title: Eruption and emplacement of the 1.3-Ma Ongatiti Ignimbrite, New Zealand:  Regional pathways, particle processes, and pumice evolution associated with a large-volume pyroclastic flow deposit

authors: Yousef Zadeh, Elham

abstract: Pyroclastic flows are the most devastating phenomena of explosive volcanic eruptions. These hazardous density currents are able to travel several tens of kilometers radially away from their source as fast, hot density currents. Due to the enveloping ash cloud, it is still impossible to directly study pyroclastic flows, however, their deposits (i.e. ignimbrites) provide useful insight into their internal processes.

The Ongatiti Ignimbrite was sourced from Mangakino caldera (1600-950 ka), the oldest volcanic center in the Taupo Volcanic Zone, North Island, New Zealand. The crystal- and pumice-rich ignimbrite offers a unique opportunity to understand an ancient and large-volume pyroclastic flow as well as more details about particle processes and pumice evolution. The minimum deposit volume for the Ongatiti Ignimbrite has been revised to approximately 720 km³, or 512 km³ dense rock equivalent. Previous studies proposed ages from 1.21 to 1.28 Ma by dating methods, such as ⁴⁰Ar/³⁹Ar, K/Ar, and zircon fission-track. This research used the (U-Th)/He method to determine an age of 1.3 Ma. 

This study has combined field facies analysis and GIS methods to characterize the Ongatiti Ignimbrite. The topographic controls on the spatial distribution of the ignimbrite has been determined to understand pyroclastic flow pathways through valleys and over hills. The Ongatiti Ignimbrite was a landscape-modifying event that covered at least the western North Island and as far away as Auckland and Wellington. It is a welded to non-welded, columnar-jointed, cliff-forming deposit, that has been divided into nine facies based on the variation in pumice and lithic clast abundance, and welding rank. 
This study also focuses on the texture and components of the ignimbrite matrix to better understand the magma fragmentation process, the transportational and depositional dynamics of the gas-ash fluid, and the post-emplacement processes. The microtextures of ignimbrite matrix were described and quantified by using optical microscopy, scanning electron microscopy, synchrotron x-ray microtomography, electron probe microanalysis, and X-ray powder diffraction (XRD).
The matrix is poorly sorted and varies in the size and texture of ash-sized pumice fragments, glass shards, crystals, and lithic clasts. Glass shards range in shape between dominantly platy, Y-shaped, and complex shapes, and in size from a few microns to about 900 microns. The matrix is crystal-rich and comprises primary subhedral to anhedral volcanic crystals (plagioclase, quartz, pyroxene, hornblende, and Ti-Fe oxides and range between 100 to a few 1000 microns in size. The degree of welding in the matrix is defined by crystal-crystal contacts and deformation of glass shards around particles, and the highest degree of welding is shown by moderately deformed shards. 
The physical characteristics and chemical composition of pumice clasts were determined, and the main pumice textures were recognised as vesicular, fibrous, microvesicular and dense type.  Tomographic imaging shows different vesicularities and local crystal arrangement within pumice and ignimbrite. Glass chemical analysis shows two different groups, higher-silica (~80-81 wt%), and high-silica (76-78 wt%). 
The Ongatiti Ignimbrite was emplaced by a super-eruption with a volcanic explosivity index (VEI) of 7, and eruptions of similar size could catastrophically bury a vast area of the North Island. The matrix of the Ongatiti Ignimbrite suggests complex particle interactions within the ash-gas fluid of pyroclastic flows. Microtomographic imaging of ignimbrite components offers new insights into understanding these complex processes.

text: Eruption and emplacement of the 1.3-Ma Ongatiti Ignimbrite, New Zealand:  Regional pathways, particle processes, and pumice evolution associated with a large-volume pyroclastic flow deposit Pyroclastic flows are the most devastating phenomena of explosive volcanic eruptions. These hazardous density currents are able to travel several tens of kilometers radially away from their source as fast, hot density currents. Due to the enveloping ash cloud, it is still impossible to directly study pyroclastic flows, however, their deposits (i.e. ignimbrites) provide useful insight into their internal processes.

The Ongatiti Ignimbrite was sourced from Mangakino caldera (1600-950 ka), the oldest volcanic center in the Taupo Volcanic Zone, North Island, New Zealand. The crystal- and pumice-rich ignimbrite offers a unique opportunity to understand an ancient and large-volume pyroclastic flow as well as more details about particle processes and pumice evolution. The minimum deposit volume for the Ongatiti Ignimbrite has been revised to approximately 720 km³, or 512 km³ dense rock equivalent. Previous studies proposed ages from 1.21 to 1.28 Ma by dating methods, such as ⁴⁰Ar/³⁹Ar, K/Ar, and zircon fission-track. This research used the (U-Th)/He method to determine an age of 1.3 Ma. 

This study has combined field facies analysis and GIS methods to characterize the Ongatiti Ignimbrite. The topographic controls on the spatial distribution of the ignimbrite has been determined to understand pyroclastic flow pathways through valleys and over hills. The Ongatiti Ignimbrite was a landscape-modifying event that covered at least the western North Island and as far away as Auckland and Wellington. It is a welded to non-welded, columnar-jointed, cliff-forming deposit, that has been divided into nine facies based on the variation in pumice and lithic clast abundance, and welding rank. 
This study also focuses on the texture and components of the ignimbrite matrix to better understand the magma fragmentation process, the transportational and depositional dynamics of the gas-ash fluid, and the post-emplacement processes. The microtextures of ignimbrite matrix were described and quantified by using optical microscopy, scanning electron microscopy, synchrotron x-ray microtomography, electron probe microanalysis, and X-ray powder diffraction (XRD).
The matrix is poorly sorted and varies in the size and texture of ash-sized pumice fragments, glass shards, crystals, and lithic clasts. Glass shards range in shape between dominantly platy, Y-shaped, and complex shapes, and in size from a few microns to about 900 microns. The matrix is crystal-rich and comprises primary subhedral to anhedral volcanic crystals (plagioclase, quartz, pyroxene, hornblende, and Ti-Fe oxides and range between 100 to a few 1000 microns in size. The degree of welding in the matrix is defined by crystal-crystal contacts and deformation of glass shards around particles, and the highest degree of welding is shown by moderately deformed shards. 
The physical characteristics and chemical composition of pumice clasts were determined, and the main pumice textures were recognised as vesicular, fibrous, microvesicular and dense type.  Tomographic imaging shows different vesicularities and local crystal arrangement within pumice and ignimbrite. Glass chemical analysis shows two different groups, higher-silica (~80-81 wt%), and high-silica (76-78 wt%). 
The Ongatiti Ignimbrite was emplaced by a super-eruption with a volcanic explosivity index (VEI) of 7, and eruptions of similar size could catastrophically bury a vast area of the North Island. The matrix of the Ongatiti Ignimbrite suggests complex particle interactions within the ash-gas fluid of pyroclastic flows. Microtomographic imaging of ignimbrite components offers new insights into understanding these complex processes.

year: 2020

title: Monitoring of subtidal reef biota off Motuotau Island in relation to dredge spoil dumping by the Port of Tauranga Ltd 2014-2018

authors: Ross, Philip M.

abstract: Background
• Dredge spoils generated from the development and maintenance of the Port of Tauranga is disposed of on the inner shelf off Mt Maunganui.
• Major channel deepening and widening programmes were undertaken in 1992 and 2015/16. Maintenance dredging is undertaken more regularly.
• Since 1990, monitoring has been conducted to determine if the dumping of dredge spoils was affecting subtidal reef communities around Motuotau Island, as island off Mt Maunganui beach and inshore of the dredge spoils disposal area.
• Three sites at Motuotau have been monitored since 1990. A site to the east of Motuotau has been monitored as a control site and two sites to the north-east and north-west of Motuotau have been monitored as impact sites.
• Monitoring has consisted of photographic monitoring of permanent transects close to the reef-sand boundary to record (a) changes in the elevation of the seabed relative to the rocky reef, and (b) changes in the biological community over time.
• Monitoring has taken place in 1990, 1991, 1993, 1995, 1998, 2001, 2006, 2009 and 2012.
• One of the difficulties of the monitoring programs was that the marker pins, which are cemented onto the reef, would often be dislodged between surveys and would need to be reinstated prior to each new survey taking place. This made it difficult to be sure that the same section of reef was monitored on each occasion and that the measurement of changes in seabed height was accurate.
• In 2013, a decision was made to reinstate marker pins for the permanent transects. In January 2014, holes were drilled into the reef and stainless steel rods were secured in these holes using epoxy cement. It is unlikely that these marker pins will ever be dislodged from the reef.
• At the same time, the monitoring methodology was reviewed and a number of changes recommended. Most significantly, it was decided that the reef being monitored as a control site did not really constitute a control against the effects of dredge spoil dumping and subsequently all three sites would be considered as impact sites.
• Monitoring took place in January and May 2014, November 2015 and April 2018 and is reported on here.
Findings
• Marker pins were reinstated in January 2014 and four and a half years later remain solidly in place.
• In general terms, the reefs around Motuotau appear to be healthy and in a state that is comparable to other reefs of similar depth, aspect and exposure along the Bay of Plenty coastline.
• The 2014, 2015 and 2018 surveys did not record any major changes in the biological communities of the reefs around Motuotau.
• Measurements of the distance between the bottom marker pin at each site and the reefsand boundary indicate that the reefs are not being inundated by dredge spoils. Seabed heights have fluctuated by as much as 30 cm since 2014 and this is not thought to be outside the scale of fluctuations that would naturally occur given the aspect and energy environment of the surveyed reefs.
• Photographic surveys continue to prove useful in capturing information about the diversity of subtidal reef communities. Although there is some evidence of minor changes in these communities over time this is to be expected. There is also evidence of stability in these communities with a number of sponges on these transects having been present for several decades.

text: Monitoring of subtidal reef biota off Motuotau Island in relation to dredge spoil dumping by the Port of Tauranga Ltd 2014-2018 Background
• Dredge spoils generated from the development and maintenance of the Port of Tauranga is disposed of on the inner shelf off Mt Maunganui.
• Major channel deepening and widening programmes were undertaken in 1992 and 2015/16. Maintenance dredging is undertaken more regularly.
• Since 1990, monitoring has been conducted to determine if the dumping of dredge spoils was affecting subtidal reef communities around Motuotau Island, as island off Mt Maunganui beach and inshore of the dredge spoils disposal area.
• Three sites at Motuotau have been monitored since 1990. A site to the east of Motuotau has been monitored as a control site and two sites to the north-east and north-west of Motuotau have been monitored as impact sites.
• Monitoring has consisted of photographic monitoring of permanent transects close to the reef-sand boundary to record (a) changes in the elevation of the seabed relative to the rocky reef, and (b) changes in the biological community over time.
• Monitoring has taken place in 1990, 1991, 1993, 1995, 1998, 2001, 2006, 2009 and 2012.
• One of the difficulties of the monitoring programs was that the marker pins, which are cemented onto the reef, would often be dislodged between surveys and would need to be reinstated prior to each new survey taking place. This made it difficult to be sure that the same section of reef was monitored on each occasion and that the measurement of changes in seabed height was accurate.
• In 2013, a decision was made to reinstate marker pins for the permanent transects. In January 2014, holes were drilled into the reef and stainless steel rods were secured in these holes using epoxy cement. It is unlikely that these marker pins will ever be dislodged from the reef.
• At the same time, the monitoring methodology was reviewed and a number of changes recommended. Most significantly, it was decided that the reef being monitored as a control site did not really constitute a control against the effects of dredge spoil dumping and subsequently all three sites would be considered as impact sites.
• Monitoring took place in January and May 2014, November 2015 and April 2018 and is reported on here.
Findings
• Marker pins were reinstated in January 2014 and four and a half years later remain solidly in place.
• In general terms, the reefs around Motuotau appear to be healthy and in a state that is comparable to other reefs of similar depth, aspect and exposure along the Bay of Plenty coastline.
• The 2014, 2015 and 2018 surveys did not record any major changes in the biological communities of the reefs around Motuotau.
• Measurements of the distance between the bottom marker pin at each site and the reefsand boundary indicate that the reefs are not being inundated by dredge spoils. Seabed heights have fluctuated by as much as 30 cm since 2014 and this is not thought to be outside the scale of fluctuations that would naturally occur given the aspect and energy environment of the surveyed reefs.
• Photographic surveys continue to prove useful in capturing information about the diversity of subtidal reef communities. Although there is some evidence of minor changes in these communities over time this is to be expected. There is also evidence of stability in these communities with a number of sponges on these transects having been present for several decades.

year: 2018

title: Ki te kapu o taku ringa - In the palm of my hand [Photography exhibition at Gallagher Academy of Performing Arts]

authors: Hill, Rodrigo

subjects: Mātauranga Māori

abstract: (Exhibition catalogue)
The title Ki te kapu o taku ringa is taken from a line of
‘Maioha mō Waikato’ by Kīngi Tāwhiao. Ejected from their
homelands he and his Waikato brethren took refuge with
their Maniapoto relations in Te Nehenehenui. The maioha
was composed expressing Kīngi Tāwhiao’s love for his
homelands, and his solastalgic holding fast to those lands
‘... in the palm of his hand.’ The words from the song-poem
were used as guide to our research and photography practice.

text: Ki te kapu o taku ringa - In the palm of my hand [Photography exhibition at Gallagher Academy of Performing Arts] (Exhibition catalogue)
The title Ki te kapu o taku ringa is taken from a line of
‘Maioha mō Waikato’ by Kīngi Tāwhiao. Ejected from their
homelands he and his Waikato brethren took refuge with
their Maniapoto relations in Te Nehenehenui. The maioha
was composed expressing Kīngi Tāwhiao’s love for his
homelands, and his solastalgic holding fast to those lands
‘... in the palm of his hand.’ The words from the song-poem
were used as guide to our research and photography practice.

year: 2025