Exogenous and endogenous growth of the Unzen lava dome examined by satellite infrared image analysis

Phase I of the most recent eruption of Unzen volcano (Japan) occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome. The temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events. However, such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the Mount St. Helens dome, in that case using detailed photogrammetry. Here we investigate these competing dome growth processes at Unzen via analysis of their thermal emission signals present in shortwave infrared (SWIR) satellite imagery. We find that both the endogenous and exogenous growth styles exhibit identifiable SWIR signatures whose magnitude can be related to their respective magma supply rates. We find that the SWIR radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate. Analysis of time series imagery indicates that, like Mount St. Helens, Unzen experienced a temporally decreasing magma discharge rate during phase 1. As time progressed and the discharge rate fell, the growth style of the Unzen dome changed from 'exogenous only', through 'exogenous decreasing and endogenous increasing' to 'endogenous only', Thus relative growth rates at the endogenous and exogenous areas of the Unzen dome appear to be at least partly controlled by the overall magma discharge rate. As with Mount St. Helens, the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent, which impeded exogenous growth more effectively as time elapsed and the discharge rate fell. These results suggest the similarity of the processes operating at the Unzen and Mount St. Helens domes, though important differences remain in that dome growth was basically continuous at Unzen, whilst at Mount St. Helens growth episodes were interspersed by significant periods of repose. (C) 2002 Elsevier Science B.V. All rights reserved