A model of ablation-dominated medial moraines and the generation of debris-mantled glacier snouts

Medial moraines form striking dark. stripes that widen non-linearly, steepe n laterally and increase in relief down-glacier from the equilibrium line. Coalescence of these low-ablation-rate features can feed back strongly on t he mass balance of a glacier snout. Ablation-dominated medial moraines orig inate from debris delivered to glacier margins, producing a debris-rich sep tum between tributary streams of ice below their confluence. Emergence of t his ice below the equilibrium line delivers debris to the glacier surface, which then moves down local slopes of evolving morainal topography. A quant itative description of moraine evolution requires specification of the debr is concentration field within the glacier, treatment of the melt-rate depen dence on debris thickness, and characterization of processes that transport debris once it emerges onto the ice surface. Debris concentration at glaci er tributary junctions scales with the erosion rates and the lengths of the tributary-valley walls, and inversely with the tributary ice speeds. Melt rate is damped exponentially by debris, with a similar to 10 cm decay scale . Debris flux across the glacier surface scales with die product of debris thickness and local slope. Analytical and numerical results show that media l moraines should develop cross-glacier profiles with parabolic crests and linear slopes, and should widen with age and hence distance down-glacier. D ebris should be both thin and uniform over the moraine. Observed faster-tha n-linear growth of moraine widths with distance reflects the increasing abl ation rate down-glacier. Increase in medial moraine cover reduces the local average ablation rate, allowing the glacier to extend further down-valley than meteorology alone would suggest. This feedback is especially effective when moraines merge.