Explosive Volcanism: Understanding Pauses and Abrupt Shifts in Intensity During the1912 Novarupta Eruption

University Of Hawaii, Honolulu

Investigators

Abstract

Houghton EAR-0106700 Pauses and/or abrupt shifts in eruptive intensity are a fundamental yet little studied part of many large explosive eruptions. Stable Plinian plumes may pause or fluctuate in intensity by several orders of magnitude on times scales of hours. The causes of such shifts remain poorly understood, at a time when our models for steady state eruptive processes become increasingly elegant and sophisticated. The quality of hazard advice and eruption 'forecasts' would be significantly improved if such changes could be explained and even anticipated. This study focuses on the origin of such 'unsteadiness' in a powerful sustained explosive eruption, for both breaks in eruptive activity of several hours duration and abrupt but short-lived changes in style/intensity. The eruption of Novarupta in 1912 event was the world's largest 20th century eruption and is widely cited as an excellent example of a historical large sustained eruption. Sustained Plinian/ignimbrite eruption at Novarupta stopped briefly and abruptly 16 and 45 hours after commencement of the eruption before finally giving way after 60 hours to effusion of lava domes (Hildreth and Fierstein 2000). Caldera collapse during the 1912 eruption occurred not at vent, but 10 km distant, thus preserving the very near-vent products of the eruption to within 100m of vent, and medial eruption products have a 'chemical stratigraphy' that enables correlation of coeval products of highly varying character between widely spaced localities. This permits us to address this fundamental problem of abrupt changes in eruption dynamics from a very well constrained perspective and for one of the five largest eruptions recorded in the last 1000 years. Goals of the proposed study are to assess the relative influences played by: a) the rates and timing of growth and collapse of gas bubbles, degassing and syneruptive microlite crystallization; b) vent roughness and stability; and c) changes in the pre-eruptive state of the magma, including pre-eruptive volatile concentrations, in altering the dynamics of the 1912 eruption. Our starting hypothesis is that the close of each episode of explosive volcanism at Novarupta was caused by 'ascent-driven' changes in the physical state of the volumetrically dominant dacitic magma whereas short-lived fluctuations in eruption intensity were the products of plume instability generated by irregularity and roughness of the vent walls and limited vent wall collapse. The effects of changing flow behavior in the conduit, as the magma rheology responds to decreases in the concentrations of dissolved volatiles, and increases in the abundances of bubbles and crystals, will be inferred from image analysis of the vesicle and crystal populations in carefully selected pumice samples from either side of the breaks in Plinian deposit and abrupt fluctuations in eruptive intensity. External environmental factors, (vent wall roughness and instability, vent migration and widening) will be studied by mapping changes in the nature and abundance of wall rock lithic clasts.