Cryospheric systems - Fundamentals of the Physical Environment - page 353

(b)

(a)

Glacier
Crevasse patterns

Cirque
glacier

Z 1

Ice sheet

Relative iceflow velocity
Zones of glaciation
(see text)
Nunatak with cirque

Z 1

Ice lobe

Valley
glacier

Outlet
glacier

Meltwater stream & lake
End moraine

Z 3

Piedmont lobe

(c)

Z 4

Ice shelf

Iceberg

Figure 15.6 Principal glacier types: (a) temperate (alpine) glaciers, (b) a cold (polar) ice sheet and (c) an ice shelf.
Source: After Addison (1983)

Meltwater drains laterally but is unable to penetrate the
frozen substrate and the layer refreezes in winter. The roles
of microclimate, relief, slope, material porosity and near-
surface drainage become increasingly important towards
the margins. Weak, sporadic permafrost activity is still
present in British mountains for these reasons, although
in other respects Britain lies outside the circumpolar
permafrost zone.

Radiative or orbital forcing of Ice Ages has long been
the prime suspect (see Chapter 9) but the magnitude and
periodicity of fluctuations in solar activity do not explain
adequately the intermittent glacial signature in the long-
term geological record. The Quaternary event is the
third Ice Age in 570 Ma of the Phanerozoic aeon, which
commenced shortly after a late Precambrian Ice Age;
earlier icehouse events at c. 430 Ma and 325-240 Ma
preceded Antarctic ice build-up after 35 Ma, leading to the
Quaternary event. Even earlier Ice Ages are known from
tillites or lithified glacigenic sediments. The Milankovich
mechanism, with its changing patterns of solar radiation
receipt due to Earth's astronomic eccentricities, offers
exciting clues to a range of geological processes. Although
it probably controls climatic oscillation within Ice Ages,
its continuing operation between Ice Ages cannot explain
the gaps. A range of possible geochemical explanations are
being explored, from Earth's passage through clouds of
cosmic dust to clear links between atmospheric SO 2 and
CO 2 levels and greenhouse-icehouse feedback processes.
Earth's supercontinental cycle is another promising
area of research (see Chapter 10). It has a major influence
on CO 2 levels through volcanic outgassing and the land
surface area exposed to weathering, which locks up CO 2
in carbonate weathering products. Tectonic forcing also
determines global distributions of sea, land and high relief

CLIMATE, TECTONICS AND ICE
AGES

Links between snowfall and temperature make the ELA
a vital measure of glacier climate. It lies close to sea level
in polar regions today, rising away from the poles so
that, for example, it lies at some 1.5 km above sea level
in northern Britain, 3 km in the European Alps and 5 km
at the equator. This is why Britain is just ice-free but there
are small glaciers on equatorial mountains in East Africa
( Plate 15.3 ). However, we know from geomorphic,
biological and oxygen isotope evidence that Quaternary
ELAs were often low enough for global or local glaciation
in about twenty individual cold stages . Why was this and
how do we explain Earth's earlier Ice Ages?