Rotomahana 1886 eruption : heterogeneous interaction between basaltic magma and the pre-existing geothermal system

Townend, John, 1972-

Results from field observations and granulometric and componentry data for samples collected in the vicinity of Lake Rotomahana suggest that the 10 June 1886 eruption of Rotomahana was dominantly phreatomagmatic with multiple vents supplying basalt. The eruption has to date been generally interpreted as a dominantly phreatic event, during which very little or no basalt was ejected from Rotomahana. Density measurements made on -2.0~ (4 mm) basalt clasts from 13 proximal samples yield maximum densities consistently less than 2.9 g cm-3 (dense rock equivalent) and typical vesicularity ranges of greater than 50%. Of the 363 clasts measured, 6% possessed rhyolitic cores, although qualitative observations suggest that cores are more common for basalt clasts of larger grainsize. These figures are interpreted to represent the termination of magma degassing by a fluid-coolant type interaction with surrounding groundwater, and the incorporation of chunks of wall rock into the magma. The premise that a significant volume of water was present during the eruption is supported by the occurrence of accretionary lapilli in outcrops in the Waimangu Forest and cauliflower surface textures on abundant basaltic bombs and lapilli. Detailed componentry of 45 proximal samples from the area reveals that the Rotomahana eruption involved spatially and temporally variable proportions of juvenile and country rock material. It is estimated that the average minimum volumetric proportion of basalt in the Rotomahana ejecta is 15%. Basalt is present throughout the deposit in various forms including disseminated lapilli and beds of coarse scoria. The proportion of basalt at any particular location varies significantly from bed to bed, suggesting that material was derived in a complex manner from several vents, including Tarawera. XRD analysis of rhyolite, pumice and tuff clasts in ~he deposit has not yet revealed conclusive evidence for appreciable hydrothermal alteration of the country rock prior to the eruption. Further work may enable the pre-eruption state of the geothermal system to be better determined and thus permit calculations of the components of heat driving the eruption These results support the conclusion that basalt was involved in the eruption throughout the Rotomahana portion of the 1886 rift in a nonuniform fashion and limited to discrete subcraters. It therefore appears that the fragmentation process involved spatially and temporally varying proportions of magma, country rock fragments and geothermal fluid.

v, 87 leaves : ill. (some col.), maps ; 30 cm.

1995Townend

POLYGON ((176.402144019061865 -38.249063279204904,176.478186295540922 -38.24325628911042,176.472904207635338 -38.283039126315451,176.400469996665919 -38.286057242936721,176.402144019061865 -38.249063279204904))