title: Products and Processes of Cone-Building Eruptions at North Crater, Tongariro

authors: Griffin, Anna Marie

subjects: Tongariro Volcano

abstract: North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre
and represents one of at least eleven vents which have been active on Tongariro since
the last glacial maximum. The most recent cone-forming activity at North Crater is
thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped
andesite cone. This project focused mainly on the cone-building eruptions at
North Crater, including stratigraphic correlations with distal tephra, interpreting
eruptive processes, and establishing the sequence of events during cone construction.
Detailed field work identified key stratigraphic sections and facies in the proximal,
medial and distal environments. These sections allowed stratigraphic correlations to
be made between proximal cone-building facies and distal sheet-forming facies at
North Crater, and established a complete North Crater eruption stratigraphy. In the
proximal environment, welded and non to poorly welded facies formed from fallout
of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the
medial and distal environment, the lithofacies consist of fallout from a convecting
plume and minor pyroclastic flow. Convective fall and non to poorly welded
pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting
explosive eruptions involving a gas-rich magma. A change to welded deposits
produced from lava-fountaining occurs later in the cone-building sequence and
suggest a change to lower explosively and eruption of gas-poor magma.
Grain size, componentry data, density, petrography and SEM analysis were carried
out on representative samples to characterise the different facies, and reveal
information about eruption processes. The non to poorly welded deposits are typically
made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and
lapilli. The welded facies are relatively dense and clast outlines are often difficult to
distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt
orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and
indicate crustal assimilation. Mingled clasts of light and dark glass were found to
have microlites present in the dark glass, but were absent in the light glass. Electron
microprobe analyses found that the dark and light glass components in a single clast
had similar compositions, showing that the contrasting physical appearance of the
glass is not due to a different chemical composition.
Forty three whole rock XRF analyses showed that the magmas ranged from basaltic
andesite to andesite, and Harker variation plots display linear trends typical of magma
mixing. Magma mixing as the most important magmatic process is supported by
disequilibrium of phenocryst compositions and phenocryst textures. Magma
viscosity, bulk density and temperature was determined using MAGMA (Kware), and
indicate that they fall into the range of typical andesites.
Eruptive activity involved vigorous lava-fountaining, minor convecting eruption
plumes and dominant collapsing eruption plumes. This activity has produced welded
and non-welded pyroclastic flow and fall deposits to form the large cone seen today.
There are significant volcanic hazards associated with this style of activity at North
Crater, characterised by lava-fountaining, eruption plume fallout, and widespread
pyroclastic flows and lahars extending beyond the ring plain. These could all be
potentially devastating to the central North Island of New Zealand.

text: Products and Processes of Cone-Building Eruptions at North Crater, Tongariro North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre
and represents one of at least eleven vents which have been active on Tongariro since
the last glacial maximum. The most recent cone-forming activity at North Crater is
thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped
andesite cone. This project focused mainly on the cone-building eruptions at
North Crater, including stratigraphic correlations with distal tephra, interpreting
eruptive processes, and establishing the sequence of events during cone construction.
Detailed field work identified key stratigraphic sections and facies in the proximal,
medial and distal environments. These sections allowed stratigraphic correlations to
be made between proximal cone-building facies and distal sheet-forming facies at
North Crater, and established a complete North Crater eruption stratigraphy. In the
proximal environment, welded and non to poorly welded facies formed from fallout
of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the
medial and distal environment, the lithofacies consist of fallout from a convecting
plume and minor pyroclastic flow. Convective fall and non to poorly welded
pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting
explosive eruptions involving a gas-rich magma. A change to welded deposits
produced from lava-fountaining occurs later in the cone-building sequence and
suggest a change to lower explosively and eruption of gas-poor magma.
Grain size, componentry data, density, petrography and SEM analysis were carried
out on representative samples to characterise the different facies, and reveal
information about eruption processes. The non to poorly welded deposits are typically
made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and
lapilli. The welded facies are relatively dense and clast outlines are often difficult to
distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt
orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and
indicate crustal assimilation. Mingled clasts of light and dark glass were found to
have microlites present in the dark glass, but were absent in the light glass. Electron
microprobe analyses found that the dark and light glass components in a single clast
had similar compositions, showing that the contrasting physical appearance of the
glass is not due to a different chemical composition.
Forty three whole rock XRF analyses showed that the magmas ranged from basaltic
andesite to andesite, and Harker variation plots display linear trends typical of magma
mixing. Magma mixing as the most important magmatic process is supported by
disequilibrium of phenocryst compositions and phenocryst textures. Magma
viscosity, bulk density and temperature was determined using MAGMA (Kware), and
indicate that they fall into the range of typical andesites.
Eruptive activity involved vigorous lava-fountaining, minor convecting eruption
plumes and dominant collapsing eruption plumes. This activity has produced welded
and non-welded pyroclastic flow and fall deposits to form the large cone seen today.
There are significant volcanic hazards associated with this style of activity at North
Crater, characterised by lava-fountaining, eruption plume fallout, and widespread
pyroclastic flows and lahars extending beyond the ring plain. These could all be
potentially devastating to the central North Island of New Zealand.

year: 2007

title: Going beyond a mean-field model for the learning cortex: second-order statistics

authors: Wilson, Marcus T.

subjects: mean-field

abstract: Mean-field models of the cortex have been used successfully to interpret the origin of features on the electroencephalogram under situations such as sleep, anesthesia, and seizures. In a mean-field scheme, dynamic changes in synaptic weights can be considered through fluctuation-based Hebbian learning rules. However, because such implementations deal with population-averaged properties, they are not well suited to memory and learning applications where individual synaptic weights can be important. We demonstrate that, through an extended system of equations, the mean-field models can be developed further to look at higher-order statistics, in particular, the distribution of synaptic weights within a cortical column. This allows us to make some general conclusions on memory through a mean-field scheme. Specifically, we expect large changes in the standard deviation of the distribution of synaptic weights when fluctuation in the mean soma potentials are large, such as during the transitions between the “up” and “down” states of slow-wave sleep. Moreover, a cortex that has low structure in its neuronal connections is most likely to decrease its standard deviation in the weights of excitatory to excitatory synapses, relative to the square of the mean, whereas a cortex with strongly patterned connections is most likely to increase this measure. This suggests that fluctuations are used to condense the coding of strong (presumably useful) memories into fewer, but dynamic, neuron connections, while at the same time removing weaker (less useful) memories.

text: Going beyond a mean-field model for the learning cortex: second-order statistics Mean-field models of the cortex have been used successfully to interpret the origin of features on the electroencephalogram under situations such as sleep, anesthesia, and seizures. In a mean-field scheme, dynamic changes in synaptic weights can be considered through fluctuation-based Hebbian learning rules. However, because such implementations deal with population-averaged properties, they are not well suited to memory and learning applications where individual synaptic weights can be important. We demonstrate that, through an extended system of equations, the mean-field models can be developed further to look at higher-order statistics, in particular, the distribution of synaptic weights within a cortical column. This allows us to make some general conclusions on memory through a mean-field scheme. Specifically, we expect large changes in the standard deviation of the distribution of synaptic weights when fluctuation in the mean soma potentials are large, such as during the transitions between the “up” and “down” states of slow-wave sleep. Moreover, a cortex that has low structure in its neuronal connections is most likely to decrease its standard deviation in the weights of excitatory to excitatory synapses, relative to the square of the mean, whereas a cortex with strongly patterned connections is most likely to increase this measure. This suggests that fluctuations are used to condense the coding of strong (presumably useful) memories into fewer, but dynamic, neuron connections, while at the same time removing weaker (less useful) memories.

year: 2007

title: A Pilot investigation into the assessment of changes in the psycholinguistic abilities of new-entrant Māori school children using the Illinois Test of Psycholinguistic Abilities

authors: St George, William Vivian Ross

abstract: The role of language as the principal medium of instruction within the New Zealand (N .Z.) educational system has recently received increasing attention both from researchers and those responsible for the formulation of our national educational policy. In particular, much attention has been directed toward the Maori pupil and the use of the English language as the medium for his instruction. The range of literature on this question includes controlled research studies on English usage by Maori children, educational policy statements concerning the - medium of instruction to be employed in New Zealand schools and writings of a more speculative, and potentially political nature, questioning educational policy and the role of language in "Maori education".

text: A Pilot investigation into the assessment of changes in the psycholinguistic abilities of new-entrant Māori school children using the Illinois Test of Psycholinguistic Abilities The role of language as the principal medium of instruction within the New Zealand (N .Z.) educational system has recently received increasing attention both from researchers and those responsible for the formulation of our national educational policy. In particular, much attention has been directed toward the Maori pupil and the use of the English language as the medium for his instruction. The range of literature on this question includes controlled research studies on English usage by Maori children, educational policy statements concerning the - medium of instruction to be employed in New Zealand schools and writings of a more speculative, and potentially political nature, questioning educational policy and the role of language in "Maori education".

year: 1969