egmont volcano, new zealand: three-dimensional structure and its implications for evolution

Joumalof volcanology and geothermal researrh

ELSEVIER Journal of Volcanology and Geothermal Research 76 ( 1997) 149- 161

Egmont Volcano, New Zealand: three-dimensional structure and its implications for evolution

Corinne A. Locke *, John Cassidy Department of Geology, The Uniuersity of Auckland, Private Bag 92019, Auckland, New Zealand

Received 3 May 1996; accepted I I October 1996

Abstract

Egmont Volcano (Mt. Taranaki), a large active andesite stratovolcano, is characterised by a well-defined large positive

residual gravity anomaly of 350 g.u. Detailed three-dimensional modelling of extensive gravity data delineates a large

subedifice intrusion of andesite density extending to at least basement depths (6 km) and a core of similar density within the edifice. These andesite bodies are attributed mainly to repeated magma injection from deeper magma chambers into both the

underlying sediments and the volcanic edifice. The dense edifice core of Egmont Volcano probably represents significant dyke intrusion; such dykes may have played a major role not only in edifice construction but also in edifice collapse.

Egmont Volcano appears to share a common evolution with the three older relict centres in the Taranaki succession since all four Taranaki volcanoes are shown to have large subedifice intrusions and similar dense edifice cores. However, intrusive

volumes are somewhat smaller below the older centres which, given their ages, suggests that magma production rates may have increased with time: the total volume of magma involved in the formation of the Taranaki volcanoes is estimated to be at least 1500 km’.

Keywords: volcanic structure; gravity survey; andesite stratovolcano; Egmont Volcano; New Zealand

1. Introduction

Egmont Volcano (Mt. Taranaki) (Fig. 1) is the

largest andesite stratovolcano in New Zealand. Of late Quatemary age, it is the youngest and only

active centre amongst the group of Taranaki volca-

noes. Recent volcanological and paleomagnetic studies of Egmont Volcano (e.g., Palmer and Neall,

1991; Downey et al., 1994) have highlighted the

periodicity of major sector collapse episodes and

* Corresponding author. Fax: 09-3737-435: e-mail:

[email protected].

also the young age (perhaps only a few hundred years) of the most recent lava flows, both important

factors in hazard assessment. Although the eruption

deposits have been well studied (e.g., Neal1 et al.,

1986; Price et al., 1992) there are no published data relating to the internal struture of Egmont Volcano;

such information is essential for models of cone development and stability.

This paper presents an investigation of the inter-

nal structure of Egmont Volcano based on three-dimensional modelling of detailed gravity data. The internal structure has implications for the role of dyke intrusion in edifice construction and, further- more, in edifice stability. Together with previous

0377-0273/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved.

PII SO377-0273(96)00074-l

150 CA. Locke, .I. Cassidy/ Journal qf Volcanology and Geothermal Research 76 (19971 149-161

Fig. 1. Egmont Volcano (25 18 m) viewed from the southeast at a distance of 15 km shows a classic profile broken by the parasitic cone of

Fanthams Peak which is just visible at the left of the snow line. At the summit the remnants of the crater and central lava dome are visible;

cliff-forming lavas can be seen on the upper northeastern and southwestern Uanks.

data (Locke et al., 1993) the results provide new information on the total volume of magma involved

in the construction of the Taranaki volcanoes.

Although gravity methods have been used widely

for investigating the structure of volcanoes (Williams

and Finn, 1985; Rymer and Brown, 1986) there have

been relatively few detailed three-dimensional mod-

elling studies reported (e.g., Rousset et al., 1989;

Deplus et al., 1995). Three aspects of the geological

setting and history of the Taranaki volcanoes make

them ideal targets for such modelling. Firstly, the

volcanoes are unusual in that they occur within a deep sedimentary basin which is well known from

seismic and borehole information (e.g., Thrasher and Cahill, 1990). This avoids some of the ambiguities

typically encountered when studying volcanoes as a

consequence of their eruption into older volcanic terrain, such as in the Cascades (e.g., Williams and

Finn, 19851, where the contrasts in physical parame-

ters between new volcanoes and their host rocks are less pronounced. Secondly, volcanicity in Taranaki has migrated southeast with time leaving behind isolated relict edifices that are presently exposed at different erosional levels and hence give direct geological and geophysical analogues for the internal

structure of the younger edifices. Finally, the Taranaki volcanoes are relatively accessible which allows an outstandingly comprehensive distribution

of data to be achieved

noes elsewhere.

2. Geological setting

compared with similar volca-

2.1. The Taranaki Basin

The Taranaki Basin, located in the west of the

North Island, is a large sedimentary basin through

which the Taranaki volcanoes have been erupted to

form the Taranaki Peninsula (Fig. 2). It comprises a

continuous sequence of sediments from Cretaceous

to Recent in age which are 5-6 km thick below the Taranaki Peninsula (Thrasher and Cahill, 1990). The basin has had a complex tectonic history since the

late Oligocene associated with the evolving Pacific-

Australian plate boundary (King and Thrasher, 1992).

The nature of the basement underlying the peninsula

is not well known but is thought to comprise plu- tonic and metamorphic rocks similar to those that crop out on northwestern South Island (Mills, 1990).

Two groups of andesite volcanoes occur within the Taranaki Basin: the outcropping Pliocene to Re- cent Taranaki volcanoes located onshore and a group of buried older Miocene volcanoes offshore to the north (Hatherton et al., 1979; Knox, 1982). Both rift-related and subduction-related origins have been

CA. Locke, J. Cassidy/Joumal of Volcanology and Geothermal Research 76 (1997) 149-161 151

174% a I 175"l

I I

0 10 km I I VOLCANO

b Fig. 2. (a) North Island New Zealand showing location of Taranaki region [rectangle delimits area shown in (b)] and the Taupo Volcanic

Zone. (b) Location of Taranaki volcanoes showing topographic surface (in metres a.s.1.) and peaks (A ). Rectangle shows area of Fig. 3Fig.

4Fig. 5.

proposed for these volcanoes (Kamp, 1986). How-

ever, King and Thrasher (1992) argue on the basis of their geochemical similarities that both groups have

a similar subduction-related history.

2.2. The Taranaki volcanoes

Volcanic activity on the Taranaki Peninsula began

at the start of the Quaternary and has migrated southeast with time along a line approximately per-

pendicular to the principal axis of volcanism in the

North Island (i.e., the Taupo Volcanic Zone). This migration has left a succession of three relict volca-

noes marking previous centres of activity: Paritutu

(1.74 Ma), Kaitake (0.58 Ma) and Pouakai (0.23 Ma), some of which are thought to have been of

similar size to the presently active Egmont Volcano

(Neall, 1979). Rock types within the Taranaki volca-

noes are predominantly pyroxene andesite, some of which are hornblende bearing; basalts are subordi- nate (Neal1 et al., 1986).

Updoming of the basement and overlying sediments by volcanic intrusion is well defined at Par-

itutu by seismic reflection data (Thrasher and Cahill,

1990). A basement plateau below Egmont and shorter

wavelength updoming under the north of Pouakai is

also indicated, however, since the data are sparse

over the high-standing parts of the edifices, neither

of these features is well defined. Similar updoming

at shallower levels is shown, for example, by the

raised Eocene reflector in the region.

2.3. Egmont Volcano

Egmont Volcano, the youngest of the Taranaki

volcanoes, rises from sea level to 2518 m in an almost perfect profile broken only by the parasitic

cone of Fanthams Peak (Fig. 1). The volcano is

considered to comprise an upper and lower section

(Neal1 et al., 1986); the upper section, which forms a

steep cone and summit crater, mostly consists of volcaniclastics but in addition numerous lava flows (I 10,000 yr old) occur at higher altitudes. The lower section comprises the ring plain, an apron of volcaniclastics covering an area of about 1000 km2 made up of debris avalanche, laharic, pyroclastic and

152 C.A. Locke, J. Cassidy / Journcrl of Volcanology and Geothermal Keseurch 76 (1997) 149-161

alluvial volcanic deposits. A number of young cumu-

lodomes (Neall, 1971) associated with radial rifts

occur on the lower flanks of the volcano but their

relationship to the main volcanic sequence is unclear.

At outcrop level, volcaniclastic rocks dominate over

lavas and dykes, by a factor of about 2O:l (Neal1 et

al., 1986).

Egmont Volcano first erupted about 120,000 years

ago with the most recent activity occurring about

1755 A.D. (Neall, 1979). The history of activity is

considered to have been dominated by cycles of cone-building followed by major collapse events

(Neal1 et al., 1986) with the bulk of the present cone

thought to have formed during the last 10,000 years.

Around 8000 years ago extensive lava flows (up to

100 m thick) were extruded that now form high cliffs

at about 1500 m elevation and comprise columnar

jointed hornblende andesite (Downey et al., 1994).

These older lava flows are overlain by an extensive

sequence of later flows and pyroclastics originating from the present day summit of the volcano which

range in composition from basaltic-andesite to an-

desite (Downey et al., 1994). Fanthams Peak first

developed some 7000 years ago (Neal1 et al., 1986)

with final construction beginning about 3300 years

ago.

3. Previous geophysical modelling

Gravity and magnetic modelling of the three older

relict centres (Locke et al., 1993, 1994) together with

their exposure at different structural levels provides

the starting point for the present study. These mod-

elling results show a commonality of subsurface

structures; in all cases the gravity data delineate significant subedifice intrusions in the form of

dyke/stock complexes within the host sediments

below sea level. Independent results from seismic

reflection data (Cope, 1965) confirm the extent of

the intrusion below the most deeply eroded Paritutu Volcano. At outcrop level in the Kaitake Volcano andesite dykes predominate (Neal1 et al., 1986) and

geophysical modelling has shown the bulk density of the edifice to be about that of andesite (i.e., 2700 kg mm3 ). In contrast, the edifice of Pouakai at outcrop level comprises predominantly volcaniclastics

with a small lava capping. However, a solid core within this edifice (somewhat larger than the total

Kaitake edifice) is required to account for the ob-

served gravity anomaly. Aeromagnetic anomalies as-

sociated with these volcanoes (Locke et al., 1994)

can be accounted for mainly by the presence of near

surface andesite lavas of moderately high magnetisation; in contrast, the subedifice dyke/stock com-

plexes have a low magnetisation which is attributed

to hydrothermal alteration.

4. Data acquisition and processing

A total of 180 new gravity stations established on and around Egmont Volcano were integrated with

300 existing stations from the surrounding area (Ferry

et al., 1974; Locke et al., 1993). Elevations of the

new gravity stations were determined either by level-

ling or by barometry. Barometric data were acquired

using two digital barometers read simultaneously at a

station and referenced to a continuous digital record of barometric variation at a base station established

in the area. A widely spaced network of more accu- rate elevations (kO.25 m per km baseline length,

maximum error of + 0.5 m) was established over the

volcano using EDM techniques. Barometrically de-

termined elevations were referenced to this network;

the root mean square discrepancy between 38 common barometric and levelled height difference mea-

surements over elevation ranges up to 600 m was 4.1

m. The uncertainties therefore in the elevations of

the stations (of about +0.5-5 m) are equivalent to

+ l-10 g.u. in the gravity data.

Gravity data were corrected using the Interna- tional Gravity Formula (1967) and a density of 2200

kgm-“, i.e., the average density of the near surface

rocks (Locke et al., 1993). The Bouguer and terrain corrections were applied out to a radius of 22 km from each station; terrain corrections were calculated

using techniques based on the methods of Hammer

(1939) and Lopez (1990). The most significant

sources of error in the corrected gravity data are in the elevation and terrain corrections, the latter being particularly difficult to quantify. The total error in the gravity data is estimated to average about 30 g.u.; this estimate is endorsed by the smoothness of the resulting anomaly data and the fact that the anomaly only partially correlates with topography.

The Bouguer gravity map (Fig. 3) is dominated by an intense positive gravity anomaly over Egmont

C.A. Locke, J. Cassidy/ Journal of Volcanology and Geothermal Research 76 (1997) 149-161 153

Fig. 3. Bouguer gravity anomaly map of Egmont and Pouakai volcanoes (contour interval is SO g.u.1. Gravity station locations shown by

dots.

7

5km

. . .

. . . .

.

. >

. . . . . \ * . . . ‘\

Fig. 4. Residual gravity anomaly map (contour interval is 50 g.u.) derived from Fig. 3 by regional removal. A-B and C-D mark profile

locations shown in Fig. 6. Other symbols as for Fig. 3.

154 C.A. Locke, J. Cassidy/Journal of Volcanology and Geothermal Research 76 (1997) 149-161

Volcano which is incompletely resolved from a simi-

lar adjacent anomaly maximum centred over the

Pouakai Volcano. These anomalies are superimposed

upon a southeasterly dipping component with a broad

and smooth nature which is particularly apparent on

a regional scale (Ferry et al., 1974). To determine

the regional field component a polynomial surface

was fitted to 134 new and existing (Ferry et al.,

1974; Locke et al., 1993) data from the entire

Taranaki Peninsula remote from and hence unaf- fected by the gravitational effect of the volcanoes.

The best approximation to the regional field was a

second-order polynomial, similar to that determined

by Locke et al. (1993) immediately to the north of

the area. The regional field dips in a south-southeast-

erly direction, steepening to the southeast to a maxi-

mum gradient of about 22 g.u./km. Subtraction of this regional field from the Bouguer

anomaly data yields the residual gravity anomaly

map shown in Fig. 4. Egmont (and Pouakai) Volcano

is characterised by a well-defined intense positive

residual gravity anomaly of about 350 g.u., indicating the presence of a significant volume of dense

rocks. The residual gravity maximum associated with Egmont Volcano is elongate to the north, whereas

that associated with Pouakai is elongate to the south

indicating that there may be a structural link between

the volcanoes at depth. The width of the anomaly

associated with Egmont Volcano is significantly less

than the width of the topographic edifice, with the SO

g.u. contour correlating approximately with the 600-

700 m topographic contour in the south and east

sector of the edifice.

5. Gravity modelling

Density measurements on 49 andesite samples

from the Taranaki volcanoes gave a range from 2400

to 2830 kg rnp3 (mean 2640 + 100 kg me3 1, with the lower values resulting from particularly vesicular

specimens. This agrees with previously published

densities for the Taranaki volcanics of 2680 + 150

kg me3 (Hunt and Woodward, 1971). Hence a bulk

density of 2700 kgm-” was assumed to represent extruded and intruded andesite in the present gravity

r-

.

.

Fig. 5. Calculated gravity effect (contour interval is 50 g.u.) of optimum model (shown in Fig. 6Fig. 7). Comer marks show location of

frame of Fig. 6. Other symbols as for Fig. 4.

C.A. Locke, J. Cassidy/ Journal of Volcanology and Geothermal Research 76 (1997) 149-161 155

400 RESIDUAL GRAVITY W.)

0 I I

0 5 10 15 20 25km

A B

I ELEVATION EGMONT

2

1

0

-1

-2

-3

-4

-5

a

4oo RESIDUAL GRAVITY

0 I I I I

0 5 10 15 20 25 30 35km

C D

Fig. 6. Cross sections along profiles given in Fig. 4 showing model of Egmont and Pouakai volcanoes with calculated (solid line) and

observed residual ( * ) gravity data. Densities of model are: dark stipple 2700 kgme3, light stipple 2200 kgm-‘. Dashed lines show

basement level, i.e., limit of model resolution below which there is no density contrast. Area of medium stipple shows connection between

dyke/stock complexes that lies behind the plane of the profile section.

156 C.A. Locke, J. Cassidy/ Journal of Volcanology and Geothermal Reseurch 76 (1997) 149-161

models. The density of the volcaniclastic material is

more variable and therefore a bulk density more difficult to quantify. Studies of similar volcanoes

elsewhere yield values of 2000-2400 kg m-j

(Budetta et al., 1983; Brown et al., 1987: Williams et

al., 1987; Fedi et al., 1991; Deplus et al.. 19951. In

the present modelling, an average value of 2200

kg mm3 has been assumed. The host sediments in-

crease in density with depth, i.e., with age. Sediment

densities used in the gravity model are: Pleistocene

2200 kg m-j; Oligocene-Miocene 2250-2550

kg mm3 ; Cretaceous-Eocene 2600 kg rn- 3 (Hather-

ton and Leopard, 1964). A density of 2700 kgm-’

was attributed to the basement, i.e., that determined for the nearest similar basement rocks to the south

(Hunt and Woodward, 197 1).

Three-dimensional modelling was carried out us-

ing a technique developed by Soengkono (19901

(after the method of Bamett, 1976) in which the

topography is closely approximated using narrow

vertical prisms. Egmont and Pouakai volcanoes are modelled simultaneously in this study because their

gravity anomalies are not fully resolved, also the

gravity effect of the model of Kaitake Volcano

(Locke et al., 1993) was included in the calculated

gravity effects. Given the inherent non-uniqueness of

gravity interpretation, an approach to modelling was taken whereby a number of ‘end-member’ geological

VlEW 262O W

Fig. 7. Model for Egmont and Pouakai volcanoes viewed from the

east (approximately). Shaded area (density 2700 kgm-“) shows

subvolcanic andesite dyke/stock complexes and cores of Egmont

and Pouakai volcanoes. Heavy lines denote simplified topographic

contours (i.e., defining the volcaniclastic mantle) at intervals of

500 m a.s.1. For clarity lava capping of Pouakai edifice is omitted.

Location of figure frame is given in Fig. 5.

models were constructed and their gravitational ef-

fects compared with the observed data. It was found

that the Egmont gravity anomaly could not be ac-

counted for adequately if either subsurface density

variations were confined solely to the edifice of the

volcano or if the edifice of the volcano were as-

sumed to have a uniform, moderate density (i.e.,

about 2500 kg me3 1. In either case both the magni-

tude and wavelength of the calculated gravity effect

differed significantly from the observed residual

anomaly. Equally, the observed anomaly could not

be accounted for if density variations were restricted to subedifice levels, i.e., involving a uniform inter-

mediate density edifice and a high-density intrusive

body within the underlying sediments; again both the

wavelength and magnitude of the calculated effect

differed significantly from those observed. Thus, it has to be concluded that high-density material occurs

both within and below the volcanic edifice.

A series of models for the Egmont and Pouakai

volcanoes were developed, using as a guide, previ-

ous models of the older Taranaki volcanoes (Locke

et al., 1993, 1994). The calculated effect (Fig. 51 of the optimum model corresponds closely with the

observed residual gravity anomaly (Fig. 4); this cor-

relation is further illustrated along the profiles across

the volcanoes (Fig. 6). The optimum model is shown both in cross sections (Fig. 6) and three-dimension-

ally (Fig. 7). The root mean square discrepancy

between calculated and observed gravity values for the whole data set is 20 gu. which is within the

estimated error in the gravity data and represents

about 6% of the maximum observed residual

anomaly.

6. Results

Within both the Egmont and Pouakai edifices, inner cores are modelled with a relatively high den-

sity of 2700 kgme3 (i.e., that of andesitel which are mantled by material with a density of 2200 kgm-” (i.e., that of volcaniclastics). Although the data could not be adequately modelled with a uniform interme- diate density for the edifices it is considered unlikely that the actual structure within the edifices is as simple as depicted but rather that high- and low-den-

CA. Locke, J. Cassidy/Joumal of Volcanology and Geothermal Research 76 (1997) 149-161 157

sity material are to some degree intermingled with a

predominance of high-density material towards the inner parts. The data distribution on the flanks of the volcanoes tends to be along radial profiles and hence

the radial symmetry of the modelled edifice cores

may be a consequence, at least in part, of this data

distribution. High-density material must extend into the upper

Egmont cone in order to account for the wavelength

of the observed anomaly; the optimum model indi-

cates that as much as about 70% of the upper cone

(i.e., levels above 1900 m) may comprise the relatively dense andesite. The modelled interface be-

tween this andesite core and the mantling low-den-

sity material has two breaks in slope (Fig. 61, one at

about 750 m (a.s.1.) and the other at 1700 m (a.s.1.).

At the latter elevation, which is about the level of the

older lavas, the high-density core narrows markedly

upwards into the upper cone and may reflect two

cycles of cone building, i.e., with the present young

upper cone having been built upon a broader feature, which may represent the remnants of an older crater.

The volume of the Egmont edifice above sea level

within a radius of about 10 km of the peak (i.e., approximately out to the 400 m contour) is about

220 km3 of which 75 km3 (i.e., about 30%) is

modelled as relatively dense andesite. In contrast, the corresponding volume of the Pouakai edifice is about

65 km3 of which 30 km” (i.e., about 50%) is mod-

elled as andesite. Further aspects of the model are

that high-density material must occur between the

two volcanoes (above sea level) to account for the observed anomalies. Also, an outcropping lava cap

on the Pouakai Volcano has been incorporated in the

model (with a volume of about 1.5 km3); other short

wavelength effects within the observed data may be fitted by similar near-surface flows.

Beneath the edifices, i.e., below sea level, large volume intrusions are modelled to the basement in-

terface. Local sedimentary structures above the basement (Thrasher and Cahill, 1990) were closely ap-

proximated in the model using the densities given earlier. A constant density of 2700 kgme3 was

maintained for the subedifice intrusive rocks. The intrusion beneath Egmont Volcano has a volume of 150 km3 and is approximately cylindrical with a diameter of about 5 km but extends north in a narrow zone to connect with the Pouakai intrusion.

An intrusion of similar volume occurs below Pouakai

and is extended slightly to the south compared with the model of Locke et al. (1993). The improved data distribution between Egmont and Pouakai has en-

abled better definition of the earlier model in this area. Further evidence for these subedifice intrusions

comes from the early arrival of seismic waves pass-

ing immediately below Egmont Volcano which indicates the presence of anomalously high-velocity ma-

terial within the sediments (Cavil1 et al., 1996). Apparent up-doming of the basement beneath the

volcanoes indicates that the subedifice intrusions may

extend into the basement. The gravity effect of extending the modelled subedifice bodies into the base-

ment to 15 km depth was determined assuming a 100

kgme3 density contrast with basement rocks (i.e.,

assuming a likely maximum density for andesite);

the resulting effects were considerably broader than

the observed anomalies. Therefore there are three

possibilities, either the intrusions do not extend into

the basement, the intrusions continue into the base-

ment at a much reduced diameter or the intrusions

extend into the basement but with no significant

density contrast with the basement rocks. Given the possible up-doming of the basement the latter of

these seems the more likely.

7. Discussion

7.1. Internal structure of Egmont Volcano

Compared to many other studies of large andesite stratovolcanoes, ambiguities in modelling Egmont

Volcano are greatly reduced because of the occur-

rence of the volcano within a sedimentary basin and the fortuitous exposure of older volcanoes in the

same sequence at successively deeper erosional levels. This is especially true of constraining the extent

and geometry of the subedifice intrusion. The model

developed for Egmont is broadly similar to some of

those of the Cascade volcanoes (summarised by Williams and Finn, 1985).

The edifice of Egmont Volcano has been clearly shown to comprise a substantial, relatively dense andesite core mantled by less dense volcaniclastic material. Some interlayering of this andesite (e.g.,

flows) within the volcaniclastic mantle may occur

158 CA. Locke, J. Causidy/Journal qf Volcanology and Geothermal Research 76 (19971 149-161

(cf. the outcropping lava cap of the eroded Pouakai

edifice); however, the gravity data indicate that dense

rocks are concentrated mainly towards the centre of

the edifice. Beneath the edifice of Egmont Volcano,

within the underlying sediments, there is an andesitic

intrusion central to the volcano but extending in a

narrow zone to the north. This intrusion extends in

depth at least to the basement interface (6 km) and probably deeper.

Thus, large subedifice intrusions, interpreted as

dyke/stock complexes (Locke et al., 1993, 1994), therefore have been shown to occur beneath all four

Taranaki volcanoes. There are well-defined connec-

tions between both the dense subedifice intrusions modelled beneath Pouakai and Egmont and between

the dense cores modelled within the volcanic edi-

fices. This connection between the volcanoes may be structurally controlled since it correlates spatially

with NW-SE-oriented lineations at the surface at-

tributed to faulting (Neall, 197 1). No similar connec- tions with or between the other Taranaki volcanoes are evident from the existing gravity or seismic data.

Seismic studies at the time of the 1991 Mt.

Pinatubo eruption defined both a deep magma reser- voir between 6 and 11 km depth and a more shallow

low-velocity zone 2-3 km wide extending from the

summit to 7 km depth (Mori and Eberhart-Phillips,

1992, 1993). Volcano-tectonic earthquakes were lo-

cated within the shallow zone which was interpreted

as interconnecting magma bodies embedded in com-

petent rock. By comparison it may be deduced that the dense subedifice dyke/stock complex and at

least part of the dense edifice core of Egmont Vol-

cano developed through repeated magma injection

and solidification in a zone of anastomising conduits

within the edifice and underlying sediments. How-

ever, on geochemical grounds (I.E.M. Smith, pers.

commun., 1996) magma appears not to have been

resident at high levels for long periods and hence the main magma chamber was probably within or below

the basement, i.e., at depths comparable with those at

Mt. St. Helens (7-14 km) (Scandone and Malone,

1985) and Mt. Pinatubo (6- 1 I km) (Mori and Eber- hart-Phillips, 1993).

7.2. Magmatic processes 7.3. Volume considerations

It is not possible to determine the relative propor- Estimates of magma volumes intruded into subed-

tion of intrusive to extrusive rocks in the dense core ifice sediments and represented in the present-day of the Egmont edifice. If the core were mainly edifices of all the Taranaki volcanoes are sum-

formed by early stage lavas then this would require a marised in Table 1. The total volume of the edifice

major change in eruption style during the evolution of Egmont Volcano approximately equates to that of

of the volcano. It seems likely therefore that the comparable volcanoes of similar longevity (Wadge,

dense core reflects significant dyke intrusion, per- 19821, although no account is taken of material lost

haps as a major contributor to edifice construction through wide dispersal or erosion. Despite all vol-

given the occurrence of radial lineation zones and a ume estimates being minima, it seems likely that the

number of related lava domes on the flanks of the amount of magma involved in edifice construction is

volcano (Neall, 1971). Also the remnant edifice of at least comparable and probably much greater than

Kaitake Volcano, which is an analogue for the dense that which solidified within the underlying sedi-

core of Egmont, predominantly consists of dykes. ments.

Table I Volume estimates for the components of the Taranaki volcanoes

Egmont

Pouakai

Kaitake

Paritutu

Total

Sub-edifice intrusion

(km’)

150

150

85

60

445

Edifice dyke/sill complex

(km’)

15

30

IO

115

Editice/ting plain volcaniclastics (DRE)

(km31

300

50

350

Total

(km3)

525

230

95

60

910

DRE = dense rock equivalent.

C.A. Locke, .I. Cassidy/Journal of Volcanology and Geothermal Research 76 (1997) 149-161 159

The similar volumes deduced here for the subedifice intrusions below Pouakai and Egmont volcanoes supports the suggestion of Neal1 (1979) that when active, the edifices were of comparable size. Intru- sion volumes below Kaitake and Paritutu however, are about half those beneath the younger volcanoes perhaps indicating that these older volcanoes were generally smaller. If true, and taking the intrusive volumes as a guide, then the total volume of magma involved in the formation of the Taranaki volcanoes is of the order of at least 1500 km3, i.e., at least 500 km3 have been removed from the Taranaki Peninsula by erosion.

The total volume of material intruded into the sediments below the Taranaki volcanoes, i.e., about 500 km3 is somewhat greater than the 200 km3 volume of mid-crustal magma bodies estimated by Allis et al. (1995) to account for the observed heat flow anomaly in the region. However, this anomaly is centred over the oldest volcanoes suggesting that a significant proportion of the heat flux has yet to reach the surface. A further implication of the above volume estimates is that given the decreasing time span between the development of the separate volcanoes, it would appear that magma production rates have increased with time.

7.4. Stability considerations

Edifice instability in large stratovolcanoes can be attributed to a number of causes relating to the mechanical strength of the edifice and the nature of both internal and external stresses (Siebert, 1984). Whilst there appears to be no correlation between eruptions and large-scale debris avalanche events at Egmont Volcano, dyke intrusion has been suggested as one possible factor contributing to major collapses of the volcano (Palmer and Neall, 1991). In this case the pressure of magma in the intruding dykes pushes the edifice outwards causing oversteepening that results in slumping and landsliding (Delaney, 1992). The model of Egmont developed here in which dyke intrusion is deduced to have played a major part in edifice construction, therefore suggests that the same process may be significant in edifice destruction. Hence it may be that collapse events are related mainly to intrusive events that have no extrusive manifestation.

Alternatively, a trigger for collapse could be earthquakes. Very strong ground shaking caused by large regional earthquakes has probably occurred many times in the history of the volcano (Smith, 1978); equally important in this regard could be local faults, some of which may pass directly beneath the volcano and have been shown from microseismic studies to be currently active at depths of lo-20 km (Cavil1 et al., 1996). Hence oversteepening, due especially to inflation by intruded dykes, coupled with seismic triggering may account for the very large volumes (up to 7 km3 Neall, 1979) of the debris avalanche deposits at Egmont Volcano. These factors clearly are very significant in terms of hazard assessment.

Borgia et al. (1992) have suggested that for major volcanic edifices, particularly those near the coast- line and erupted onto weak clay-rich sediments, instability may result from fault movement within the immediately underlying sediments induced by load- ing. The significance of such an effect in Taranaki is unknown but it is possible that the subedifice dyke/stock complex and central core within the edifice of Egmont Volcano would act as a structural pedestal supporting much of the load of the edifice and hence considerably reducing this effect.

Acknowledgements

Financial support from Auckland University Re- search Committee and Earthwatch (Centre for Field Research, Watertown, Mass.) is gratefully acknowl- edged. We thank numerous Earthwatch volunteers, students of Auckland University and members of the Taranaki Branch of the Geological Society of N.Z. for field and logistic assistance, the New Zealand Department of Conservation for their cooperation, C. Yong for technical support, R.F. Keam for valuable discussion, D.C. Nobes for useful comments on the manuscript and L. Cotterall for assistance with manuscript preparation.

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