“Global Dynamics” of a Temperate Valley Glacier, Mer De Glace, and Past Velocities Deduced from Forbes’ Bands

• Published in: Journal of Glaciology
• Main Authors: Lliboutry, Louis, Reynaud, Louis
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1981
• Subjects: Earth-Surface Processes; Tilting; Forbes; Weertman; Journal of Glaciology
• Online Access: http://dx.doi.org/10.1017/s0022143000015367; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143000015367

Abstract Transverse profiles and velocities which have been measured on the ablation zone of Mer de Glace more or less continuously since 1891 contradict the Weertman–Nye theory of glacier kinematic waves. Faint broad waves, which undoubtedly result from fluctuations in the balance, travel down the glacier faster than this theory predicts. (This theory having first been completed by taking changes in width with time and with distance down the glacier into account.) On the other hand, velocity fluctuations are synchronous and more or less the same over the entire length studied (6 km). These discrepancies result from bottom friction being of the solid type, i.e. independent of sliding velocity. Friction should also be almost insensitive to discharge in subglacial waterways, since in the steady state energy for keeping them open, not entirely flooded, and at atmospheric pressure, is superabundant. The sliding velocities at all cross-profiles are thus controlled by some areas where the body of the glacier suffers strong deformation because the valley shape is far from cylindrical. One such controlling zone exists on Mer de Glace owing to the existence of a subglacial transverse shoulder. A new perturbation equation and a new rough expression for wave velocity are given. Intervals between Forbes’ bands were plotted on seven aerial surveys between 1939 and 1979. Progressive tilting of the slices of blue, dusty ice from the position from which these dark bands proceed and progressive lowering of their exposed edge must be taken into account. This analysis confirms the validity of our simple model for velocity fluctuations and allows us to estimate the entire series since the year 1888.