The Rotokawau eruption.
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The Rotokawau eruption occurred approximately 13 kilometres northeast of Rotorua about 4000 years ago, and produced two overlapping tephra rings of pyroclastic deposits. The western vent is now occupied by Lake Rotokawau, while a chain of aligned and coalesced craters 2l kilometres east of Lake Rotokawau mark the vents for the eastern activity.
The pyroclastic deposits are documented and described; they comprise airfall, base surge and ballistic ejecta of essential and lithic material, 0.7 km 3 in volume.
Eruption style of the western vent, a maar type volcano, was surtseyan, or phreatomagmatic. Deposits are widely dispersed over 70 square kilometres. A maximum thickness on the crater rim of nearly ten metres decreases to one metre four kilometres from source. Proximal base surge deposits are present on the lake shore, dipping towards the centre of the crater. These are irregular beds up to one metre thick of hard and very poorly sorted block and ash material. All other exposures are made up of many thin (l-10 cm) layers of alternating base surge (type A: (distal) fine, ashy, poorly sorted and coherent tephra) and airfall (type B: loose-,c fJ:Table, well sorted tephra) deposits. Grain size analysis provided a distinction between type A and type B deposits based on size and sorting parameters. Grain size characteristics of Rotokawau samples are presented.
Activity from the eastern vents was both surtseyan and strombolian in style, reflecting lesser interaction between magma and water .. Deposits around these craters are more cone-shaped, maximum thickness is about 20 metres. Tephra is generally coarser than that of western deposits, with a lesser amount of lithic component. Particle morphology reveals a contrast between dense, blocky fragments of phreatomagmatic origin, and light, highly vesiculated scoriaceous fragments which reflect the greater degree of importance of magmatic gas release in the franentation of the magma. Both morphologies are found in eastern samples, only phreatomagmatic fragments are present in western samples.
Essential, or juvenile, material is high alumina basalt derived from partial melting of anOsubsequent segregation from the mantle at depths of 30-35 km. Geochemical data are presented and these indicate a temperature of crystallisation (and eruption) of ll50°C.
Parameters of magnitude are calculated for the eruption. Rates of emission were about 6.4 m3/s at initial velocities of about 150 m/s. Column heights are inferred to have averaged 4.5 km and reached a maximum of 10 km.
The Rotokawau eruption occurred from the Otataina centre of the Taupo Volcanic Zone. Although this volcano is unlikely to re-erupt it can be seen as representative of an eruption style of this volcanically active zone. An eruption sequence of Rotokawau type would be extremely violent near source, characterised by frequent base surges, and would deposit perhaps a metre of tephra up to 4 km from the vent.
The pyroclastic deposits are documented and described; they comprise airfall, base surge and ballistic ejecta of essential and lithic material, 0.7 km 3 in volume.
Eruption style of the western vent, a maar type volcano, was surtseyan, or phreatomagmatic. Deposits are widely dispersed over 70 square kilometres. A maximum thickness on the crater rim of nearly ten metres decreases to one metre four kilometres from source. Proximal base surge deposits are present on the lake shore, dipping towards the centre of the crater. These are irregular beds up to one metre thick of hard and very poorly sorted block and ash material. All other exposures are made up of many thin (l-10 cm) layers of alternating base surge (type A: (distal) fine, ashy, poorly sorted and coherent tephra) and airfall (type B: loose-,c fJ:Table, well sorted tephra) deposits. Grain size analysis provided a distinction between type A and type B deposits based on size and sorting parameters. Grain size characteristics of Rotokawau samples are presented.
Activity from the eastern vents was both surtseyan and strombolian in style, reflecting lesser interaction between magma and water .. Deposits around these craters are more cone-shaped, maximum thickness is about 20 metres. Tephra is generally coarser than that of western deposits, with a lesser amount of lithic component. Particle morphology reveals a contrast between dense, blocky fragments of phreatomagmatic origin, and light, highly vesiculated scoriaceous fragments which reflect the greater degree of importance of magmatic gas release in the franentation of the magma. Both morphologies are found in eastern samples, only phreatomagmatic fragments are present in western samples.
Essential, or juvenile, material is high alumina basalt derived from partial melting of anOsubsequent segregation from the mantle at depths of 30-35 km. Geochemical data are presented and these indicate a temperature of crystallisation (and eruption) of ll50°C.
Parameters of magnitude are calculated for the eruption. Rates of emission were about 6.4 m3/s at initial velocities of about 150 m/s. Column heights are inferred to have averaged 4.5 km and reached a maximum of 10 km.
The Rotokawau eruption occurred from the Otataina centre of the Taupo Volcanic Zone. Although this volcano is unlikely to re-erupt it can be seen as representative of an eruption style of this volcanically active zone. An eruption sequence of Rotokawau type would be extremely violent near source, characterised by frequent base surges, and would deposit perhaps a metre of tephra up to 4 km from the vent.
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169 p. : ill. (some col.), maps ; 31 cm.
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1981Beanland
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Beanland, Sarah., “The Rotokawau eruption.,” Otago Geology Theses, accessed May 12, 2024, http://theses.otagogeology.org.nz/items/show/141.