Environmental Engineering Reference
In-Depth Information
Figure 1
Principal glacier types: (a) temperate
(alpine) glaciers, (b) a cold (polar) ice sheet and
(c) an ice shelf.
Source: After Addison (1983).
in about twenty individual
cold stages
. Why was this and how do we explain Earth's
earlier Ice Ages?
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. 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 and two
broad correlations are observed. Polar supercontinents, fragmentary oceans and their
circulation systems may induce icehouse conditions. Equatorial supercontinents, well
