Tholeiitic-alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Abstract

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice-impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water, degassed subaerial lava flows were erupted. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice-impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile-rich alkalic melts, causing them to rise to the top of the chamber and erupt. -from Authors