title: Diverse patterns of ascent, degassing, and eruption of rhyolite magma during the 1.8 ka Taupo eruption, New Zealand: Evidence from clast vesicularity

authors: Houghton, Bruce F.

subjects: explosive volcanism

abstract: The 22 km³ (DRE) 1.8 ka Taupo eruption ejected chemically uniform rhyolite in a wide range of eruptive styles and intensities. The 7 eruptive units include the 'type examples' of phreatoplinian (units 3, 4) and ultraplinian fall (unit 5) deposits, and low-aspect-ratio ignimbrite (unit 6). Contrasts in bulk vesicularity, vesicle (and microlite) number densities and the size distributions of bubbles (and crystals) in the Taupo ejecta can be linked to the influence of shallow conduit processes on volatile exsolution and gas escape, before and during eruption, rather than changes in pre-eruptive chemistry. Existing work has modeled the individual phases of this complex eruption but not fully explained the abrupt shifts in style/intensity that occur between phases. We link these rapid transitions to changes in vent position, which permitted contrasts in storage, conduit geometry, and magma ascent history.
Samples in the study show that coalescence of bubbles was a late-stage process in the pre-fragmentation degassing of even the most rapidly ascending magma, but, in most cases, nucleation of new bubbles continued until close to fragmentation. In the two phases of the Taupo eruption linked to dome or cyptodome formation, we can also recognize an influence of onset of permeability, partial outgassing, limited syn-eruptive crystallization, and bubble collapse. Post-fragmentation expansion or contraction of vesicles was only marked in some pumices within the deposits of the two heat-retentive yet nonwelded pyroclastic density currents (units 5B and 6).
We can recognize three different types of history of ascent, bubble nucleation, and degassing for the Taupo eruption. Units 1, 2 3, 5, and 6 involved magma that ascended rapidly under the southern and central portions of the vent system and underwent late-stage closed system, coupled vesiculation without syn-eruptive crystallisation of microlites. The limited contrasts in textures amongst these units reflect only slightly different ascent histories, including slower rise permitting extended bubble coalescence (unit 1), rapid, accelerating ascent (units 2, 3 and 5), and exceptionally rapid decompression and ascent at the time of highest mass discharge rates (unit 6).
In contrast to all these units, the unit 4 magma also rose rapidly initially but then underwent some degree of limited and shallow storage, permitting variable degrees of prolonged bubble maturation (growth and coalescence), development of permeability and outgassing, under the northern portion of the 10-km-long vent system, probably beginning at the time of eruption of units 1-3 from the southern portion of the fissure. Finally, the magma which formed the late-stage dome (unit 7), remained deeper in the plumbing system for an extended time permitting limited growth of microphenocrysts, (perhaps in response to partial depressurisation occurring during the earlier phases of the eruption), before ascending and continuing to outgas in equilibrium fashion.

text: Diverse patterns of ascent, degassing, and eruption of rhyolite magma during the 1.8 ka Taupo eruption, New Zealand: Evidence from clast vesicularity The 22 km³ (DRE) 1.8 ka Taupo eruption ejected chemically uniform rhyolite in a wide range of eruptive styles and intensities. The 7 eruptive units include the 'type examples' of phreatoplinian (units 3, 4) and ultraplinian fall (unit 5) deposits, and low-aspect-ratio ignimbrite (unit 6). Contrasts in bulk vesicularity, vesicle (and microlite) number densities and the size distributions of bubbles (and crystals) in the Taupo ejecta can be linked to the influence of shallow conduit processes on volatile exsolution and gas escape, before and during eruption, rather than changes in pre-eruptive chemistry. Existing work has modeled the individual phases of this complex eruption but not fully explained the abrupt shifts in style/intensity that occur between phases. We link these rapid transitions to changes in vent position, which permitted contrasts in storage, conduit geometry, and magma ascent history.
Samples in the study show that coalescence of bubbles was a late-stage process in the pre-fragmentation degassing of even the most rapidly ascending magma, but, in most cases, nucleation of new bubbles continued until close to fragmentation. In the two phases of the Taupo eruption linked to dome or cyptodome formation, we can also recognize an influence of onset of permeability, partial outgassing, limited syn-eruptive crystallization, and bubble collapse. Post-fragmentation expansion or contraction of vesicles was only marked in some pumices within the deposits of the two heat-retentive yet nonwelded pyroclastic density currents (units 5B and 6).
We can recognize three different types of history of ascent, bubble nucleation, and degassing for the Taupo eruption. Units 1, 2 3, 5, and 6 involved magma that ascended rapidly under the southern and central portions of the vent system and underwent late-stage closed system, coupled vesiculation without syn-eruptive crystallisation of microlites. The limited contrasts in textures amongst these units reflect only slightly different ascent histories, including slower rise permitting extended bubble coalescence (unit 1), rapid, accelerating ascent (units 2, 3 and 5), and exceptionally rapid decompression and ascent at the time of highest mass discharge rates (unit 6).
In contrast to all these units, the unit 4 magma also rose rapidly initially but then underwent some degree of limited and shallow storage, permitting variable degrees of prolonged bubble maturation (growth and coalescence), development of permeability and outgassing, under the northern portion of the 10-km-long vent system, probably beginning at the time of eruption of units 1-3 from the southern portion of the fissure. Finally, the magma which formed the late-stage dome (unit 7), remained deeper in the plumbing system for an extended time permitting limited growth of microphenocrysts, (perhaps in response to partial depressurisation occurring during the earlier phases of the eruption), before ascending and continuing to outgas in equilibrium fashion.

year: 2010

title: The nature, composition and distribution of sediment in Lake Rotorua, New Zealand

authors: Pearson, Lisa Kyle

subjects: Lake Rotorua

abstract: Lake Rotorua has become increasingly eutrophic over the past 2 to 3 decades. The sediments of the lake have been shown to exert an important influence on this eutrophication process. Chemistry of the sediments has been studied to determine the nature, composition and distribution of elements, through a 1.5 year coring programme. A geophysical survey together with sub-bottom profiling has provided stratigraphic information related to the bathymetry of the lake.
Lake Rotorua has two types of sediments: coarse, dense (density c. 0.5 g/cm3) sediments comprised of clastic erosion products and coarse rhyolitic airfall components covering approximately 60% of the lake area; and fine, low-density (approximatly 0.02 g/cm3) diatomaceous ooze that covers the remaining 40% of the lake, accumulated from deposition of biota, predominantly diatom frustules of Aulacoseira granulata. The sediment contains a record of volcanic eruptions, with the Tarawera Tephra typically found 0.5 m below the sediment water interface and Kaharoa Tephra typically between 2 to 3 m depth, in water depths of 10-15 m.
Phosphorus concentration in Lake Rotorua sediments decreases with sediment depth. In the centre of the lake phosphorus concentrations in the top 2 cm can exceed 2500 g/tonne and decline to 800 g/tonne at 20 cm depth. Accumulation rate of phosphorus in the sediment based on the nutrient budget is approximatly 29.6 t/yr. Iron and manganese concentrations in the sediment depend on the availability of the element and the sedimentation rate of diatom frustules, and are controlled by the redox conditions in the sediment. The average concentration of iron and manganese in the sediment is approximately 8000 g/tonne and between 300 and 400 g/tonne, respectively. Iron accumulates in the sediment at a rate of 385 t/yr and manganese at 17.9 t/yr. Maximum concentrations of arsenic in the sediment are 250 g/tonne but are generally between 50-100 g/tonne, depending on the water depth. Lead concentrations are typically below 15 g/tonne. Sediment concentrations of both arsenic and lead are highly correlated with iron and manganese concentrations in the sediment and mimic the respective concentration profiles. Arsenic and lead accumulate in the sediment at a rate of 3.71 and 0.49 t/yr, respectively. All elements show a peak in concentration in the tephra layers.
The bathymetry of Lake Rotorua is dominated by a curved depression extending from Sulphur Point and almost reaching the Ohau Channel. This depression is probably a structural feature likely associated with the collapse of the caldera, but could be an ancient drainage channel. A series of conical depressions clustered to the north of Sulphur Point and to the east of Mokoia Island are likely to be hydrothermal explosion creators. In the north in the lake at water depths less than 10 m, a series of near-shore terraces are preserved in the sediment.
Sub-bottom echo-sounding shows no return of sonic and seismic signals from most of the lake floor, indicating total absorption by the methane gas-filled sediment. In the shallow lake margin environments, generally less than 10 m water depth, gas is absent and a detailed stratigraphy of multiple reflectors from tephra layers was observed with sub-bottom profiles. The basin sediments of Lake Rotorua are significantly pockmarked, with deep, circular flat-bottomed depressions c. 20-60 m diameter and 0.5-6 m depth. The pockmarks are located on the lake floor in areas where the sediment is saturated with gas.

text: The nature, composition and distribution of sediment in Lake Rotorua, New Zealand Lake Rotorua has become increasingly eutrophic over the past 2 to 3 decades. The sediments of the lake have been shown to exert an important influence on this eutrophication process. Chemistry of the sediments has been studied to determine the nature, composition and distribution of elements, through a 1.5 year coring programme. A geophysical survey together with sub-bottom profiling has provided stratigraphic information related to the bathymetry of the lake.
Lake Rotorua has two types of sediments: coarse, dense (density c. 0.5 g/cm3) sediments comprised of clastic erosion products and coarse rhyolitic airfall components covering approximately 60% of the lake area; and fine, low-density (approximatly 0.02 g/cm3) diatomaceous ooze that covers the remaining 40% of the lake, accumulated from deposition of biota, predominantly diatom frustules of Aulacoseira granulata. The sediment contains a record of volcanic eruptions, with the Tarawera Tephra typically found 0.5 m below the sediment water interface and Kaharoa Tephra typically between 2 to 3 m depth, in water depths of 10-15 m.
Phosphorus concentration in Lake Rotorua sediments decreases with sediment depth. In the centre of the lake phosphorus concentrations in the top 2 cm can exceed 2500 g/tonne and decline to 800 g/tonne at 20 cm depth. Accumulation rate of phosphorus in the sediment based on the nutrient budget is approximatly 29.6 t/yr. Iron and manganese concentrations in the sediment depend on the availability of the element and the sedimentation rate of diatom frustules, and are controlled by the redox conditions in the sediment. The average concentration of iron and manganese in the sediment is approximately 8000 g/tonne and between 300 and 400 g/tonne, respectively. Iron accumulates in the sediment at a rate of 385 t/yr and manganese at 17.9 t/yr. Maximum concentrations of arsenic in the sediment are 250 g/tonne but are generally between 50-100 g/tonne, depending on the water depth. Lead concentrations are typically below 15 g/tonne. Sediment concentrations of both arsenic and lead are highly correlated with iron and manganese concentrations in the sediment and mimic the respective concentration profiles. Arsenic and lead accumulate in the sediment at a rate of 3.71 and 0.49 t/yr, respectively. All elements show a peak in concentration in the tephra layers.
The bathymetry of Lake Rotorua is dominated by a curved depression extending from Sulphur Point and almost reaching the Ohau Channel. This depression is probably a structural feature likely associated with the collapse of the caldera, but could be an ancient drainage channel. A series of conical depressions clustered to the north of Sulphur Point and to the east of Mokoia Island are likely to be hydrothermal explosion creators. In the north in the lake at water depths less than 10 m, a series of near-shore terraces are preserved in the sediment.
Sub-bottom echo-sounding shows no return of sonic and seismic signals from most of the lake floor, indicating total absorption by the methane gas-filled sediment. In the shallow lake margin environments, generally less than 10 m water depth, gas is absent and a detailed stratigraphy of multiple reflectors from tephra layers was observed with sub-bottom profiles. The basin sediments of Lake Rotorua are significantly pockmarked, with deep, circular flat-bottomed depressions c. 20-60 m diameter and 0.5-6 m depth. The pockmarks are located on the lake floor in areas where the sediment is saturated with gas.

year: 2007

title: Ahumai

authors: Temara, Pou

subjects: Pātere

abstract: E tika ana kia punarua te wānanga i ēnei pātere e rua nā te mea he mārama te kite iho o whatu he ōrite ētahi o ngā rārangi kupu, ngā whakaaro me ngā kareāroto ki te pātere a Ahumai. Ahakoa kāore i te takoto te ingoa o te kaitito i tēnei pātere, nā te ōrite o ētahi o ngā kupu, e taea te whakapae, nā te ringa kotahi ēnei pātere i waituhi? Kai roto hoki i te pātere nei e takaoto ana ngā rohe o Raukawa pērā i Wharepūhunga, me ngā tangata whai mana o tērā iwi pērā i a Tīkao. Hei taituarā ka hahua ake anō te kōrero a Turanga Te Mauparaoa ki te tata o te rohe o te iwi nāna te waiata nei, ki tōna rohe. Kāore te Māori e kī ‘nō tua ake nei tēnei pātere’ mēnā he tawhiti atu te iwi i tōna. Nō reira, ki taku wānanga, ehara tēnei pātere nō ngā iwi o Taranaki. He tawhiti rawa a Taranaki i Murupara. Hāunga a Raukawa, koia te iwi kai ‘tua ake nei’.

E tuku ana ahau i taku ihu kia puta. Ki te tapahia mai, e pai ana. E kī ana hoki au, ko te mea poto te titonga tuatahi, ko te mea roa te titonga o muri mai. He ariā te mea tuatahi nō te mea tuarua.

text: Ahumai E tika ana kia punarua te wānanga i ēnei pātere e rua nā te mea he mārama te kite iho o whatu he ōrite ētahi o ngā rārangi kupu, ngā whakaaro me ngā kareāroto ki te pātere a Ahumai. Ahakoa kāore i te takoto te ingoa o te kaitito i tēnei pātere, nā te ōrite o ētahi o ngā kupu, e taea te whakapae, nā te ringa kotahi ēnei pātere i waituhi? Kai roto hoki i te pātere nei e takaoto ana ngā rohe o Raukawa pērā i Wharepūhunga, me ngā tangata whai mana o tērā iwi pērā i a Tīkao. Hei taituarā ka hahua ake anō te kōrero a Turanga Te Mauparaoa ki te tata o te rohe o te iwi nāna te waiata nei, ki tōna rohe. Kāore te Māori e kī ‘nō tua ake nei tēnei pātere’ mēnā he tawhiti atu te iwi i tōna. Nō reira, ki taku wānanga, ehara tēnei pātere nō ngā iwi o Taranaki. He tawhiti rawa a Taranaki i Murupara. Hāunga a Raukawa, koia te iwi kai ‘tua ake nei’.

E tuku ana ahau i taku ihu kia puta. Ki te tapahia mai, e pai ana. E kī ana hoki au, ko te mea poto te titonga tuatahi, ko te mea roa te titonga o muri mai. He ariā te mea tuatahi nō te mea tuarua.

year: 2017