GLACIOVOLCANISM IN BRITISH COLUMBIA'S MOUNT CAYLEY VOLCANIC FIELD: THE INTERPLAY OF MAGMA, TOPOGRAPHY, AND ICE IN A CONVERGENT MARGIN SETTING

British Columbia's Garibaldi volcanic belt (GVB), the northern segment of the Cascade volcanic arc, consists of multiple centres grouped into at least 8 volcanic fields extending from 49°N to 51.6°N. Eruption styles range from effusive to explosive, with compositions from basalt to rhyolite. Morphologically, centres include cinder cones, stratovolcanoes, calderas, and small isolated lava masses. Due to repeated continental and alpine glaciations, many of the volcanic deposits in the GVB reflect complex interactions between magma composition, topography, and changing ice configurations.

Lava properties are controlled by composition, but when lavas erupt in contact with ice, topography and ice characteristics play additional roles. Topography provides barriers that may confine eruption products, promotes or inhibits meltwater escape, and influences the geometry and hydrology of overlying ice. Ice is the source for meltwater; it also serves as a potential barrier for eruptive materials and meltwater, acts as an erosive agent, and exerts pressure on vents. The GVB has steep, irregular topography with many high altitude vents. Ice configurations have ranged from continental scale sheets (>1500 m thick), covering all but the highest peaks, to isolated alpine glaciers.

In the central GVB's Mount Cayley volcanic field (MCVF), the combined effects of composition, topography, and ice result in glaciovolcanic landforms different from those in other tectonic settings (e.g. Iceland). The high elevation of the MCVF, coupled with its cluster of mostly high altitude, non-overlapping vents, has resulted in numerous eruptions under ice <800 m thick (commonly much less). Due to the steep underlying topography and vent-ice geometries during eruptions, ice associated with most eruptions is likely to have been permeable, promoting meltwater escape. This has resulted in glaciovolcanic landforms which lack evidence for abundant water during eruption (i.e. pillows, hyaloclastite).