Jean-Baptiste Rosseel
Email pitcairn@es.co.nz
PhD title: Phreatic and phreatomagmatic processes of the 1886 Rotomahana eruption: destruction of an active hydrothermal system along an eruption fissure
Supervisor: Assoc. Prof. James White
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Figure 1: Map of the Tarawera-Rotomahana area, adapted from Nairn, 1979 |
On 10th June 1886, Tarawera and Rotomahana (Okataina Volcanic Centre, Taupo Volcanic Zone) erupted in New Zealand's largest historic volcanic event, opening a fissure 7 km long across Tarawera mountain and extending along 8 km across Rotomahana basin to Waimangu valley, and destroying the internationally known "White and Pink silica Terraces".
The volcanic complex lies in the Taupo Volcanic Zone, on the North Island of New Zealand, within the Haro-Haro caldera (fig. 1). The volcanic stratigraphy around Tarawera comprises pyroclastic air-fall and flow deposits, which have accumulated over the past 500 ky.
It is possible to divide the fissure into three parts, the deposits in these three locations being drastically different: the Tarawera Volcanic Complex, the Rotomahana basin, and the Waimangu Geothermal Valley.
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Figure 2: Picture of Mount Tarawera, from the North. |
Mount Tarawera itself (fig. 2) has been formed by the extrusion of rhyolite domes and flows and by the accumulation of pyroclastic debris over the past 18 ka. The most recent rhyolite emplacement took place ~900 yr ago (Kaharoa pyroclastic succession). The 1886 eruption was basaltic on Mount Tarawera, and occurred in three phases. The first and third phases were phreatomagmatic and the second, very powerful, was dry and subplinian.
Before the 1886 eruption, Rotomahana was an intensely active geothermal field, with many boiling springs, geysers and two large siliceous sinter aprons forming the internationally known Pink and White Terraces. Two small lakes - Rotomahana (hot lake) and Rotomakariri (cold lake) - occupied part of the site of the present lake Rotomahana. The interaction of basalt with the geothermal fluids in this area triggered a phreatomagmatic eruption, with turbulent pyroclastic density currents extending 6 km from the eruptive centre. The Terraces and the lakes were destroyed during the eruption and the deposits cropping along the shores of the new lake contain remains of the silica sinter. The eruption destroyed the surface features, but the deep convection system was probably not greatly perturbed. The shallow part of the geothermal system was excavated and fractured and new fluid conduits have been created. The new system, unstable at first, has evolved towards the reestablishment of a steady state flow pattern and the Waimangu valley is an active geothermal area nowadays (fig. 3).
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Figure 3: Geyser on the shore of lake Rotomahana, near Waimangu valley. |
The study of numerous outcrops along the shores of modern lake Rotomahana, as close to the vents as possible, allows recognition of a number of features. There are basalt pyroclasts in all samples, in very variable amounts (up to 90% per layer), and the pyroclasts show a wide range of textures and shapes. The percentage and the composition of the lithics vary drastically from the SW to the NE, recording inhomogeneity of the vent wall rocks. The variation within the layers, from layer to layer and along the length of the fissure, suggests several sources or fragmentation mechanisms, and the impossibility of establishing any clear correlation between the different outcrops, demonstrates a high degree of complexity for this 3-hour eruption. Data extracted by different methods from the numerous samples will help us to understand the mechanisms of the apparent interaction of magma and water in the geothermal area.
The results of grainsize analysis are shown on figure 4, where it is possible to see that the sorting (standard deviation) is poor to very poor all along the fissure, and that there is a fine-skewed tendency, due to the important proportion of mud.
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Figure 4. Skewness vs Standard deviation chart, showing the poor sorting of most samples and the fine-skewed tendency. |
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Figure 5: Density measurements vs Distance from Waimangu valley. |
Several kinds of basalt particles are present in the deposits. The vesicularity, color and shape vary. This suggest several sources for the basalt. Density measurements on these populations (fig. 5) show that in the SW the basalt, mostly grey and blocky, is very dense and the range of densities is very small. And towards the NE, different populations show various densities.
The "grey" population is a consistent feature along the Rotomahana part of the fissure and is not seen elsewhere or in very little quantities. It may have participated directly to the explosions in this area, taking its peculiar texture from the contact with the geothermal fluids.
The extreme diversity of the deposits all along the fissure, and the vertical and horizontal variations at very small scales show that several vents erupted different materials at the same time, with pauses and multiple climaxes at different times through the few hours of the eruption. Consequently, plumes and density currents of different material and of different behaviours rose together and mixed, giving the deposits their very peculiar and unique characteristics.