Large-scale interaction of lake water and rhyolitic magma during the 1.8 ka Taupo eruption, New Zealand

Abstract

The 1.8 ka Taupo eruption was one of the largest and most powerful of eruptions world wide in the last 5000 years, and notable for its uniform chemical composition, yet diversity of eruptive styles. Three phreatomagmatic phases (phases 1,3,4) involved extensive interaction with a pre-existing caldera lake. The eruption shifted abruptly between wet and dry eruptive conditions. These shifts accompanied changes in vent position that led to discharge of melt that had undergone quite different ascent and degassing histories. Vesicle size distributions and morphologies in the 1.8 ka juvenile ejecta supply constraints on these processes. Pumice clasts in units 1 and 3 have microtextures reflecting vesicle growth and coalescence during steady ascent of magma, with slight contrasts probably due to contrasting faster (phase 3) and slower (phase 1) rates of rise. There is no unambiguous evidence for magma:water interaction triggering, or contributing to fragmentation during these phases and we suspect that the principal role of external water was after fragmentation. Unit 4 clasts have microtextures reflecting bubble collapse. It is clear that this melt was disrupted after its peak of vesiculation; a scenario most compatible with a staged ascent of the melt. Low vesicularities and vesicle number densities and ample evidence for bubble collapse suggest that magma:water interaction played the dominant role in fragmentation for this phase. It is clear from the deposits of the Taupo eruption that interaction between surface water and silicic magma took place under widely varying conditions and that the physical state of the melt (in turn reflecting the ascent and degassing histories) was the determinant variable. At Taupo, extensive interaction of the rising magma with the caldera lake only occurred when slow final magma ascent permitted egress of lake water to the shallow conduit.