993). The Auckland field providesfavourable conditions for geophysical studies since low-density, non-magnetic sediments host the volcanoes,giving rise to strong geophysical contrasts between thebasaltic and sedimentary rocks. Complementary gravity

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Fig. 1. Volcanoes and associated deposits of the Auckland volcanic field (after Kermode, 1992). Maar volcanoes discussed in this work are numbered:1. Domain, 2. Pukekiwiriki, 3. Waitomokia and 4. Pukaki. The inset shows North Island, New Zealand with the area covered by the main map(arrowed) and the present-day plate boundary marked (teeth on the upper plate).

154 J. Cassidy et al. / Journal of Volcanology and Geothermal Research 159 (2007) 153–163

155J. Cassidy et al. / Journal of Volcanology and Geothermal Research 159 (2007) 153–163

and magnetic measurements are used in this study toinvestigate a number of typical maar volcanoes withinthe field and model their subsurface geometry, espe-cially any sub-crater volcanic structures (e.g. feederdykes or diatremes) which may be present.

A key aspect of this study is the use of aeromagneticdata. This has the potential advantage of better iden-tifying anomalies caused by deeper structures, since itavoids the typically short-wavelength, high amplitudeground-based anomalies associated with near-surfacerocks. Thus, the depths of investigation of both gravityand magnetic methods in this study are comparable.Furthermore, combining gravity and magnetic methodsreduces the ambiguities inherent in potential field in-terpretation and allows the identification of rock unitswhich have only density or magnetisation contrasts.Also, the fact that all volcanoes in the Auckland field areyoung, and hence are exposed at the surface, makesgeophysical modelling of volcanic structures morestraightforward compared to situations where thevolcanoes are buried by younger sediments. Finally,an important aspect of the study is that there is boreholecontrol on the thicknesses of key rock types at all thevolcanoes described in detail, which therefore providesgood constraints for the geophysical models.

Two pairs of case studies are reported here that allowdetailed comparisons of structures beneath the maarcraters: one pair (Pukaki and Pukekiwiriki volcanoes,Fig. 1) comprises near-identical maars, each consistingof a simple tuff ring and sediment-filled crater, hostedwithin loosely compacted Plio-Pleistocene sediments;the second pair (Domain and Waitomokia volcanoes)are similar but each has, in addition, a small centralscoria cone within its maar crater. In the case of the latterpair, Waitomokia is hosted by the Plio-Pleistocene sedi-ments and Domain is hosted by Miocene sandstones.This paper presents new data from the Pukekiwiriki,Domain and Waitomokia volcanoes and uses existingdata from Pukaki (Rout et al., 1993) which is re-inter-preted in the light of new borehole information.

2. Geological setting

The geology of the Auckland region (Fig. 1) consistsprimarily of sequences of alternating sandstones andmudstones (with occasional grit and conglomerate ho-rizons) of Miocene age, approximately half a kilometrethick below the Auckland field (Edbrooke et al., 1998).These sediments overlie Mesozoic metasedimentarybasement rocks which are up-lifted to the east as aconsequence of extensional block-faulting (marked by arhombic pattern of NNW and ENE striking normal

faults), that occurred throughout the region during Plio-cene–Quaternary times (Kermode, 1992; Hochstein andBallance, 1993). This faulting may possibly continue upto Recent times (Wise et al., 2003). Locally overlyingthe Miocene sequences are loosely consolidated Plio-Pleistocene fluviatile sands (occasionally pumiceous) upto several tens of metres thick, which occur mostly in thesouth of the field, especially in paleo-valleys within theMiocene rocks (Kermode, 1992).

The Auckland volcanic field occurs in a region ofpossible intraplate extension (Smith, 1989; Sporli andEastwood, 1997), 400 km west and behind the present-day plate boundary through New Zealand. The fieldconsists of small monogenetic volcanic centres whoseages are poorly known but that are thought to extendfrom Recent to possibly 250,000 yr BP (Allen andSmith, 1994; Shane et al., 2002). The volcanic rocks aredominantly alkali basalts or basanites with less commontholeiite, transitional basalt and nephelinite (Hemingand Barnet, 1986). There is a wide range of eruptionstyles in the Auckland field, from dominantly phreato-magmatic (producing maars and tuff rings) to domi-nantly magmatic (producing Strombolian/Hawaiianscoria cones and lava flows), typical of many monoge-netic fields (Schmincke et al., 1983; Houghton et al.,1996). The total estimated eruptive volume of depositsis about 4 km3 (Allen and Smith, 1994).

Volcanoes in the Auckland field exhibit no obviousspatio-temporal patterns that might indicate structuralcontrols on the sites of eruptions or on the evolution ofvolcanism within the field. The limited age data do,however, indicate that eruption frequency and magni-tude have been variable during the life of the field, butwith a clear tendency for eruption magnitude to increasewith time (Allen and Smith, 1994). Hence, hazard as-sessment for Auckland City, which is New Zealand'slargest population centre and geographically coincideswith the Auckland volcanic field, is problematic, espe-cially given the likely rapid rise of magma from mantledepths (Allen and Smith, 1994).

3. Geology of the maar volcanoes

The four maars studied here (Figs. 2a and 3a) havesimilarly sized, approximately circular raised tuff rings,with diameters of about 700 m. The tuff rings typicallyconsist of well-bedded lithic tuff (dominantly countryrocks with a minor basaltic component) occurring asboth fall and surge deposits (Kermode, 1992). Themaars are hosted by Plio-Pleistocene sediments up to50 m in thickness, except for Domain volcano, which ishosted by Miocene sedimentary rocks. All these maars

Fig. 2. (a) Geology of the Pukaki and Pukekiwiriki maars (after Kermode, 1992), geology key as in Fig. 1; A–A′ etc. are profile lines for modelling,borehole locations are shown as dots. (b) Corresponding residual Bouguer gravity anomalies (station locations as dots), contour interval 2 g.u., outlineof lithologies from (a) dashed. (c) Corresponding residual aeromagnetic anomalies (station locations as dots), contour interval 10 nT, outline oflithologies from (a) dashed.



have slightly depressed craters containing recent sedi-ment infill and two have been breached by the sea sinceformation (Pukaki and Pukekiwiriki).

In Pukaki crater, the probable maximum thickness ofcrater sediments is known from stratigraphic drilling in thecentre of the crater to be about 75m (Dickinson et al., 2002)and consists of 50 m of marine muds overlying 25 m of

Fig. 3. (a) Geology of the Domain and Waitomokia maars (after Kermode, 19borehole locations are shown as dots. (b) Corresponding residual Bouguer gravof lithologies from (a) dashed. (c) Corresponding residual aeromagnetic anlithologies from (a) dashed.

lacustrine organic-rich muds, below which lies basalticejecta (to at least 88 m depth). The thickness of thesurrounding Plio-Pleistocene sediments is at least a fewmetres and possibly some tens ofmetres (Rout et al., 1993);a possible occurrence of scoria mapped in the centre of thePukaki crater (Kermode, 1992) is unconfirmed. Hence thecrater is excavated into the Miocene sedimentary rocks

92), geology key as in Fig. 1; C–C′ etc. are profile lines for modelling,ity anomalies (station locations as dots), contour interval 5 g.u., outlineomalies (station locations as dots), contour interval 50 nT, outline of


indicating that magma–water interaction penetrated todepths of at least a few tens of metres into the Miocenecountry rocks. At Pukekiwiriki, engineering boreholesdrilled both into and outside the tuff ring encounter amaximum thickness of 20 m for the remanent of the tuffring and a maximum thickness of 30 m for the Plio-Pleistocene sediments across the volcano (France, 2003).

The Domain andWaitomokia maars both have, or had,central scoria cones, 20 m and 40 m high, respectively(although the cone of Waitomokia has now beencompletely removed by quarrying). At Domain volcano,a secondary scoria cone on the outer edge of the tuff ring(some 600 m southwest of the present cone) was pos-tulated by Searle (1981) based on a low scoria mound, butno current trace exists. Engineering drilling within the tuffring has revealed basalt flows and breccias up to 95m thickconcealed beneath the tuff, with the tuff depositsthemselves being up to 20 m thick. Borehole data atWaitomokia are more scarce and show only thin post-eruptive sediments within the maar, with the Plio-Pleistocene sediments approximately 50 m thick acrossthe whole area.

The only reported age determinations for the vol-canoes studied here are an unconfirmed luminescencedate of 125,000 yr BP from sediment core at the base ofthe Pukaki crater (Rieser and Shulmeister, 2002) and anunconfirmed K–Ar date of 148,000 yr BP from basalt atDomain volcano (MacDougall et al., 1969).

4. Geophysical data acquisition and processing

Gravity data were collected over an area in the vicinityof the Domain and Pukekiwiriki maars and along ortho-gonal profiles at Pukaki (Rout et al., 1993) and Wait-omokia, using a station spacing of approximately 50 macross the central anomalies and 100–200 m elsewhere.Elevation control was by differential GPS or preciselevelling (at Pukaki) to an accuracy of better than ±20 cm.The Bouguer gravity data were corrected with the appro-priate density of the near-surface host sedimentary rocks.Errors in the corrected gravity data are estimated to be±1 g.u. (1 g.u.= 0.1 mgal). Residual anomalies werecalculated by subtraction of a smooth ‘regional’ field localto each volcano, determined from nearby stations un-affected by the gravity effects of the volcanoes. Residualvalues are close to zero distant from the volcanoes showingthat the regional field was successfully removed in allcases (Figs. 2b and 3b).

The aeromagnetic data are part of a more extensivedataset (Rout et al., 1993;Cassidy et al., 1999; Cassidy andLocke, 2002) and were collected along SW–NE linesevery 100 m, at a nominal elevation of 330 m and line

spacing of 500 m (except at Domain volcano where ele-vation was 430m and line spacing 250 m). Positional datawere provided by differential GPS to an accuracy of ±5 m(except for over Pukaki where positioning was based onvideo recording to an estimated accuracy of ±50 m). Totalfield magnetic intensity data were calculated by subtract-ing diurnal variations recorded at a local base station andremoval of the International Geomagnetic ReferenceField (IGRF). Errors in the corrected magnetic data areestimated to be at most ±5 nT. As with the gravity data, aregional field at each volcano was defined from stationsdistant from the volcano and subtracted to give theresidual anomaly; in all cases, the residual values arearound the zero level distant from the volcano indicatingsuccessful removal of the regional field (Figs. 2c and 3c).

5. Results

Residual gravity and aeromagnetic anomalies are shownin Figs. 2 and 3 together with the corresponding geology. Itcan be seen that these maars (apart from Pukaki) havesignificant, though contrasting, positive geophysical anoma-lies which are broadly coincident with their mapped centres.This indicates the occurrence of dense, magnetic rocks atdepth below the maar craters. The polarity and location ofthe magnetic anomalies relative to their correspondinggravity anomalies indicates a magnetisation directionsimilar to that of the present-day geomagnetic field.

5.1. Pukaki and Pukekiwiriki

Despite their very similar geology and setting, thesetwo maars have contrasting geophysical signatures. Pu-kaki has a subdued negative gravity anomaly (−6 g.u.),located in the centre of the crater, and an indistinctmagnetic expression (b+20 nT) indicating low-densitycrater-filling sediments but no dense magnetic materialat depth. Pukekiwiriki, however, has a small, somewhatcomplex positive gravity anomaly (+8 g.u.) and a cir-cular-shaped moderate magnetic anomaly (+160 nT),indicating the presence of a subsurface volume of densemagnetic rocks. These contrasting results emphasise thevalue of geophysical methods in studying maars sincethe geological mapping provides no information on suchdifferences between these maars.

5.2. Domain and Waitomokia

The Domain and Waitomokia maars both have sig-nificant gravity and magnetic anomalies indicating sub-stantial subsurface volumes of dense magnetic rocksbeneath their craters. These results contrast with those


found at Pukaki and to some extent Pukekiwiriki, per-haps as might be expected given their central scoriacones (though as noted Waitomokia no longer has acone). Waitomokia has a circular gravity anomaly of+21 g.u. located in the crater centre and a strong mag-netic anomaly of +310 nT, shifted somewhat north ofthe gravity anomaly, indicating that the dense magneticrocks must extend to some depth (cf. Pukekiwiriki). TheDomain maar has a stronger, rather elongate, gravityanomaly of +32 g.u. which, in contrast to the othermaars, is offset from the centre of the crater (as definedby the present-day tuff ring and cone). The location ofthe gravity anomaly centre is coincident with the postu-lated secondary cone referred to earlier and its generallygreater magnitude is consistent with the thick subsurfacebasalts penetrated by boreholes. A similarly strong butbroader magnetic anomaly of +300 nT is located to the

Fig. 4. Cross sections through 3Dmodels of the subsurface structure (vertical exagandB–B′ (Fig. 2). (a, b)Observed residual (dashed lines) and calculated (solid linessections (geology key as Fig. 1) showing borehole depth control. Modelled dePleistocene and crater fill (1800–2200 kgm−3) andMiocene sediments (2200–2301.0 A m−1), other rocks have zero magnetisation; directions of magnetisation are

north of the gravity anomaly, again indicating the oc-currence of dense magnetic rocks at some depth.

6. Geophysical modelling and interpretation

Residual gravity and magnetic data were modellediteratively in 3D (with the exception of Waitomokiawhich was modelled in 2.75D, i.e. 2D models that canhave non-infinite and uneven ‘half-widths’); cross sec-tions through the resulting models are shown in Figs. 4and 5. Rock densities and magnetisations were based onpreviously published values for volcanic and sedimen-tary rocks within the Auckland field (summarised inRout et al., 1993; Affleck et al., 2001). Given the mag-netisation intensities required to fit the observed data, itis probable that remanent magnetisation is dominant (atleast in the basalts). Magnetisation directions are likely

geration of 3) beneath the Pukaki and Pukekiwiriki maars along lines A–A′)magnetic and gravity data projected along the profile lines. (c)Model crossnsities are: basalt (2800–2900 kg m−3), tuff (1900–2200 kg m−3), Plio-0 kgm−3); modelledmagnetisations are: basalt (5.5–6.0Am−1), tuff (0.5–as present day (see text).


to be similar to that of the present-day geomagnetic field(i.e. −63° inclination and 20°E declination) given theage of the basalts, and any uncertainty in these directionsis probably insignificant with respect to the modellingerrors. Models were adjusted to optimise the fit betweencalculated values and observed anomalies for both thegravity and magnetic data, using the borehole data asindependent control on the thicknesses of lithologicalunits. Complementary gravity and aeromagnetic data,together with extensive borehole control, is rarelyavailable for such studies and has significantly reducedthe ambiguities in these models. For the optimummodels

Fig. 5. Cross sections through 3D models of the subsurface structure (verticalines C–C′ and D–D′ (Fig. 3). (a, b) Observed residual (dashed lines) and calclines. (c) Model cross sections (geology key as Fig. 1) showing borehole detopographic profiles of the projected (Domain) and former (Waitomokia) sc(2000–2100 kg m−3), Plio-Pleistocene and crater fill (1500–2000 kg m−3) aare: basalt (4.0–7.5 A m−1 for Domain and 17 A m−1 for Waitomokia), tufmagnetisation are as present day (see text).

(Figs. 4 and 5), fits between the calculated and observedvalues are generally within, or close to, the measurementerrors.

It is apparent from both the observed anomalies andthe models that the tuff deposits and scoria cone haveonly minor gravity and magnetic effects and that thestrong anomalies are a consequence of rocks which areentirely concealed beneath the surface. To account for theobserved anomalies at most maars it is necessary to havesubstantial subsurface volumes of rocks with densitiesranging from 2800 to 3000 kg m−3 and magnetisationsof 4–17 A m−1. These ranges of values are typical of

l exaggeration of 3) beneath the Domain and Waitomokia maars alongulated (solid lines) magnetic and gravity data projected along the profilepth control. Insets show true-scale cross sections and the approximateoria cones. Modelled densities are: basalt (2800–3000 kg m−3), tuffnd Miocene sediments (2200–2300 kg m−3); modelled magnetisationsf (0.5–1.0 A m−1), other rocks have zero magnetisation; directions of


basaltic rocks (France, 2003) like those encountered inboreholes at depth beneath the Domain crater.

6.1. Pukaki and Pukekiwiriki

At Pukaki, there is no gravity or magnetic evidencefor any significant volume of basalt below the crater(Figs. 2 and 4), a result consistent with the drill hole datain the centre of the crater, at least up to the maximumdepth penetrated (88 m) where basaltic sands and lapilliwere reached below the lacustrine sediments. The oc-currence of only a weak negative gravity anomaly im-plies that the density of the marine muds is about1900 kg m−3; it is very unlikely that any lower-densitysediments within the crater offset the gravity effects of adeeper, dense basaltic layer because there is no accom-panying magnetic anomaly. This demonstrates the valueof having both gravity and magnetic data in interpretingpotential field data over such maars.

At Pukekiwiriki (Figs. 2 and 4), the only availableborehole data were collected from the tuff ring depositsand outside the crater (Fig. 2a), where no basalt wasencountered, but they are useful in constraining themaximum lateral extent of any basalt present and tuffthicknesses. A 20–40 m thick basalt layer, extendingbeneath the width of the central crater, is required toaccount for the gravity and magnetic anomalies. Thisbasalt body extends below the level of the surroundingPlio-Pleistocene sediments (assuming these sedimentsare the same thickness as shown by the boreholes out-side the crater), just penetrating the Miocene sediments.

6.2. Domain and Waitomokia

At Waitomokia (Figs. 3 and 5), borehole data withinthe tuff ring (Fig. 3a) show a maximum 5 m thickness ofpost-eruption deposits, but no basalt. To account for thestrong gravity and magnetic anomalies, a body of basaltup to 150 m thick is required, extending over about halfthe diameter of the tuff ring (i.e. not much beyond theextent of the former scoria cone). This basalt bodypenetrates 100 m into the underlying Miocene sediments.

The more extensive gravity anomaly at Domainvolcano (Figs. 3 and 5) requires a more complex sub-surface model. About 30 boreholes have been drilledwithin the perimeter of the tuff ring, mostly concentratedinto two groups in the west and south (Fig. 3a); 17 ofthese penetrated through to the underlying Miocenesediments. These borehole data have provided veryvaluable controls on the models, especially in defininglateral limits and thicknesses of the basalt flows underthe tuff ring, the thickness of the tuff ring (up to 40–

45 m), and a minimum thickness for the scoria cone(25 m). In order to account for the geophysical ano-malies and also be consistent with the borehole data, twodistinct bodies of basalt are required, one centred belowthe postulated location of a former ‘secondary’ cone andthe other approximately beneath the present-day cone.These bodies extend across most of the crater and pe-netrate about 50–100 m into the Miocene rocks.

7. Discussion

The bowl-shaped geometries of the modelled sub-surface basalt bodies below the Pukekiwiriki, Domainand Waitomokia maars are similar to those of cratersexcavated by phreatomagmatic explosions, for exampleat Ukinrek (Kienle et al., 1980), and in volcanic fieldssuch as the Eifel (Lorenz, 1986) and Pinacate (Gutmann,2002). Given the high density and magnetisation ofthese modelled bodies, they are unlikely to consist of asignificant proportion of volcaniclastic deposits orrepresent inter-fingering feeder systems. Rather, lavaponding from the continuing extrusion of magma intoearly-formed craters is a more likely explanation (c.f.Lorenz, 1986). An example from the Auckland field ofsuch ponding within a crater occurs at Crater Hill vol-cano (Houghton et al., 1999) and similar features arecommon in monogenetic fields elsewhere (e.g. Lavineand Aalto, 2002).

It is perhaps surprising that the geophysical dataprovide no clear evidence for diatreme structures belowthe maars. Such structures might be expected to havesome geophysical expression on account of their poten-tially lower density and higher magnetisation than thehost rocks (cf. Lorenz, 1986; Buechel, 1988). For ex-ample, taking the tuffs as a geophysical proxy for dia-treme fill (density and magnetisation contrasts withMiocene host rocks of −200 kg m−3 and 0.5 to 1 A m−1,respectively; France, 2003) would give anomalies ofabout −10 g.u. and 10 to 20 nT, respectively, assumingthe diatreme geometries envisaged by Lorenz (1986).Therefore, if such a diatreme were present, it should bedetectable using the present gravity and magnetic data,particularly at Pukaki. It should be noted that the grit andconglomerate beds within theMiocene rocks (mentionedearlier) are both thin and discontinuous, and thereforeinsignificant with respect to the bulk of the country rock.However, for those maars modelled here with substantialsubsurface bodies of basalt, the geophysical effects ofany underlying diatreme might be masked. Therefore, ifdiatreme structures do occur beneath these Aucklandmaars, they must have minor geophysical contrast withthe host Miocene sandstones.


There is no evidence in these data for near-surfacefeeder dykes in the vicinity of the volcanoes that mightbe indicative of a local structural control on the magmaconduits. Indeed, both the geometry of the tuff rings andtheir associated anomalies are quite circular. Dykes mayexist at depths beyond the resolution of the methods, butany shallow feeder conduits appear to be confined cen-trally to the maars. Elsewhere in the Auckland field,evidence for alignment of vents or a structural control onvolcanism on any scale is scarce, although occasionalexamples are reported (Sporli and Eastwood, 1997;Houghton et al., 1999). This contrasts with many similarbasaltic fields elsewhere where deep-seated structuralcontrols are commonly reported, for example at Eifel(Schmincke et al., 1983), Pinacate (Lutz and Gutmann,1995) and the Mexican Volcanic Belt (Ort and Carrasco-Nunez, 2004). The centres of the two separate modelledbasalt bodies below Domain volcano do, however, havean alignment that is concordant with one of two majorstructural trends in the region.

These case studies provide three examples that pos-sibly reflect a balance between magma supply andgroundwater availability. In the case of Pukaki, all in-coming magma interacted explosively with groundwaterto create a crater 70 m deep which subsequently filledwith terrestrial, then marine, muds. This suggests thatmagma supply was either limited or possibly withdrewto greater depths. Alternatively, pressure release andhence degassing of the magma (due to its explosiveinteraction with groundwater) may have sufficientlyreduced its buoyancy to prevent further eruption, how-ever, the geophysical data show that any such residualsolidified magma must be either of small volume or atdepths beyond the resolution of these data. In the case ofPukekiwiriki, groundwater availability appears to havebeen more limited relative to magma supply and hence ashallower crater was formed (30 m deep), not muchdeeper than the Plio-Pleistocene sediments; further up-welling magma ponded in the crater to form a thin lensof basalt. In contrast, at both the Domain and Wait-omokia maars, magma supply was substantially greater,which, together with a continuing availability ofgroundwater, allowed phreatomagmatic excavation tomuch greater depths (100–150 m). Eventually, however,the groundwater supply was exhausted and effusivemagmatism predominated, forming lava lakes in thecraters. For Domain volcano, the geophysical modelprovides evidence that the focus of eruption shiftedconsiderably during its lifetime.

It is notable that the depth of excavation for both theDomain and Waitomokia maars is similar, despite the50 m thickness of porous and presumably water-satu-

rated Plio-Pleistocene sediments at the surface at Wai-tomokia. Clearly, although these sediments undoubtedlyplayed a role in early crater-forming events, it was theMiocene sedimentary rocks that provided the majoraquifer for ongoing phreatomagmatic excavation ofthese craters. Since the permeability of the Miocenesedimentary rocks is only poor to moderate (according toKermode, 1992), apart from that due to fracture-inducedpermeability (which may be developed as a consequenceof magma pressure, e.g. Germanovich et al., 2000), it islikely that groundwater starvation occurred at a relativelyearly stage and that magmatic activity then dominated.Interestingly, within the Auckland field, those tuff ringswith central scoria cones have a very restricted range ofdiameters (around 700 m), which may be related to aspatio-temporal limit on the ingress of water from theMiocene sedimentary rocks; this may include a limitimposed by the maximum depth of the Miocene rocks,which is between 0.5 and 1 km in the region (Kermode,1992).

The geophysical anomalies described in these casestudies are similar to those noted elsewhere in the Auck-land field (e.g. Cassidy et al., 1999), including subduedanomalies associated with simple maars, moderateanomalies over tuff rings with central scoria cones andwith the largest anomalies characterising volcanoes withthe largest cones (and lava flows). These observations,which indicate that the largest subsurface volumes ofsolidified magma are associated with volcanoes domi-nated by magmatic deposits, are consistent with thenotion that the Auckland volcanoes typically progressedfrom phreatomagmatic to magmatic (Allen and Smith,1994; Houghton et al., 1999). Such a progression iscommonly described from other monogenetic basalticvolcanoes (e.g. Lorenz, 1986; White, 1991).

This study illustrates the value of complementarygravity and magnetic data in investigating the subsurfacevolcanic structure at maars. The aeromagnetic method isparticularly valuable in identifying those magneticanomalies that reflect structure at depth below thevolcanoes by avoiding the complexities of ground-basedmeasurements which are dominated by very near-surfacerocks. The resulting geophysical models also providemore accurate assessments of magma volumes, whichwould be underestimated if based solely on surfacemapping. For example, the volume of subsurface basaltmodelled below Domain volcano is equal to the totalvolume of eruptive products estimated from surfacedeposits (Allen and Smith, 1994). Geophysical methodsare under-utilised in the investigation of maars; thepresent study shows how potential field methods can beused to recognise significant differences in subsurface


structure that may not be apparent from geologicalmapping. Hence they provide valuable new informationabout the magmatic evolution of such volcanoes.

Acknowledgements

We thank the University of Auckland ResearchCommittee for financial support, the Auckland RegionalCouncil and Tonkin and Taylor Ltd for providingborehole data, Colin Yong for technical assistance andLouise Cotterall for assistance with manuscript prepa-ration. Valuable comments by Michael Ort and AndrewGorman significantly improved the manuscript.

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