title: Precursory eruptions, magma priming and eruption dynamics associated with the climactic 19th February 1975 eruption of Ngauruhoe

authors: Geraghty, Krystal Joan

subjects: Ngauruhoe

abstract: Ngauruhoe is an active central North Island composite volcano, located at the south end of the Taupo Volcanic Zone. This basaltic andesite to andesite cone is one of the most active vents of the larger Tongariro Volcanic Complex; the active summit scoria cone (c. 150 m in diameter) was constructed within the older crater (c. 400 m in diameter) during the eruptions of 1954 to 1975. Approximately 9 m of volcanic material was added to the inner crater rim during the 1975 eruption. Past eruptions from Ngauruhoe have been well-documented in various studies; however, these generally lack detailed observation and analyses of the 1975 eruption deposit meaning that pre-eruptive conditions and explosion dynamics associated with this eruption were left poorly constrained.

This research investigates the physical parameters and eruption controls that led up to the 1975 eruption, including the activity observed from 1968. Specifically, this study examines the pre-eruptive conditions, triggers and eruption mechanisms that led to eruption and emplacement of the 1975 inner crater rim deposit. A key part of this study involved identifying the relationship between, and source of, different clast types comprising the deposit, in addition to determining whether a solid plug and/or magma-water interaction contributed to the high explosivity of the 1975 eruption. By improving our understanding of magmatic and volcanic processes associated with this eruption, this study may assist with more reliable interpretation of monitoring data prior to and during such eruptions (not only at Ngauruhoe, but also basaltic-andesitic volcanoes worldwide). Fieldwork involved stratigraphic logging, high-resolution photography, and sample collection, in addition to sub-sample collection of the 1954-75 volcanic ash deposits stored on archive at GNS Science. Outcrop photos were subsequently point-counted to discriminate between the components, and collected samples described and characterised. Further laboratory analyses included optical microscopic petrography, component point counting, vesicularity studies, scanning electron microscopy, electron microprobe analysis, laser sizing, and quantitative software shape analysis.

This study has demonstrated the importance of crystal fractionation and magma mixing in the evolution of the magma that erupted during 1975. In particular, the transporting magma may have interacted with, and entrained, exotic crystals from a cooler, more evolved crystal mush zone formed by an antecedent magma. Further, magma ascent was associated with efficient gas segregation and transport to the surface along preferential percolation pathways. Efficient degassing in low viscosity magmas typically results in effusive activity; however, this study shows that the high explosivity of the climactic 19th February 1975 eruption was linked to the presence of a coherent, degassed, and hydrothermally altered caprock resulting in accumulation and coalescence of pressurised gas beneath the plug. This study discounts the previously ascribed theory that magma-water interaction controlled the high explosivity of this event. Instead, limited magma-water interaction likely produced the steam observed in the eruption column.

Based on the interpretations outlined above, it is now possible to explain the inner crater stratigraphy. The 1975 inner crater rim deposit is now divided into four facies, A through D, based on textural characteristics and componentry. Facies A (comprising dense sub-angular lapilli and vesicular scoria lapilli) and B (comprising black scoria lapilli/bombs and dense lapilli) were emplaced during vulcanian activity associated with disruption of a caprock, leading up the climactic eruption on the 19th of February 1975. Facies C (comprising loose scoria, dense lapilli, and large irregular domains of densely coalesced/agglutinated pyroclasts) and D (comprising dense lapilli to blocks, scoria lapilli, and bombs) were deposited during the continuous ‘sub-plinian’ eruption on the 19th of February 1975, from strombolian fire-fountaining activity at the base of the large eruption plume. Evidence for this is recorded by the mixture of scoria, agglutinated spatter, and dense clasts throughout these facies; which are indicative of a spatter-fed eruption.

text: Precursory eruptions, magma priming and eruption dynamics associated with the climactic 19th February 1975 eruption of Ngauruhoe Ngauruhoe is an active central North Island composite volcano, located at the south end of the Taupo Volcanic Zone. This basaltic andesite to andesite cone is one of the most active vents of the larger Tongariro Volcanic Complex; the active summit scoria cone (c. 150 m in diameter) was constructed within the older crater (c. 400 m in diameter) during the eruptions of 1954 to 1975. Approximately 9 m of volcanic material was added to the inner crater rim during the 1975 eruption. Past eruptions from Ngauruhoe have been well-documented in various studies; however, these generally lack detailed observation and analyses of the 1975 eruption deposit meaning that pre-eruptive conditions and explosion dynamics associated with this eruption were left poorly constrained.

This research investigates the physical parameters and eruption controls that led up to the 1975 eruption, including the activity observed from 1968. Specifically, this study examines the pre-eruptive conditions, triggers and eruption mechanisms that led to eruption and emplacement of the 1975 inner crater rim deposit. A key part of this study involved identifying the relationship between, and source of, different clast types comprising the deposit, in addition to determining whether a solid plug and/or magma-water interaction contributed to the high explosivity of the 1975 eruption. By improving our understanding of magmatic and volcanic processes associated with this eruption, this study may assist with more reliable interpretation of monitoring data prior to and during such eruptions (not only at Ngauruhoe, but also basaltic-andesitic volcanoes worldwide). Fieldwork involved stratigraphic logging, high-resolution photography, and sample collection, in addition to sub-sample collection of the 1954-75 volcanic ash deposits stored on archive at GNS Science. Outcrop photos were subsequently point-counted to discriminate between the components, and collected samples described and characterised. Further laboratory analyses included optical microscopic petrography, component point counting, vesicularity studies, scanning electron microscopy, electron microprobe analysis, laser sizing, and quantitative software shape analysis.

This study has demonstrated the importance of crystal fractionation and magma mixing in the evolution of the magma that erupted during 1975. In particular, the transporting magma may have interacted with, and entrained, exotic crystals from a cooler, more evolved crystal mush zone formed by an antecedent magma. Further, magma ascent was associated with efficient gas segregation and transport to the surface along preferential percolation pathways. Efficient degassing in low viscosity magmas typically results in effusive activity; however, this study shows that the high explosivity of the climactic 19th February 1975 eruption was linked to the presence of a coherent, degassed, and hydrothermally altered caprock resulting in accumulation and coalescence of pressurised gas beneath the plug. This study discounts the previously ascribed theory that magma-water interaction controlled the high explosivity of this event. Instead, limited magma-water interaction likely produced the steam observed in the eruption column.

Based on the interpretations outlined above, it is now possible to explain the inner crater stratigraphy. The 1975 inner crater rim deposit is now divided into four facies, A through D, based on textural characteristics and componentry. Facies A (comprising dense sub-angular lapilli and vesicular scoria lapilli) and B (comprising black scoria lapilli/bombs and dense lapilli) were emplaced during vulcanian activity associated with disruption of a caprock, leading up the climactic eruption on the 19th of February 1975. Facies C (comprising loose scoria, dense lapilli, and large irregular domains of densely coalesced/agglutinated pyroclasts) and D (comprising dense lapilli to blocks, scoria lapilli, and bombs) were deposited during the continuous ‘sub-plinian’ eruption on the 19th of February 1975, from strombolian fire-fountaining activity at the base of the large eruption plume. Evidence for this is recorded by the mixture of scoria, agglutinated spatter, and dense clasts throughout these facies; which are indicative of a spatter-fed eruption.

year: 2019

title: Computational Bayesian inference using low discrepancy sequences

authors: Brown, Paul Tamataatoi

subjects: Bayesian Inference

abstract: The Integrated Nested Laplace Approximation (INLA) provides fast and accurate Bayesian inference for complex hierarchical models. For INLA, and other deterministic methods, the hyperparameter space is explored and points are laid out in a grid structure. These points are used in some numerical integration scheme for which marginal posterior distributions are computed. The main drawback is that the number of points increase exponentially with the number of hyperparameters. The grid is a type of quasi-Monte Carlo (QMC) point set. Low discrepancy sequences (LDS) are QMC point sets that are well known to have significant advantages over grids in terms of convergence and accuracy, and suffer less from the so-called curse of dimensionality.

This work makes several important contributions. We introduce a new method using LDS to compute marginal posterior distributions for hyperparameters, discuss the convergence properties of the approximations and show that they converge to the true posterior. We also show how these methods can be incorporated into the INLA inference framework, and we outline important extensions that improve the accuracy of our approximations with little extra computational effort needed. Lastly, we build a unique spatio-temporal model of residential crime in Hamilton, using INLA’s stochastic partial differential equation approach to a Log-Gaussian Cox Process, and use an LDS to approximate the latent parameters of the model.

Our results show that for a fixed number of points or computational time, LDS methods can outperform general grid-based methods, leading to better marginal posterior approximations. Modifying the method for the purposes of incorporation to INLA, we show that we can outperform INLA’s grid with respect to computational speed, and obtain accurate and flexible approximations to the model hyperparameters.

text: Computational Bayesian inference using low discrepancy sequences The Integrated Nested Laplace Approximation (INLA) provides fast and accurate Bayesian inference for complex hierarchical models. For INLA, and other deterministic methods, the hyperparameter space is explored and points are laid out in a grid structure. These points are used in some numerical integration scheme for which marginal posterior distributions are computed. The main drawback is that the number of points increase exponentially with the number of hyperparameters. The grid is a type of quasi-Monte Carlo (QMC) point set. Low discrepancy sequences (LDS) are QMC point sets that are well known to have significant advantages over grids in terms of convergence and accuracy, and suffer less from the so-called curse of dimensionality.

This work makes several important contributions. We introduce a new method using LDS to compute marginal posterior distributions for hyperparameters, discuss the convergence properties of the approximations and show that they converge to the true posterior. We also show how these methods can be incorporated into the INLA inference framework, and we outline important extensions that improve the accuracy of our approximations with little extra computational effort needed. Lastly, we build a unique spatio-temporal model of residential crime in Hamilton, using INLA’s stochastic partial differential equation approach to a Log-Gaussian Cox Process, and use an LDS to approximate the latent parameters of the model.

Our results show that for a fixed number of points or computational time, LDS methods can outperform general grid-based methods, leading to better marginal posterior approximations. Modifying the method for the purposes of incorporation to INLA, we show that we can outperform INLA’s grid with respect to computational speed, and obtain accurate and flexible approximations to the model hyperparameters.

year: 2019

title: The anthropologist as tribal advocate

authors: Hopa, Ngapare K

subjects: economic conditions

abstract: My colleague has outlined some of the major issues facing New Zealand and its indigenous Polynesians - the Maaori - who not so long ago were ranked among the most documented in the world and the country as THE laboratory of race relations.
The present disadvantaged and dependent status of Maaori people show how illusory that notion is and, for New Zealand anthropologists, the challenge it presents in helping Maaori to attain socio-economic sovereignty within the 'partnership' promised by the Treaty of Waitangi.
In the late 60s and since, a few 'radical' mostly 'native' anthropologists can be said to have risen to the challenge that Gough (1968) and others, laid down, but it cannot be said, even now, that the profession as a whole in New Zealand, has been responsive.
Perhaps it is still committed to the myth of scientific objectivity, or, after regarding themselves as champions of a powerless and marginal people, has not recovered from the accusation of impotency during the social ferment of the 70s and since. To be fair, there were a few individuals who were allied with the protest movement but whose contributions were not acknowledged. They rode out the storm however and have continued to contribute significantly to the Maaori cause.

text: The anthropologist as tribal advocate My colleague has outlined some of the major issues facing New Zealand and its indigenous Polynesians - the Maaori - who not so long ago were ranked among the most documented in the world and the country as THE laboratory of race relations.
The present disadvantaged and dependent status of Maaori people show how illusory that notion is and, for New Zealand anthropologists, the challenge it presents in helping Maaori to attain socio-economic sovereignty within the 'partnership' promised by the Treaty of Waitangi.
In the late 60s and since, a few 'radical' mostly 'native' anthropologists can be said to have risen to the challenge that Gough (1968) and others, laid down, but it cannot be said, even now, that the profession as a whole in New Zealand, has been responsive.
Perhaps it is still committed to the myth of scientific objectivity, or, after regarding themselves as champions of a powerless and marginal people, has not recovered from the accusation of impotency during the social ferment of the 70s and since. To be fair, there were a few individuals who were allied with the protest movement but whose contributions were not acknowledged. They rode out the storm however and have continued to contribute significantly to the Maaori cause.

year: 1988