Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 199

rasion. Plucking also operates when ice reaches the pressure melting point on the up-
stream side of obstacles and the water moves downslope and refreezes (regelation) in
cracks in the bedrock, creating a bond between the glacial ice and the rock; the con-
tinued movement of the ice plucks the individual segments from the bedrock. This pro-
cess gives an asymmetric profile to the underlying obstacles: The stoss (upstream) side
is smoothed and gentle, while the lee (downstream) side becomes steep and irregular,
owing to the quarrying which has taken place. Such features (called roche moutonées )
provide excellent evidence for the direction of glacial movement.
The landscape that extends above the glacial ice is a product of both frost shattering
and glacial erosion (Russell 1933). Frost-shattered rocks eventually tumble onto the ice
surface for further transport. Glacial erosion takes place constantly as well. A glacier
can be thought of as a huge malleable mass completely smothering the surface and pick-
ing up loose rock and soil as it moves along. In this way, a new surface is continually
being exposed to the erosive power of the ice. The load a glacier can carry is almost
unlimited; a large glacier can easily transport rocks as big as a house.
MECHANISMS OF GLACIAL TRANSPORT
Rock material incorporated into the flow of glacial ice can be transported in one of
three modes: supraglacial (on top of the ice surface), englacial (within the glacial ice),
or basal (at the bottom of the glacier). The most important notion to keep in mind about
glacially transported and deposited sediment is that ice can carry any size particle any-
where in its flow, including huge boulders right on the surface! In addition to material
carried directly by the ice, sediment can be transported by meltwater through the com-
plex plumbing system within the glacier. Sediment carried in this way is subject to the
same hydraulics as sediment in rivers, and hence displays different characteristics from
sediments laid down directly from the melting ice. The most pronounced difference is
the sorted nature of the glaciofluvial sediments compared with the unsorted nature of
the ice-laid deposits (Benn and Evans 2010).
Glaciers generally continuously bury surface rock material under deepening layers
of snow and ice in the accumulation zone, and expose melted-out rock material in the
ablation zone. All of this takes place while glacier ice is moving from the accumulation
zone to the ablation zone. The net result is a set of curved flow streamlines that are
nested from the surface at the ELA to the sole of the glacier at the head and toe of the
glacier (Fig. 4.31; Sharp 1988; Benn and Evans 2010). This has the practical effect of
taking rocks falling onto the glacier surface near the head on a long trip deep into the
glacier at or near the sole of the ice before releasing them from their icy surroundings
at the surface near the snout. On the other hand, rocks falling onto the surface of the
glacier just above the ELA are taken on a short, shallow ride into the glacier before
reemerging just downslope of the ELA. A dramatic demonstration of this effect can be
found in the disappearance and subsequent discovery of the infamous missing airliner
named Stardust. Stardust vanished without a trace in 1947 during a trans-Andean flight
from Buenos Aires, Argentina, to Santiago, Chile. The disappearance was so immediate
and complete that the loss of Stardust was attributed to a UFO abduction. In reality, the
plane had crashed in poor weather at the head of the Tupangato Glacier and became
quickly entombed by an avalanche triggered by the impact. Thus Stardust vanished and