John Cassidy

Magma movements in Etna volcano associated with the major 1991–1993 lava eruption: evidence from gravity and deformation

The 1991–1993 eruption was probably the largest on Mt. Etna for 300 years. Since then the volcano has entered an unusually quiescent period. A comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before, during and after the eruption. The gravity and deformation changes observed before the eruption (1990–1991) record the intrusion of magma into the summit feeder and the SSE-trending fracture system which had recently been active in 1978, 1979, 1983 and 1989, creating the feeder dyke for the 1991–1993 eruption. In the summit region gravity changes between 1992 and 1993 (spanning the end of the eruption) reflect the withdrawal of magma from the conduit followed more recently (1993–1994) by the re-filling of magma in the conduit up to pre-eruption levels. In contrast, in the vicinity of the fracture zone, gravity has remained at the 1991–1992 level, indicating that no withdrawal has occurred here. Rather, magma has solidified in the fracture system and sealed it such that the 1993–1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks. Mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano.

Geophysical imaging of the Quaternary Wairoa North Fault, New Zealand: a case study

Abstract An integrated geophysical investigation, including gravity, vertical electric sounding (VES), 2D resistivity and seismic reflection/refraction techniques, was carried out along two profiles across the extensional Wairoa North Fault, New Zealand, prior to a trenching and paleoseismic study. The geophysical data clearly imaged the fault plane lying directly beneath a suspected fault scarp on one profile. The location of the intersection of the fault plane with the surface was deduced from 2.5D gravity and 2D resistivity modeling and subsequently corroborated by trenching to within 5 m. A 60° dip (±10–20°) for the fault plane along both profiles, similar to that revealed in the trench, was modeled to about 70 m depth at both locations, indicating a minimum vertical displacement. Seismic methods, together with a trial ground penetrating radar (GPR) profile, were less successful in imaging deformation above the fault plane, partly because of a lack of coherent reflectors within the upper Quaternary sequence, but mainly because of the deep weathering profiles on both sides of the fault. Despite this deeply weathered environment, geophysical methods (especially 2D resistivity) have proved very effective in confirming the significance of a suspected fault scarp, facilitating the accurate siting of a trench and providing valuable deeper information about fault morphology.

Geophysical evidence for temporal and structural relationships within the monogenetic basalt volcanoes of the Auckland volcanic field, northern New Zealand

Abstract Detailed gravity, aeromagnetic and geochemical studies have been used to investigate late Pleistocene monogenetic basaltic volcanoes in the Auckland volcanic field. The styles of eruption range from purely phreatomagmatic to purely magmatic, with some volcanoes displaying a range of eruptive characteristics. Appropriately scaled three-dimensional geophysical modelling methods have successfully delineated the subsurface structure of these small volcanoes and have been used to interpret the controls on their eruptive styles. This modelling has shown that a variety of structural styles exists in close proximity in the Auckland volcanoes, ranging from shallow effusive volcanoes to deeper-seated explosive volcanoes. The main control on the style developed appears to be the nature of the near-surface geology at the site of the eruption. Evidence for contemporaneous eruptions from multiple centres within this monogenetic field has been provided by both the geophysical and geochemical data. This information bears on the behaviour of monogenetic fields in general and in particular has important implications for modelling the past behaviour of the Auckland field and for the assessment of future activity which will form the basis of any contingency planning for volcanic hazard in the region.

A geomagnetic excursion in the Brunhes epoch recorded in New Zealand basalts

Abstract Anomalous geomagnetic directions are recorded in the thermoremanent magnetisation (TRM) of alkali basalt lavas from the Auckland volcanic field, New Zealand. The concordance of paleomagnetic directions both in different sites from individual eruptive centres and in sites from several different centres, as well as their relatively short age span compared with that of the field as a whole, indicates that these anomalous directions have a geomagnetic origin. The directions, which are dated between approximately 25 ka and 50 ka, do not represent full reversals of the geomagnetic field and are interpreted as excursions. Very few excursions have been reported from the Southern Hemisphere; this is the first such record known from igneous rocks and further supports the notion that excursions are global phenomena. The excursions here are tentatively correlated with those reported from Lake Mungo, Mono Lake and Laschamp.

Post-eruptive gravity changes from 1990 to 1996 at Krafla volcano, Iceland

The 1975–1984 Krafla rifting episode was a major lava- and dyke-producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in North Iceland. The present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano. After height correction, persistent net gravity decreases over the source of observed Mogi-type deflation occur with gravity increases occurring further away from this centre of deformation. Gravity decreases are interpreted in terms of drainage from a shallow magma chamber. The net gravity decreases require that at least 4×1010 kg of magma must have been drained during the 6-year observation period. Assuming a density of 2700 kg m−3, this magma would occupy 1.5×107 m3 and by analogy with results obtained for Kilauea, this implies a magma chamber volume change of 4.1×106 m3. This is consistent with the chamber volume change deduced from ground deformation data assuming a Poissons ratio of 0.25 and a Mogi source. Net gravity increases are more spatially extensive and are most likely caused by migration of the steam–water interface and/or closure of micro-fractures in lavas above the magma chamber during post-eruptive cooling and contraction. We present a model for the Krafla magma chamber in which a cooling, contracting and draining magma body causes subsidence at the surface. These results contrast with observations from the Askja caldera, Iceland, where post-eruptive deflation has been shown to be accompanied by negligible net gravity changes above the Mogi-type source in the caldera. Long-term post-eruptive deflation and magma drainage have not been observed at subduction-related volcanoes; this may be a function of a difference in magma viscosity.

K-Ar ages of the Auckland geomagnetic excursions

K-Ar age determinations were made on two monogenetic volcanoes in the Auckland volcanic field, New Zealand, which have recorded the Auckland geomagnetic excursions. For the Wiri volcano with the north-down intermediate paleomagnetic direction, five samples gave a weighted mean age of 27 ± 5 (1σ) ka. For the Hampton Park volcano with the west-up intermediate direction, three samples gave a weighted mean of 55 ± 5 (1σ) ka. Since these two K-Ar ages are distinguished at 2σ level, it is inferred that at least two geomagnetic excursions can be recognized in Auckland. The age of the Hampton Park is barely distinguished from the established age range of the Laschamp excursion (39–45 ka) at 2σ level. The age of the Wiri coincides with the age of c. 30 ka in which excursions have been found from sedimentary and volcanic records. The reported excursions from volcanic rocks show a VGP cluster in the central to northern Pacific region which is distinct from the VGP paths or clusters during polarity reversals.

Magma transfer processes at persistently active volcanoes: insights from gravity observations

Magma transfer processes at persistently active volcanoes are distinguished by the large magma flux required to sustain the prodigious quantities of heat and gas emitted at the surface. Although the resulting degassed magma has been conjectured to accumulate either deep within the volcanic edifice or in the upper levels of the sub-edifice system, no direct evidence for such active accumulation has been reported. Temporal gravity data are unique in being able to quantify mass changes and have been successfully used to model shallow magma movements on different temporal scales, but have not generally been applied to the investigation of postulated long-term accumulation of magma at greater spatial scales within volcanic systems. Here, we model the critical data acquisition parameters required to detect mass flux at volcanoes, we review existing data from a number of volcanoes that exemplify the measurement of shallow mass changes and present new data from Poas and Telica volcanoes. We show that if a substantial proportion of degassed magma lodges within the sub-edifice region, it would result in measurable annual to decadal gravity increases occurring over spatial scales of tens of kilometres and propose that existing microgravity data from Sakurajima and, possibly, Etna volcanoes could be interpreted in these terms. Furthermore, such repeat microgravity data could be used to determine whether the accumulation rate is in equilibrium with the rate of production of degassed magma as calculated from the surface gas flux and hence identify the build-up of gas-rich magma at depth that may be significant in terms of eruption potential. We also argue that large magma bodies, both molten and frozen, modelled beneath volcanoes from seismic and gravity data, could represent endogenous or cryptic intrusions of degassed magma based on order of magnitude calculations using present-day emission rates and typical volcano lifetimes.

The Auckland volcanic field, New Zealand: geophysical evidence for its eruption history

Abstract The Late Quaternary monogenetic basalt volcanoes of the Auckland volcanic field exhibit styles of eruption ranging from phreatomagmatic to magmatic. New detailed aeromagnetic and other geophysical data from the southern half of the field provide constraints on the style and relative timing of eruptions. Concealed basalt bodies are shown to be common beneath maars indicating deep excavation by phreatomagmatic events and subsequent filling by magma. Depth of excavation is unrelated to the presence of surficial and potentially saturated Plio-Pleistocene sediments but commonly involves magma-water interaction in widespread aquifers within the underlying Miocene sediments. Coincident anomalous bulk magnetization directions show that at least three volcanoes were active contemporaneously and suggest a very short duration of activity which is probably typical of other centres in the field. These results emphasize the spasmodic nature of activity in the field over the last 140 ka culminating in the most recent centre, Rangitoto, which erupted after a long quiescent period and represents a large spasm of activity, confined to a single centre. The locations of contemporaneous centres are tentatively correlated with the regional NNW-SSE structural trend.

Three-dimensional structure of relict stratovolcanoes in Taranaki, New Zealand: evidence from gravity data

Abstract The andesite stratovolcanoes of Taranaki occur in a spatially distinct age progression, being eroded to different levels and situated within a large well-known sedimentary basin, thus providing particularly favourable circumstances for geophysical investigation. New gravity data presented here show that the Kaitake and Pouakai volcanoes, two of the older relict centres, are associated with large positive residual gravity anomalies (240 g.u. and 160 g.u., respectively). Detailed three-dimensional gravity models define confined high-density dyke/stock complexes below the volcanoes which constrain the likely locus of eruptive activity and extend to at least basement depths (about 5 km). The older Kaitake edifice is modelled as solid andesite whereas the younger Pouakai edifice comprises a solid andesite core mantled by lower-density volcaniclastics. These andesite bodies are interpreted as regions of extensive dyke injection, a process clearly important in cone development and possibly also in cone stability.

Tectonic implications of Paleomagnetic data from the Northland ophiolite, New Zealand

Abstract Paleomagnetic sampling of 12 sites from two separate massifs of the Northland ophiolite, New Zealand, yields consistent results. Two characteristic NRM directions are recognised: one of normal polarity associated with weakly magnetised rocks (particularly intrusive dolerites), and another of reversed polarity associated with strongly magnetised rocks (particularly basaltic pillow lavas). Thermal and AF demagnetisation shows that the normal directions are due to a secondary viscous component which reflects the present-day field direction, whilst the reversed directions are stable (typically to 500°C) reflecting either the primary TRM or a hard secondary component. The declination and inclination of this stable magnetisation differ substantially from that of the present-day field implying significant tectonic movement or deformation of the ophiolite since formation. Anomalous declinations and inclinations are interpreted here in terms of rotations about vertical axes and, tentatively, latitudinal translation since no widespread significant tilting has been recognised within the massifs. Declinations are consistent with clockwise rotations of 50° and 120° for the Cape Reinga and Mangakahia massifs, respectively, probably associated with obduction at a convergent margin with a right-lateral slip component, situated to the northeast of Northland. The anomalously low inclination vectors (30–40°) are consistent with an origin at lower than present latitudes which, if true, implies long-lived N-S translation of oceanic crust in the southwest Pacific region between Cretaceous and Oligocene times.