Bruce F. Houghton

Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review

The Taupo Volcanic Zone (TVZ) in the central North Island is the main focus of young volcanism in New Zealand. Andesitic activity started at c. 2 Ma, joined by voluminous rhyolitic (plus minor basaltic and dacitic) activity from c. 1.6 Ma. The TVZ is c. 300 km long (200 km on land) and up to 60 km wide, as defined by vent positions and caldera structural boundaries. The total volume of TVZ volcanic deposits is uncertain because a sub-volcanic basement has not been identified, but present data suggest bulk volumes of 15–20,000 km3, and that faulted metasediments form most of the immediate subvolcanic basement. Rhyolite (≥15,000 km3 bulk volume, typically 70–77% SiO2) is the dominant magma erupted in the TVZ (mostly as calderaforming ignimbrite eruptions), andesite is an order of magnitude less abundant, and basalt and dacite are minor in volume (< 100 km3 each). The history of the TVZ is here divided into ‘old TVZ’ from 2.0 Ma to 0.34 Ma, and ‘young TVZ’ from 0.34 Ma onwards, separated by the Whakamaru eruptions, which obscured much of the evidence for older activity within the zone. The TVZ shows a pronounced segmentation into northeastern and southwestern andesite-dominated extremities with composite cones and no calderas, and a central 125-km-long rhyolite-dominated segment. Eight rhyolitic caldera centres have so far been identified in the central segment, of which two (Mangakino and Kapenga) are composite features, and more centres will probably be delineated as further data accumulate. These centres account for 34 inferred caldera-forming ignimbrite eruptions, in the c. 1.6-Ma lifetime of the central TVZ. The modern central TVZ is the most frequently active and productive silicic volcanic system on Earth, erupting rhyolite at c. 0.28 m3 s−1, and available information suggests this has been so for at least the past 0.34 Ma. The rhyolites show no major compositional changes with time, though the extent of magma chamber zonation may have changed with the incoming of rifting and crustal extension in the past c. 0.9 Ma. Within the central TVZ, non-rhyolitic compositions have been erupted apparently irregularly in time and space; in particular there is no evidence for a geographic separation of basalts from andesites. Between 0.9 and 0.34 Ma, a major episode of uplift affected areas around the TVZ, while at the same time the main focus of activity may have migrated eastwards within the TVZ accompanying rifting along the axis of the zone. The modern TVZ is rifting at rates between 7 and 18 mm a−1 and restoration of the thin (15km) ‘crust’ (Vp ≤ 6.1 km s−1) beneath the central TVZ to its pre-rifting thickness (25 km) implies that rifting at such rates may have begun only at c. 0.9 Ma. The TVZ is a rifted arc, but its longitudinally segmented nature, high thermal flux and voluminous rhyolitic volcanism make it unique on Earth.

A vesicularity index for pyroclastic deposits

The vesicularity of juvenile clasts in pyroclastic deposits gives information on the relative timing of vesiculation and fragmentation, and on the role of magmatic volatiles versus external water in driving explosive eruptions. The vesicularity index and range are defined as the arithmetic mean and total spread of vesicularity values, respectively. Clast densities are measured for the 16–32 mm size fraction by water immersion techniques and converted to vesicularities using measured dense-rock equivalent densities. The techniques used are applied to four case studies involving magmas of widely varying viscosities and discharge rates: Kilauea Iki 1959 (basalt), Eifel tuff rings (basanite), Mayor Island cone-forming deposits (peralkaline rhyolite) and Taupo 1800 B.P. (calc-alkaline rhyolite). Previous theoretical studies suggested that a spectrum of clast vesicularities should be seen, depending on the magma viscosity, eruption rate, and the presence and timing of magma: water interaction. The new data are consistent with these predictions. In magmatic “dry” eruptions the vesicularity index lies uniformly in the range 70%–80% regardless of magma viscosity. For high viscosities and eruption rates the vesicularity ranges are narrow (< 25%), but broaden to between 30% and 50% as the viscosity and eruption rates are lowered and the volatiles and magma can de-couple. In phreatomagmatic “wet” eruptions, widely varying clast vesicularities reflect complex variations in the relative timing of vesiculation and water-induced fragmentation. Magma:water interaction at an early stage greatly reduces the vesicularity indices (< 40%) and broadens the ranges (as high as 80%), whereas late-stage interaction has only a minor effect on the index and broadens the range to a limited extent. Clast vesicularity represents a useful third parameter in addition to dispersal and fragmentation to characterise pyroclastic deposits.

Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand

The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo. 11 refs., 4 figs., 1 tab.

Primary volcaniclastic rocks

We propose a classification scheme that unifies terminology for all primary volcaniclastic deposits, assigns initial depositional mechanism as the basis for classifying them, and refines the grain-size classes used to pigeonhole samples. By primary volcaniclastic deposits and rocks, we mean the entire range of fragmental products deposited directly by explosive or effusive eruption. This definition thus focuses on the primary transport and deposition of particles, rather than those processes by which the particles form or the nature of the fluid in which they are carried. We favor this approach for all primary volcaniclastic deposits because they typically contain assemblages of clasts formed by different processes and/or at different times that are subsequently brought together during eruption.

Volcanic hazard perceptions: comparative shifts in knowledge and risk

Residents of two North Island, New Zealand, communities were surveyed in March 1995 to measure their understanding of volcanic hazards. This was repeated in November 1995, following the Ruapehu eruptions of September‐October 1995. Both communities were subjected to intense media coverage during the 1995 Ruapehu eruption. Whakatane was spared any direct effects, whereas Hastings experienced the hazard directly, in the form of ash falls. Only Hastings’ respondents showed a significant change in threat knowledge and perceived volcanic risk. While experiencing the direct and indirect impacts of the 1995 Ruapehu eruption may make subsequent warnings and information releases more salient, thereby enhancing the likelihood of engaging in successful protective actions or other forms of response, the characteristics of hazard impacts may increase susceptibility to a “normalisation bias”, reducing future community preparedness.

Impacts of the 1945 and 1995–1996 Ruapehu eruptions, New Zealand: An example of increasing societal vulnerability

Over the past 50 yr the risk to society from volcanic eruptions has increased sharply due to an increased population, more developed and diversified economies, and a more technologically advanced infrastructure. This fact is demonstrated vividly by the impacts from the two largest eruptions of the twentieth century from the cone volcano of Ruapehu, which suggest that the vulnerability of key sectors in New Zealand society has increased by one to two orders of magnitude over this period. Both the 1945 and 1995–1996 eruptions included explosive phases that dispersed ash over a wide area of the North Island for a period of several months. Individual ash falls were only a few millimeters thick in communities within 150 km of the volcano and only trace amounts were found in communities farther away. The 1995–1996 eruption caused similar physical effects to the 1945 eruption but had considerably greater social and economic impacts. The greatest contribution to the cost of the 1995–1996 eruption, estimated in excess of

Physical mingling of magma and complex eruption dynamics in the shallow conduit at Stromboli volcano, Italy

130 million (New Zealand dollars), was the impact on the alpine tourist industry in the central North Island, essentially nonexistent in 1945. Other significant impacts were felt by the rapidly growing aviation and electricity-generation sectors. The cost of any future eruption of the same magnitude is likely to grow as the vulnerability of our society increases at a rapid rate.

The perception of volcanic risk in Kona communities from Mauna Loa and Hualālai volcanoes, Hawai‵i

Strombolian eruptions are caused by the bursting of large gas bubbles through essentially stagnant melt that resides in the uppermost part of the conduit. We investigate the physical properties of this shallow melt via a detailed analysis of vesicularity textures in lapilli ejected during a period of moderate-intensity activity at Stromboli volcano (Italy) in 2002. The lapilli show clear evidence that the erupted material is the product of a late-stage dynamic mingling of melts that are distinct in terms of density and rheology. Vesicle-volume distributions for two end-member melts indicate contrasting degrees of outgassing that can be linked to different residence times in the shallow conduit. We propose a model in which actively vesiculating melt rises with the gas phase and mingles with more mature stagnant melt en route to the magmas free surface. We suggest that the complex rheology of this mingled melt feeds back to strongly influence eruption dynamics.

A new radiometric age estimate for the Rotoehu Ash from Mayor Island volcano, New Zealand

Abstract Volcanic hazards in Kona (i.e. the western side of the island of Hawai‵i) stem primarily from Mauna Loa and Hualālai volcanoes. The former has erupted 39 times since 1832. Lava flows were emplaced in Kona during seven of these eruptions and last impacted Kona in 1950. Hualālai last erupted in ca. 1800. Society’s proximity to potential eruptive sources and the potential for relatively fast-moving lava flows, coupled with relatively long time intervals since the last eruptions in Kona, are the underlying stimuli for this study of risk perception. Target populations were high-school students and adults (n=462). Using these data, we discuss threat knowledge as an influence on risk perception, and perception as a driving mechanism for preparedness. Threat knowledge and perception of risk were found to be low to moderate. On average, fewer than two-thirds of the residents were aware of the most recent eruptions that impacted Kona, and a minority felt that Mauna Loa and Hualālai could ever erupt again. Furthermore, only about one-third were aware that lava flows could reach the coast in Kona in less than 3 h. Lava flows and ash fall were perceived to be among the least likely hazards to affect the respondent’s community within the next 10 years, whereas vog (volcanic smog) was ranked the most likely. Less than 18% identified volcanic hazards as amongst the most likely hazards to affect them at home, school, or work. Not surprisingly, individual preparedness measures were found on average to be limited to simple tasks of value in frequently occurring domestic emergencies, whereas measures specific to infrequent hazard events such as volcanic eruptions were seldom adopted. Furthermore, our data show that respondents exhibit an ‘unrealistic optimism bias’ and infer that responsibility for community preparedness for future eruptions primarily rests with officials. We infer that these respondents may be less likely to attend to hazard information, react to warnings as directed, and undertake preparedness measures than other populations who perceive responsibility to lie with themselves. There are significant differences in hazard awareness and risk perception between students and adults, between subpopulations representing local areas, and between varying ethnicities. We conclude that long time intervals since damaging lava flows have occurred in Kona have contributed to lower levels of awareness and risk perceptions of the threat from lava flows, and that the on-going eruption at Kīlauea has facilitated greater awareness and perception of risk of vog but not of other volcanic hazards. Low levels of preparedness may be explained by low perceptions of threat and risk and perhaps by the lack of a clear motivation or incentive to seek new modes of adjustment.

The petrology, phase relations and tectonic setting of basalts from the taupo volcanic zone, New Zealand and the Kermadec Island arc - havre trough, SW Pacific

Abstract The age of the Rotoehu Ash is estimated as 64 ± 4 ka from whole‐rock K‐Ar age determinations on older and younger lava flows on Mayor Island volcano. This age is compatible with an existing U/Th disequilibrium age of 71 ± 6 ka and both are significantly older than the currently accepted value of 50 ka.