Intrusive and extrusive growth of the Mount St Helens lava dome

Abstract

LAVA domes grow by a combination of endogenous (intrusive) and exogenous (extrusive) addition of magma, either as single continuous events^1,2 or through a series of relatively small pulses which together build a larger construct³. Evaluating the hazards that attend growth of a dome requires an assessment of the processes controlling whether lava is added to its interior or surface, because many of these dangers depend on how volatiles are admitted to or released from hot magma. If the volatile pressure inside an inflating dome becomes high enough, sudden exposure provided by flow-front slumping can lead to explosive decompression and the generation of pyroclastic flows^4,5. On the other hand, lava erupted on the oversteepened surface of a dome may collapse, fragment and mix with snow or ice to form pyroclastic flows, surges and mudflows^6,7. Here we provide a quantitative assessment of the partitioning of magma into endogenous intrusions and exogenous lobes, using high-resolution, digital topographic maps of the Mount St Helens dome derived from aerial photographs taken periodically between 1980 and 1986. Endogenous, exogenous and total volume production rates follow distinct trends which provide important clues about the nature of eruption mechanisms. Calculating endogenous and exogenous components for active domes like those recently formed at Redoubt Volcano may help quantify magma-chamber processes and provide another tool for understanding the re-equilibration of shallow magmatic systems.