Ice at the land surface - Fundamentals of the Physical Environment - page 468

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.
GLACIER MASS BALANCE
systems
Glaciologists refer to the difference between a glacier's annual accumulation and ablation
as its mass balance . It eventually forms glacier ice wherever mass accumulation exceeds
all forms of mass ablation or loss through melting, iceberg calving and sublimation. Mass
gain or loss may also occur by deflation (wind drifting) of snow and avalanching of snow
or ice between the glacier and its surrounds (Figure 1). Zones of net accumulation in
higher, colder parts of the glacier environment and net ablation towards the terminus
meet at the equilibrium line , whose annual average altitude (ELA) remains fixed in a
steady-state glacier. Mass is measured in water-equivalent terms (water has a density of
1·0 g cm −3 or 1000 kg m −3 ) because of the wide range of densities encountered (see Plate
14.1). Initial snowfall densities of 0·04-0·06 increase to 0·4-0·5 in firn or granular snow,
beyond which it becomes impermeable, with a density of about 0·6 as bubbly glacier ice
and about 0·9 as polycrystalline glacier ice . A positive mass balance permits the glacier
to thicken and extend its terminus, whilst a negative mass balance causes thinning and
retreat without necessarily halting internal ice flow. Ice flow in steadystate glaciers
transfers just enough ice from the accumulation zone through the equilibrium line to
match ablation zone losses. Ice cut off from its accumulation area by the reappearance of
bedrock during thinning becomes stagnant and experiences downwasting in situ .