Cryospheric systems - Fundamentals of the Physical Environment - page 348

Glacier ice
Glacier ice is fashioned from snow, hail, sublimation
(direct deposition) and rain or dew which subsequently
freezes. Snow is transformed through several recognized
stages before becoming mature glacier ice capable of
substantial geomorphic activity. Snowpack is highly
porous and held together by a frozen crystal lattice;
pores contain air and, depending on temperature, perhaps
water vapour and/or water. In essence, transformation
progressively expels air and reduces void space through
autocompaction . This process is assisted by the lowering
of freezing point under higher pressure. Further snowfall
increases overburden pressure on the snowpack, which
reaches its pressure-melting point at delicate snowflake
tips. Supercooled meltwater diffuses to areas of lower
pressure in the pack before regelation or refreezing/
recrystallization occurs. This process has some affinity
with rock crystallization and metamorphism. In addi-
tion, localized melt through insolation, advection or
geothermal heating assists consolidation if water then
regelates in colder parts of the pack. Tiny geometrically
complex snowflakes are transformed into assemblages of
progressively larger, amorphous ice crystals. Gravity tends
to draw this downhill as regelation creep , which initiates

Plate 15.1 Break-up and retreat of the floating portion of a
tidewater glacier on the Davis Strait, south-east Baffin Island.
Photo: Ken Addison

of 2-2·5 m. Sea water freezes in each hemisphere's winter
and shrinks by half in summer ( Figure 15.2 ). Ground ice
forms when pore water freezes in terrestrial substrates and
accumulates as perennial permafrost . It usually occurs
where intense cold and aridity stifle glacier development
and underlies a further 25 per cent of modern land
surfaces.

(a) (b) 180° 0°

Bering

45°E

135°W 135°E 45°W

Beaufort East

Siberian

Larsen

Laptev

Ronne-
Filchner

Hudson
Bay

Amery

90°W Kara 90°E 90°W 90°E

80°S

80°N Ross

60°N 80°S

Barents

40°N 40°S

60°S

Baltic 45°W

45°E 135°W 135°E

0

2000

0° 180°

km

Late Pleistocene
winter ice

Modern
winter ice

Modern
summer ice Modern

ice shelf

Ice limit

Figure 15.2 Global sea ice and ice shelf distribution. Polar seas are named in (a) the northern hemisphere and major modern
ice shelves in (b) the southern hemisphere.