Surface-velocity and strain-rate variations at the glacier Austre Okstindbreen, Okstindan, Norway, 1976-95

Long term observations of surface velocities and strain rates at the Norwegian glacier Austre Okstindbreen revealed both temporal and spatial variations. During a period of 6 years, the amount of ice passing through a cross-section slightly below the mean equilibrium-line altitude (1250 m) was some 30% less than the amount which accumulated above the equilibrium line. The mean horizontal component of surface velocity at the centre of the cross-section was of the order of 45-50 m a(-1), whilst the thinner marginal ice moved less rapidly. At an altitude of about 1230-1200 m, surface velocities generally increased as the ice entered a steep icefall. In the lower part of the icefall, mean surface velocities again were of the order of 50 m a(-1). From there, a general decrease down-glacier was evident, and longitudinal compression along the curving centre line of flow was accompanied by lateral extension. The contribution of internal deformation to surface flow at the lower part of the glacier, which was less than 150 m thick, is likely to have been relatively small, and between-year variations of the horizontal component of surface flow which affected a large area probably were a response to changes of basal sliding rates, reflecting variations of mass balance and water availability.