Geology Reference
In-Depth Information
Plate 10.4
Glacial trough with valley glaciers at head, East Greenland.
(
Photograph by Mike Hambrey
)
Water
gain
Mass balances may also be drawn up for continental
ice sheets and ice caps. In an ice sheet, the accumu-
lation zone lies in the central, elevated portion and is
surrounded by a skirting ablation zone at lower eleva-
tion. In Antarctica, the situation is more complicated
because some ice streams suffer net ablation in the arid
interior and net accumulation nearer to the wetter coasts.
Accumulation
zone
Water
loss
Ablation
zone
Quaternary glaciations
Glacier surface at
start of budget year
It is important to realize that the current distribution of
ice is much smaller than its distribution during glacial
stages over the last million years or so. Oxygen iso-
tope data from deep-sea cores (and loess sequences) has
revealed a sequence of alternating frigid conditions and
warm interludes known as glacial and interglacial stages
(Figure 10.4). The coldest conditions occurred at high
latitudes, but the entire Earth seems to have cooled down,
with snowlines lower than at present even in the tropics
(Figure 10.5). Palaeoglaciology deals with the reconstruc-
tion of these Quaternary, and older, ice sheets, mainly by
analysing the nature and distribution of glacial landforms
(see Glasser and Bennett 2004).
Figure 10.3
Glacier mass balance: schematic changes
in the geometry of a glacier during an equilibrium
budget year.
Source:
Adapted from Marcus (1969)
net losses of water in the ablation zone and the glacier
will retain its overall shape and volume from year to year.
If there is either a net gain or a net loss of water from
the entire glacier, then attendant changes in glacier shape
and volume and in the position of the firn line will result.
