Rotomahana—Waimangu eruption, 1886: base surge and basalt magma

Abstract

Steam explosions ejecting country rock and lake-floor sediments from Rotomahana-Waimangu accompanied eruption of basaltic scoria from Tarawera Volcanic Complex during the early morning of 1886 June 10. The near-source Rotomahana ejecta displays wave-like internal bed forms typical of base surge eruption mechanisms. Deposition occurred from high velocity, steam-fluidised density currents, which carried ejecta laterally outwards from the base of a large vertical eruption column. The surges extended at least 6 km west of the source and passed over steep hills that rise to 350 m above the eruptive vents. Controversy has existed as to whether basalt magma was erupted from craters in the previously highly active Rotomahana hydrothermal area. New exposures clearly demonstrate that basalt was erupted from Waimangu in 1886; pre-1886 basic scorias outcrop in the Rotomahana area; much near-source Rotomahana ejecta consists of little or non-altered rock. These data are consistent with the existence of a continuous basaltic dike at depth beneath Rotomahana and suggest that inagmatic energy may have contributed to the 1886 Rotomahana explosions.