Glaciovolcanism encompasses all forms of interaction between volcanism and ice. The resulting deposits can be probed forensically to recover properties of prehistoric ice sheets on Earth, including: i) thicknesses, ii) hydrology, iii) timing and iv) distributions. The growth and emergence of subaqueous volcanoes above their englacial lakes forms passage zones (cf. Jones 1969). Passage zones are diachronous surfaces marking transitions between subaqueous and subaerial environments. In glaciovolcanic settings the elevations of passage zone surfaces unequivocally record the height and depth of the paleo-englacial lake at a specific point in time and space. This lake depth fixes the minimum thickness of the enclosing ice sheet. Kima’Kho Mountain is a 1.8 Ma Pleistocene basaltic glaciovolcano (i.e. subglacial) situated in northern British Columbia (Ryane et al. 2011). The volcanic edifice rises 460 m from its base and comprises a central tephra cone and a surrounding plateau underlain by a sequence of flat-lying, subaerial lava flows capping dipping beds of pillow lavas and associated pillow lava breccias. The stratigraphic sequence hosts several discrete "passage zones" (cf. Jones, 1969). The first passage zone, a pyroclastic passage zone, is preserved entirely within pyroclastic deposits, marking the growth of a tephra cone above the englacial lake level during the explosive onset of volcanism (Russell et al. 2013). Later passage zones are associated with effusive volcanism and mark the transition from subaqueously deposited pillow lava-breccia deposits and overlying subaerial lavas. Here, we describe the properties of these diverse passage zones; their presence and geometry are used to constrain ice thicknesses at the time of eruption and inform on the englacial lake dynamics.
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