Motutapu Research Articles

Gravity and magnetic models at Rangitoto Volcano, Auckland Volcanic Field, New Zealand: Implications for basement control on magma ascent

In small-volume volcanism, pre-existing crustal structures can influence magma ascent processes. Rangitoto volcano in the Auckland Volcanic Field, New Zealand, provides a possible example of magma-structure interaction as this volcano was emplaced adjacent to an outcrop of a regional-scale basement fault, the Islington Bay Fault. In this study, we evaluated the magmatic plumbing system of Rangitoto using gravity and magnetic data acquired over the volcano and the adjacent, non-volcanic Motutapu Island. We modelled the Rangitoto internal architecture and magma plumbing using 2.5D forward and 3D inverse modelling methods. Both models are constrained by petrophysical data, while drill hole logs are only used to constrain the 2.5D model. Our model endmembers suggest that parallel magma pathways are present below the Rangitoto summit cones. In 3D magnetic models, this is evidenced by a fault-aligned pair of high-susceptibility bodies. Interpreting our models in conjunction with previously published geological and geophysical models allows us to hypothesise that the fault-parallel alignment of Rangitoto magma pathways reflects the primary influence of the Islington Bay Fault over the Rangitoto magma ascent. Magma diversion at shallow levels by other finer structures intersecting and adjacent to the fault could explain why Rangitoto erupted 3.5 km west of the Islington Bay Fault surface trace.

Archaeological excavations at Pig Bay (N38/21, R10/22), Motutapu Island, Auckland, New Zealand, in 1958 and 1959

Archaeological excavations at Pig Bay (N38/21, R10/22), Motutapu Island, Auckland, New Zealand, in 1958 and 1959

Archaeological excavations at Pig Bay (N38/21, R10/22), Motutapu Island, Auckland, New Zealand, in 1958 and 1959

Archaeological excavations at Pig Bay (N38/21, R10/22), Motutapu Island, Auckland, New Zealand, in 1958 and 1959

Observation of sedimentation process of Middle Triassic bedded cherts by a high-resolution SEM from Motutapu Island, New Zealand

Observation of sedimentation process of Middle Triassic bedded cherts by a high-resolution SEM from Motutapu Island, New Zealand

Measuring connectivity of invasive stoat populations to inform conservation management

Context Effective design of conservation management programs for long-term population control requires an accurate definition of the spatial extent of populations, along with a proper understanding of the ways that landscape patchiness influences demography and dispersal within these populations. Aims In the present study, genetic techniques are used to describe the population genetic structure and connectivity of invasive stoats (Mustela erminea) across the Auckland region, New Zealand, so as to assist planning for mainland stoat control, and define potential future eradication units. Methods A sample of stoats from across the region (n = 120), was genotyped at 17 microsatellite loci, and a combination of clustering, genetic population assignment and various migration estimation methods were applied to these data. Key results Moderate population structure was observed (FST = 0.03–0.21), with five geographic populations defined by genetic clustering. Almost all individuals were correctly assigned to the location of origin, and recent migration rates among forest patches were found to be low. Conclusions It is possible to define the origin of stoats at this regional scale using genetic measures. From this, we show that the stoat incursion on Rangitoto Island that occurred post-eradication in 2010 probably came from East Auckland (P < 0.0001), whereas the 2014 stoat incursion on Motutapu Island probably originated from a population linked to the Waitakeres. Also, the Waiheke Island stoat population has minimal connection to all other populations and it is therefore a potential eradication unit. Implications The low migration rates among forest patches indicated that if thorough control is imposed on a discrete forest patch, reinvasion from other forest patches will be relatively low. Importantly, for stoat control in the region, the isolation of the Waiheke Island stoat population means that eradication here is likely to be feasible with low reinvasion pressure.

Sedimentation process of Middle Triassic bedded cherts from Motutapu Island, North Island, New Zealand

Sedimentation process of Middle Triassic bedded cherts from Motutapu Island, North Island, New Zealand

Sequential eruption of alkaline and sub-alkaline magmas from a small monogenetic volcano in the Auckland Volcanic Field, New Zealand

Rangitoto Volcano is the youngest and largest eruptive centre in the monogenetic intraplate Auckland Volcanic Field (AVF). The stratigraphy of Rangitoto pyroclastic deposits that have been preserved in swamps on nearby Motutapu Island and in Lake Pupuke on the mainland reveals that the volcano erupted twice; radiocarbon dating of 10 samples from the two tephra units in the swamps indicates eruption ages of 553 ± 7 and 504 ± 5 Cal years BP, for the lower and upper tephra layers, respectively. Geochemistry of the lava field and various scoria cones on Rangitoto Island itself reveals two distinct compositional groups: an alkaline olivine basalt group (that correlates geochemically with the lower tephra layer) and a group that is sub-alkaline and transitional to tholeiite (that correlates geochemically with the upper tephra layer). Based on this data, we infer that, following a phreatomagmatic vent-clearing phase, the early magmatic eruption of Rangitoto Volcano was Strombolian in character and produced an alkaline olivine basalt scoria cone and an associated thick ash deposit on nearby Motutapu Island. This was followed by a time gap of up to several decades, after which a second eruptive phase built the current summit scoria cone together with an encircling lava field. We suggest this later, sub-alkaline eruptive period was associated with the deposition of the thin upper tephra layer on Motutapu Island. The two suites of Rangitoto samples are chemically quite distinct, and each is associated with a distinct parental composition. Trace element modelling indicates the alkaline and sub-alkaline parental melts could have been derived by ~ 1 and 6 wt.% partial melting of an anhydrous garnet peridotite source at ~ 80 and 65 km depth, respectively. The compositional range within each suite is similar, and can be explained by mainly olivine together with minor clinopyroxene fractionation within a relatively simple conduit system in which mixing and mingling were not important. Significant olivine fractionation (< 25%) suggests that the magma may have spent some time in the upper conduit during ascent. This contrasts with a recently published model for the Crater Hill centre in the AVF, in which deep-seated fractionation of clinopyroxene followed by relatively rapid ascent to the surface has been invoked to explain the compositions seen there. The polycyclic nature of Rangitoto and in particular the reuse of the conduit system after a period of quiescence have implications for the concept of monogenetic volcanism. There are also implications for hazard assessment, such that when a future eruption occurs in the AVF, it will be necessary to consider the conduit a possible pathway for another eruption for up to several decades afterwards.

Ecology of the black shore skink,Leiolopisma suteri(Lacertilia: Scincidae), in boulder beach habitats

The ecology of the nocturnal black shore skink, Leiolopisma suteri, was studied on a boulder beach at Cable Bay, Motutapu Island, for 19 months. Distributions over a stable beach (Motutapu Island) and an exposed beach (Cuvier Island) were compared, and the effects of storms on the Cable Bay population were noted. Density, aggression, thermal preference, feeding, and fat reserve fluctuations were also studied. The L. suteri population at Cable Bay has, in favourable areas, one of the highest densities (13/m2) recorded for a lizard species. Spatial distribution of this population varies between the different age classes: adults congregate in the most stable areas, whereas hatchlings are commonest in the least stable areas. Storms cause changes in the beach structure and in the spatial limits of the population.

Reproduction and growth of the black shore skink,Leiolopisma suteri(Lacertilia: Scincidae), in north‐eastern New Zealand

Reproduction and growth of a boulder beach population of the nocturnal skink Leiolopisma suteri on Motutapu Island were studied for 19 months. Epididymes contain sperm from April to November, and copulation probably occurs in October or November (spring). Ovulation occurs in late October or early November, and three or four (mean, 3.7) eggs are laid in late December. Incubation takes 3 months. Total development time from ovulation to hatching is 5 months, one of the longest development periods recorded for Scincidae. Growth to sexual maturity is slow, most females depositing their first clutch 33 months after hatching; males mature at 29 months.

Factors causing losses of clover seed

Abstract Crops of white clover (Trifolium repens L.), alsike clover (Trifolium hybridum L.), and red clover (Trifolium pratense L.) were studied during 1958–60 to determine the effect of shrivelling, insect damage, and harvesting operations on seed yields. In individual crops the percentages of shrivelled seed ranged from 1–28 per cent in white clover, 1–44 per cent in alsike clover, and 23–54 per cent in red clover. Seed damage, caused mainly by clover case-bearer (Coleophora spp.) varied from 0–46 per cent in white clover and 0–17 per cent in alsike clover. No such damage occurred in red clover. Red thrips (Haplothrips niger Osborne) caused little darnage except on Motutapu Island where white clover was severely affected. Losses during harvesting varied from 42–75 per cent of plump seed in white clover and 64–74 per cent in red clover.

Geological and archaeological interpretation of a section in Rangitoto ash on Motutapu island, Auckland

Summary A foredune section at Pig Bay, Motutapu, has been excavated to expose nonvolcanic beach sand below a single Rangitoto ash layer which, illi turn, is overlain by a series of wind-blown and water-laid sediments containing reworked ash. The date of the eruption was determined by carbon sample; as A.D. 1188 ± 50 years. The sediments above the Rangitoto ash include a number of well defined human occupation levels, some of which have yielded adzes of moa-hunter type.