GLACIALLY TRANSPORTED BOULDERS ARE MOVING OBLIQUE TO ICE FLOW ON A HIGH-LATITUDE GLACIER: MATANUSKA GLACIER, ALASKA, USA

Field data collected near the terminus of the Matanuska Glacier, Alaska document transport of coarse-grained sediments oblique to the direction of ice flow in response to sun-driven ice ablation. The direction and amount of movement of glacial boulders were measured for 66 days near the terminus of the glacier in May-July 2001. Near the terminus, ice flows generally towards the northwest (~250° to 310°; mean=283.5°). Daily measurements indicate that boulders moved 181 to 870 cm total (mean=405.0 cm) towards the southwest (188.4° to 212.6°; mean=198.5°). The azimuth of incoming solar radiation at the glacier during May-July ranges from the southeast to southwest (161° to 210°; mean=186°; Lloyd 2000). Azimuth of boulder movement is skewed to the west in response to increased intensity of solar radiation during the afternoon. The ice surface surrounding the studied boulders slopes downhill (mean gradient=4.6%) towards the northwest (278°). However, boulders moved to the southwest up or down-slope (6.1 % to – 5.2 %). The variance in slope oblique to the ice surface slope is attributed to small-scale topographical changes, i.e. drainage networks that trend in the same direction.

Our measurements document a two-phase cycle of boulder movement: (1) prolonged episodes of reduced movement (0 to 10 cm/day), and (2) rapid episodes of accelerated movement (8 to 210 cm/day). Boulders experienced 2 to 9 cycles (mean=6.8) during the study period. Although we have not performed rigorous analyses, boulders > 5 cm tall and 20 cm long appear to insulate the underlying ice from incoming solar radiation, reducing the rate of ablation beneath the boulder compared to the surrounding ice, which is covered by a thin sediment layer (< 1.0 cm thick). Sediment cover thickness < 1.0 cm conducts solar radiation increasing ablation (Ostrem, 1959). Thus, a pedestal of ice (mode of transportation) forms beneath the boulder due to a difference in ablation rates between the ice surrounding and beneath the boulder. Collectively, these observations indicate that boulders were transported nearly perpendicular (~85° difference) to ice flow in response to ablation via incoming solar radiation. Glacially transported sediments have been only previously documented moving in the direction of ice flow.