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Propagation of a glacier surge into stagnant ice

The propagation of the surge of Variegated Glacier into its terminal lobe was observed by daily surveying of a longitudinal line of closely spaced markers extending up glacier from a “stagnant zone” of thin (≈40 m), nearly motionless ice near the terminus, across a “front zone,” to a “surge zone” of thicker (≈100 m), rapidly moving (>20 m d−1) ice above. Within the front zone there were large angles of upward motion (>30°), compressive longitudinal strain rates (>0.1 d−1), and upward vertical velocities (>5 m d−1). Within the surge zone the ice motion was nearly parallel to the ice surface, and longitudinal gradients were small. The deformation pattern produced microcracking, exfoliation, and buckling of surface ice, longitudinal cracking, longitudinal chasms, and an increase in glacier volume. The front zone propagated at an average speed of about 40 m d−1 or roughly twice the surge speed of the ice above the front. This factor is determined by the thickness jump across the front. Finite element modeling constrained by measured geometry and surface velocity was used to deduce the velocity and stress at depth. The velocity in the surging zone occurs almost entirely by sliding. The sliding rate drops abruptly beneath the center of the front zone to zero in the stagnant zone. A comparison of basal velocity and shear stress distributions indicates a highly “lubricated” area beneath the upper part of the front zone and a “locked” area beneath the lower part of the front zone. Normal stress at the bed is not significantly different from overburden, and there is no high compressive normal stress on the bed to act as a pressure dam to obstruct water flow through the front zone.