Cryospheric systems - Fundamentals of the Physical Environment - page 351

interstitial ice at 0 C but attractive forces lower the freezing
point below 0 C for

water bonded to soil or rock particles. However, ice crystal
growth exerts its own strong attractive force and draws
water to the freezing plane. Expansion on freezing
generates heaving pressures which displace loose soil
particles. In these conditions, segregated ice lenses can
form at the freezing front. Its continuing downward
penetration passes through the ground layer thus
desiccated and into the next zone of pore water, beyond
initial reach. The process is repeated to depths at which
downward penetration of the cold wave ceases or is
matched by geothermal heat flow ( Figure 15.5 ). Ground
contraction on cooling and local desiccation counter
expansion due to frost heave and generate cracks
which may become sites of vertical lenses or ice wedges .
Heaving, contraction and seasonal melt drive permafrost
processes, as we shall see below.
Permafrost, or perennially frozen ground, consisting of
segregated and interstitial ice zones and desiccated lenses
up to 400 m thick, is found in the Arctic basin. It forms
continuous cover on non-glacial polar land surfaces and
cold, arid continental interiors but thins equatorwards and
coastward. Discontinuous or sporadic forms occur as the
extent of talik or unfrozen ground increases and account
for 45 per cent of approximately 40 M km 2 of global
permafrost ground (see Figure 15.16 ). Seasonal melt
during summer months with temperatures above 0 C
develops a saturated, surface active layer 0·1-3·0 m thick.

C for capillary water and below -20

Plate 15.2 The Antarctic Ice Sheet and continent, compiled
from AVHRR data from NOAA satellites. The Transantarctic
mountains are seen snaking across the left centre of the
composite image and the deep pink-purple areas in the
adjacent coastline denote the principal ice shelves separating
the West (left) and East (right) Antarctic Ice Sheets.
Photo: © Infoterra Ltd

depends on the porosity and thermal conductivity of
Earth materials and the behaviour of pore water, outlined
first in Chapter 14. Gravitational water freezes to form

Thermodynamic character of glaciers

KEY CONCEPTS

There are clear links between glacier climate, mass balance and rates of flow which collectively define a glacier's
thermodynamic state( Table 15.2 ) and hence geomorphic activity. Polar climates are so cold that relatively little snow
falls or melts, ice takes longer to accumulate and flow velocity varies from zero (where the glacier is frozen to its
bed) to a few tens of metres per year. Low-energy polar or cold glaciers are consequently large and stable, capable
of surviving relatively large climatic fluctuations. The Antarctic Ice Sheet and Greenland Ice Cap are the largest modern
polar ice bodies but during Earth's glacial maxima two or more similar ice sheets form over much of North America
and Eurasia.

By contrast, warmer alpine climates experience heavier snowfalls and more rapid melt. Ice forms quickly and flow
velocities are measured in 10 1-3 m yr -1 . However, these high-energy alpine or temperate glaciers are much smaller
and far more susceptible to even small climatic changes. They are restricted today to high mountains like the Alps,
Himalayas, Andes and North American cordillera, where they are topographically constrained within the valleys they
have eroded. Thermodynamic character therefore also determines glacier configuration ( Figure 15.6 ). Larger and
thicker ice sheets are less constrained spatially but lower average velocity reduces their geomorphic impact except
in the vicinity of outlet glaciers , where large ice volumes accelerate towards ice sheet margins and create the most
impressive erosional landforms.