title: Volcanic Generation of Tsunamis: Two New Zealand Palaeo-Events

authors: de Lange, Willem P.

subjects: Science & Technology

abstract: Rapid emplacement of a mass via pyroclastic flows, or edifice failure, generates volcanic tsunamis. Physical modelling demonstrates that the efficiency of tsuna-mi generation is influenced by the angle the mass enters the ocean. Efficiency de-creases with increasing slope angle from 20° to 60°, before increasing to a maxi-mum at 90°, which corresponds to a mass falling directly into the ocean without interacting with the slope (impact tsunami). Further, in the case of surging pyro-clastic flows or regressive failures, successive closely spaced events may generate larger tsunami waves than a single event of comparable volume.
It is difficult to assess if physical model results are meaningful for real world tsu-nami events due to limited observational data. This paper compares numerical models developed from physical simulations with palaeotsunami deposits from two New Zealand palaeo-events – pyroclastic flows from Mt Tarawera and edi-fice failure at Whakaari (White Island) – which constrains numerical simulations of the source mechanisms. The Mt Tarawera event involved multiple pyroclastic flows entering a lake during the AD 1314±12 Kaharoa Eruption. The interaction of multiple closely spaced pyroclastic flows is necessary to generate the 6-7 m maximum wave height inferred from near source tsunami deposits. Tsunami de-posits in the Bay of Plenty, dated to 2962±52 BP, are consistent with edifice fail-ure at Whakaari. In this case a single event with a volume of 0.23 km3 is suffi-cient to account for the tsunami deposits. Hence, if the failure was regressive, the successive stages were sufficiently close together to be indistinguishable from a large single event.

text: Volcanic Generation of Tsunamis: Two New Zealand Palaeo-Events Rapid emplacement of a mass via pyroclastic flows, or edifice failure, generates volcanic tsunamis. Physical modelling demonstrates that the efficiency of tsuna-mi generation is influenced by the angle the mass enters the ocean. Efficiency de-creases with increasing slope angle from 20° to 60°, before increasing to a maxi-mum at 90°, which corresponds to a mass falling directly into the ocean without interacting with the slope (impact tsunami). Further, in the case of surging pyro-clastic flows or regressive failures, successive closely spaced events may generate larger tsunami waves than a single event of comparable volume.
It is difficult to assess if physical model results are meaningful for real world tsu-nami events due to limited observational data. This paper compares numerical models developed from physical simulations with palaeotsunami deposits from two New Zealand palaeo-events – pyroclastic flows from Mt Tarawera and edi-fice failure at Whakaari (White Island) – which constrains numerical simulations of the source mechanisms. The Mt Tarawera event involved multiple pyroclastic flows entering a lake during the AD 1314±12 Kaharoa Eruption. The interaction of multiple closely spaced pyroclastic flows is necessary to generate the 6-7 m maximum wave height inferred from near source tsunami deposits. Tsunami de-posits in the Bay of Plenty, dated to 2962±52 BP, are consistent with edifice fail-ure at Whakaari. In this case a single event with a volume of 0.23 km3 is suffi-cient to account for the tsunami deposits. Hence, if the failure was regressive, the successive stages were sufficiently close together to be indistinguishable from a large single event.

year: 2016-01-01

title: Improvement in soil water availability in pastures by excavating and mixing buried soil horizons from multilayered Pumice Soils (Vitrands) at Galatea, central North Island, New Zealand

authors: Laubscher, Nadia

subjects: Pumice Soil

abstract: Pumice Soils (Typic Udivitrands) in the Galatea Basin in the eastern Bay of Plenty, central North Island, New Zealand, are formed on weakly weathered, coarse textured, glassy, pumice tephra deposits and associated buried soil horizons. The Galatea Basin tends to experience summer drought and decreased summer pasture production. 

Observations that refilled holes had better summer pasture growth in the disturbed materials compared to growth in adjacent undisturbed soils led to trials where the underlying finer-textured, buried soil horizons were “flipped” (excavated and mixed into the pumiceous surface horizons) using a mechanised digger. The main objective of this study was to test the hypothesis that the mixing of the finer textured buried horizons into the pumiceous surface soils will give increased water holding capacity and thus improve water availability to plants during dry summer periods. 

The top 2.5 m of a reference sequence of four tephra deposits and associated buried soil horizons in an exposed section through a terrace at the southern end of Galatea Basin was described. The tephras and their dates or ages of deposition are(from top) Kaharoa (AD 1314 ± 12), Taupo (AD 232 ± 10), Whakatane (5526 ± 145 calendar years BP), and Rotoma (9423 ± 120 calendar years BP). Undisturbed cores from each soil horizon and the pumiceous tephra layers were collected for water retention and bulk density determination. Four sites were identified that had previously been modified by flipping. Each site contained a flipped area comprising excavated and mixed soil materials and an adjacent “undisturbed” (control) area comprising undisturbed soil. At each of site, three pits were excavated in the flipped area and three in the undisturbed area. Detailed soil descriptions were made and undisturbed soil cores were taken from all horizons in each pit to a depth of 0.6 m. Water retention was determined using a hanging water column and a pressure plate extractor. 

The water retention curves for the undisturbed horizons from the reference profile show that most of the water was held in the soil at lower tensions (<100 kPa). The 3bBw horizon (Whakatane Tephra) had the highest readily available water (25 % v,v) while the 2bAB horizon (Taupo Tephra) had the highest total available water capacity (37 % v,v). The mean profile readily-available water contents to 600 mm depth from the flipped soils were higher (P<0.01) than those of the undisturbed soils. Plant readily-available water content was calculated in the flipped soils to 600 mm depth and to 450 mm depth in the undisturbed soils (based on plant root observations in the field over summer). Plant readily-available water content was greater in the flipped soils compared to that in the undisturbed soils at sites 1, 2 and 3 (P<0.05) and at site 4 (P≤ 0.1).

Assuming an evaporation rate of 4 mm per day and that plant rooting depth is deeper in flipped soils than in undisturbed soils (~600 mm compared to ~450 mm), the flipped soils can hold more readily-available water (mean of 39 days) than the undisturbed soils (mean of 19 days) between rainfall/irrigation events (assuming soils were wet to field capacity at the start) before the soil water drops below the readily available limit.

The hypothesis that “flipping” of the multilayered Pumice Soils will give increased water holding capacity in modified surface horizons (mixed buried soil horizon materials), and thus improve moisture availability to plants during dry summer periods, was accepted.

text: Improvement in soil water availability in pastures by excavating and mixing buried soil horizons from multilayered Pumice Soils (Vitrands) at Galatea, central North Island, New Zealand Pumice Soils (Typic Udivitrands) in the Galatea Basin in the eastern Bay of Plenty, central North Island, New Zealand, are formed on weakly weathered, coarse textured, glassy, pumice tephra deposits and associated buried soil horizons. The Galatea Basin tends to experience summer drought and decreased summer pasture production. 

Observations that refilled holes had better summer pasture growth in the disturbed materials compared to growth in adjacent undisturbed soils led to trials where the underlying finer-textured, buried soil horizons were “flipped” (excavated and mixed into the pumiceous surface horizons) using a mechanised digger. The main objective of this study was to test the hypothesis that the mixing of the finer textured buried horizons into the pumiceous surface soils will give increased water holding capacity and thus improve water availability to plants during dry summer periods. 

The top 2.5 m of a reference sequence of four tephra deposits and associated buried soil horizons in an exposed section through a terrace at the southern end of Galatea Basin was described. The tephras and their dates or ages of deposition are(from top) Kaharoa (AD 1314 ± 12), Taupo (AD 232 ± 10), Whakatane (5526 ± 145 calendar years BP), and Rotoma (9423 ± 120 calendar years BP). Undisturbed cores from each soil horizon and the pumiceous tephra layers were collected for water retention and bulk density determination. Four sites were identified that had previously been modified by flipping. Each site contained a flipped area comprising excavated and mixed soil materials and an adjacent “undisturbed” (control) area comprising undisturbed soil. At each of site, three pits were excavated in the flipped area and three in the undisturbed area. Detailed soil descriptions were made and undisturbed soil cores were taken from all horizons in each pit to a depth of 0.6 m. Water retention was determined using a hanging water column and a pressure plate extractor. 

The water retention curves for the undisturbed horizons from the reference profile show that most of the water was held in the soil at lower tensions (<100 kPa). The 3bBw horizon (Whakatane Tephra) had the highest readily available water (25 % v,v) while the 2bAB horizon (Taupo Tephra) had the highest total available water capacity (37 % v,v). The mean profile readily-available water contents to 600 mm depth from the flipped soils were higher (P<0.01) than those of the undisturbed soils. Plant readily-available water content was calculated in the flipped soils to 600 mm depth and to 450 mm depth in the undisturbed soils (based on plant root observations in the field over summer). Plant readily-available water content was greater in the flipped soils compared to that in the undisturbed soils at sites 1, 2 and 3 (P<0.05) and at site 4 (P≤ 0.1).

Assuming an evaporation rate of 4 mm per day and that plant rooting depth is deeper in flipped soils than in undisturbed soils (~600 mm compared to ~450 mm), the flipped soils can hold more readily-available water (mean of 39 days) than the undisturbed soils (mean of 19 days) between rainfall/irrigation events (assuming soils were wet to field capacity at the start) before the soil water drops below the readily available limit.

The hypothesis that “flipping” of the multilayered Pumice Soils will give increased water holding capacity in modified surface horizons (mixed buried soil horizon materials), and thus improve moisture availability to plants during dry summer periods, was accepted.

year: 2014

title: E kui Rāwinia

authors: Maxwell, Te Kahautu

subjects: Waiata tangi

abstract: He waiata tangi tēnei mō Rāwinia Te Kani he uri nō Te Whakatōhea.  I whānau mai a Rāwinia ki roto o Ngāti Rua he hapū nō Te Whakatōhea, i pakeke mai ia ki roto o Ngāti Rua ki Ōmarumutu, ā, i kuraina ia ki reira.  He wahine rongonui a Rāwinia ki roto o Tūranga i moe ia i a Tokorua Te Kani he ariki nō roto o Tūranga.  Hoi anō he wahine haka a Rāwinia he kuia tāngia ki te moko kauae me te tautoko i ngā mokopū wānanga puta i te motu.  He waiata tangi tēnei mōna nā tana iwi a Te Whakatōhea.

text: E kui Rāwinia He waiata tangi tēnei mō Rāwinia Te Kani he uri nō Te Whakatōhea.  I whānau mai a Rāwinia ki roto o Ngāti Rua he hapū nō Te Whakatōhea, i pakeke mai ia ki roto o Ngāti Rua ki Ōmarumutu, ā, i kuraina ia ki reira.  He wahine rongonui a Rāwinia ki roto o Tūranga i moe ia i a Tokorua Te Kani he ariki nō roto o Tūranga.  Hoi anō he wahine haka a Rāwinia he kuia tāngia ki te moko kauae me te tautoko i ngā mokopū wānanga puta i te motu.  He waiata tangi tēnei mōna nā tana iwi a Te Whakatōhea.

year: 2016