THE MORPHOLOGY OF GLACIOVOLCANIC CAVES

Glacial ice mantles some volcanoes at high latitudes or altitudes. In these settings, thermal flux at the ice-rock boundary forms glaciovolcanic caves. The morphology of these caves balances ice accumulation and ablation, the movement of glacial ice, volcanic heat flux, and liquid and gas flow through interconnected voids. These caves are an understudied part of the hydrogeology and mechanical weathering of volcanic edifices.

Longitudinal studies of glaciovolcanic caves in the Cascade Volcanic Arc reveal a window into the underlying processes. At Mt. Hood, we have observed a decade-long reduction in cave passage contemporaneous with the retreat of the Sandy Glacier. On the summit of Mt. Rainer, a persistent circum-crater conduit in the glacial plug connects fumaroles to the surface through a web of dynamic rising vents. In the crater of Mt. Saint Helens, a complex array of recently formed caves is arranged astride the 2004-2008 lava dome. The caves are clearly associated with fumaroles and are evolving into persistent conduits in a growing glacier.

Comparative assessment between glaciovolcanic caves of the Cascades and other examples reveals generalized morphological patterns: 1) thermally influenced englacial conduits, where warm water creates melt-void caves that are enlarged by atmospheric advection; 2) isolated ‘steam domes’, whose size and shape are dictated by the rate of convective fumarole emissions; 3) lateral conduits in glacial ice are often chains of steam domes positioned around fumaroles with a size interannually maintained by atmospheric advection; 4) chimneys and rising conduits venting fumaroles with size and shape guided by accumulation or ablation of firn; 5) crevasses and moulins intersecting glaciovolcanic caves maintained by heat flux and atmospheric advection; and 6) ice-marginal melt at the ice-rock interface enhanced at the glacial margin and maintained by fluid movement but with highly variable morphology and persistence governed by bedload.