Page 361 - Laboratory Manual in Physical Geology

Map of Regional Variations SEASONAL SNOW: Snow
in the Cryosphere and ice may accumulate here
in winter, but it melts over the
following summer.
ICE SHELF: A sheet of ice PERMAFROST CONTINUOUS:
attached to the land on one The ground is permanently
side but afloat on the ocean on frozen over this entire area.
the other side.
SEA ICE: A sheet of ice that • North Pole PERMAFROST
originates from the freezing of DISCONTINUOUS: The ground
seawater. is permanently frozen in
isolated patches within this
no/graphicslib/detail/the-cryosphere-world-map_e290.) (Courtesy of The Cryoshphere, world map, UNEP/GRID-Arendal, www.grida. • South Pole mountain sides (cirques),
area.
MOUNTAIN GLACIERS AND
ICE CAPS: This area contains
permanent patches of ice on
river-like bodies of ice that flow
down and away from
mountains (valley and
piedmont glaciers), and
dome-shaped masses of ice
and snow that cover the
summits of mountains so that
no peaks emerge (ice cap).
ICE SHEET: A pancake-like
mound of ice covering a large
part of a continent (more than
2
50,000 km ).

FIGURE 13.1 Cryosphere components. You can also download a complete world map of cryosphere components from this UNEP

(United Nations Environment Programme) website: http://www.grida.no/graphicslib/detail/the-cryosphere-world-map_e290
Glaciers (tons of rock debris), and deposit their load “down-
Glaciers are large ice masses that form on land areas stream” (down-glacier).
that are cold enough and have enough snowfall to sus- The downslope movement and extreme weight of
tain them year after year. They form wherever the win- glaciers cause them to abrade and erode (wear away)
ter accumulation of snow and ice exceeds the rock materials that they encounter. They also pluck rock
summer ablation (also called wastage ). Ablation (wast- material by freezing around it and ripping it from bedrock.
age) is the loss of snow and ice by melting and by The rock debris is then incorporated into the glacial ice
sublimation to gas (direct change from ice to water and transported many kilometers by the glacier. The debris
vapor, without melting). Accumulation commonly also gives glacial ice extra abrasive power. As the heavy
occurs in snowfields —regions of permanent snow cover rock-filled ice moves over the land, it scrapes surfaces like
( FIGURE 13.2 ). a giant sheet of sandpaper. Rock debris falling from valley
Glaciers can be divided into two zones, walls commonly accumulates on the surface of a moving
accumulation and ablation ( FIGURE 13.2 ). As snow and glacier and is transported downslope. Thus, glaciers

ice accumulate in and beneath snowfields of the zone transport huge quantities of sediment, not only in, but
of accumulation , they become compacted and highly also on the ice.
recrystallized under their own weight. The ice mass When a glacier melts, it appears to retreat up
then begins to slide and flow downslope like a very the valley from which it flowed. This is called glacial
viscous (thick) fluid. If you slowly squeeze a small piece retreat , even though the ice is simply melting back
of ice in the jaws of a vise or pair of pliers, then you (rather than moving back up the hill). As melting occurs

can observe how it flows. In nature, glacial ice formed ( FIGURE 13.3 ), deposits of rocky gravel, sand, silt, and
in the zone of accumulation flows and slides downhill clay accumulate where there once was ice. These deposits
into the zone of ablation , where it melts or sublimes collectively are called drift . Drift that accumulates
(undergoes sublimation) faster than new ice can form. directly from the melting ice is unstratified (unsorted by
The snowline is the boundary between the zones of size) and is called till . However, drift that is transported
accumulation and ablation. The bottom end of the by the meltwater becomes more rounded, sorted by size,

glacier is the terminus . layered, and is called stratified drift . Wind also can
It helps to understand a glacier by viewing it as a transport the sand, silt, and clay particles from drift.
river of ice. The “headwater” is the zone of accumula- This wind-transported sediment can form dunes or loess
tion, and the “river mouth” is the terminus. Like a river, deposits (wind-deposited, unstratified accumulations
glaciers erode (wear away) rocks, transport their load of clayey silt).

Glaciers and the Dynamic Cryosphere ■ 331