Glacial and geothermal dynamics in Sherman Crater, Mount Baker, Washington

Abstract

Although quiescent since a significant thermal event in 1975, Mount Baker, in Washington, continues degassing from fumaroles in Sherman Crater, indicating the presence of a connection to an active magmatic system at depth. The apparent equilibrium condition of the crater glacier between 2003 and 2008, despite lying well above the regional equilibrium line altitude, suggests that melting of basal ice by heat flux from fumaroles and heated ground must balance the glacier's positive surface mass-balance. My investigation of glacial and geothermal dynamics in Sherman Crater between 2009 and 2010 provides the first rigorous quantitative assessment of the Sherman Crater glacier: ice volume, flow direction and velocity, annual mass balance and characteristics of the material below its base. Heat flux is derived from these constraints by two methods: the glacier calorimetry method of Welch et al. (2007) and the thermal grounds method of Frank et al. (1977). Ground-Penetrating Radar (GPR) transects across the crater glacier yield a maximum ice thickness of ~40 m W.E. and volume of ~1.3 million m3 W.E. for 2009 and ~66 m W.E. and volume of ~2.1 million m3 W.E. for 2010, assuming a standard velocity of EM-waves through ice of 0.151 m/ns and an average ice density of 700 kg/m3. The measured annual mass balance in 2009 of ~4.0 m of snow and ice and derived heat flux range of ~18 to ~28 W/m2 are comparable to values at active but quiescent volcanoes. The glacier flows northward at a rate of ~3 cm/day and rests on a base of hydrothermally altered regolith with thin meltwater layers. Because the glacier's total volume in 2009 is ~2/3 of the inferred volume of ice melted during the 1975 unrest event (which did not produce a debris flow), the risk from meltwater-induced debris flow from Sherman Crater is low. The large fluctuation in calculated glacier volume between 2009 and 2010, however, highlights the potential for this situation to change rapidly. An increase in annual-mass balance or decrease in heat flux could affect both ice volume and the distribution of thermally exposed surfaces in the crater. The present glacier mass-gradient driving the flow of ice northward towards large areas of thermally exposed ground could be overwhelmed by an eastward directed mass-gradient, towards the East Breach, a source of debris flows since the mid-Holocene. Sherman Crater might therefore become more of a threat if volcanic activity subsides.