Debris-covered glaciers and rock glaciers in the Nanga Parbat Himalaya, Pakistan

The origin and mobilization of the extensive debris cover associated with t he glaciers of the Nanga Parbat Himalaya is complex. In this paper we propo se a mechanism by which glaciers can form rock glaciers through inefficienc y of sediment transfer from glacier ice to meltwater. Inefficient transfer is caused by various processes that promote plentiful sediment supply and d ecrease sediment transfer potential. Most debris-covered glaciers on Nanga Parbat with higher velocities of movement and/ or efficient debris transfer mechanisms do not form rock glaciers, perhaps because debris is mobilized quickly and removed from such glacier systems. Those whose ice movement act ivity is lower and those where inefficient sediment transfer mechanisms all ow plentiful debris to accumulate, can form classic rock glaciers. We document here with maps, satellite images, and field observations the pr obable evolution of part of a slow and inefficient ice glacier into a rock glacier at the margins of Sachen Glacier in c. 50 years, as well as several other examples that formed in a longer period of time. Sachen Glacier rece ives all of its nourishment from ice and snow avalanches from surrounding a reas of high relief, but has low ice velocities and no efficient system of debris removal. Consequently it has a pronounced digitate terminus with fou r lobes that have moved outward from the lateral moraines as rock glaciers with prounced transverse ridges and furrows and steep fronts at the angle o f repose. Raikot Glacier has a velocity five times higher than Sachen Glaci er and a thick cover of rock debris at its terminus that is efficienctly re moved. During the advance stage of the glacier since 1994, ice cliffs were exposed at the terminus. and an outbreak flood swept away much debris from its margins and terminus. Like the Sachen Glacier that it resembles, Shaigi ri Glacier receives all its nourishment from ice and snow avalanches and ha s an extensive debris cover with steep margins close to the angle of repose . It has a high velocity similar to Raikot Glacier and catastrophic breakou t floods have removed debris from its terminus twice in the recent past. In addition, the Shaigiri terminus blocked the Rupal River during the Little Ice Age and is presently being undercut and steepened by the river. With hi gher velocities and more efficient sediment transfer systems, neither the R aikot nor the Shaigiri form classic rock-glacier morphologies.