VOLCANIC HAZARD ON MAUNA LOA'S NORTHWEST FLANK: AN INTEGRATED APPROACH

Historic eruptions at Mauna Loa have issued primarily from the volcano’s summit and rift zones, regions in which magma transport paths are well developed. The hazards posed by lava flows initiating at summit and rift locations are therefore well known. Less commonly, however, eruptions initiate at off-rift locations, such as the volcano’s northwest flank. The 1859 eruption, the only historic eruption to issue from a vent on the subaerial northwest slope, produced a paired pahoehoe and ‘a‘a flow notable for its great length (51 km). During the earliest phase of the eruption, the flow developed extensive pahoehoe-lined channels, which, according to contemporary accounts, traveled from the source near the summit (2850 m) to the ocean in eight days. Glass geothermometry indicates a high eruption temperature of ~1215 °C. The flow’s ability to develop such long channels may therefore be related to the degree of cooling required prior to the onset of microlite crystallization. Crystallization impedes flow by increasing lava viscosity, prompting crustal tearing and increasing the rate of radiative flow cooling.

Temperatures comparably high to those estimated for the 1859 eruption (1200-1239 °C) are found in historic picrites and in several xenolith-bearing submarine lavas. Examination of mineral abundances, compositions, and textures in these lavas suggests that they may have bypassed a shallow storage region and reached the surface by an alternate route. It is possible that the high temperature 1859 magma also bypassed this storage region. Perhaps because of its temperature, this flow was able to quickly transport lava long distances.

Unfortunately, petrologic assessments of internal volcanic processes are often divorced from hazard assessments based on an understanding of surface flow dynamics. Integrating these assessments, we explore the possibility that the northwest flank provides an outlet for hot, fluid lavas that have bypassed a shallow reservoir, as well as the hazard implications of this scenario.