Geoscience Reference
In-Depth Information
KEY POINTS
1
Earth experiences both icehouse and greenhouse extremes of global climate. Over geological time scales,
icehouse conditions generally coincide with fragmented continent-ocean stages of the supercontinental
cycle, with some continents in polar locations. Each Ice Age lasts millions of years, comprising separate
cold (glacial) stages interspersed with temperate (interglacial) stages. During the current Quaternary Ice
Age, longer cold stages (10
4-5
yr) and shorter temperate stages (10
3-4
yr) coincide with regular orbital
(Milankovich) cycles, with shorter stadial and interstadial episodes.
2
Large continental ice sheets develop slowly and alpine glacier systems expand during cold stages -
particularly in the northern hemisphere, with its greater landmass - but recede or disappear altogether
during temperate stages. The Antarctic Ice Sheet, Earth's largest ice mass, has endured the past 35 Ma.
Global changes in albedo and sea level accompany ice sheet growth, initially intensifying the cold stage;
ice sheet-ocean-atmosphere coupling is sensitive enough to cause rapid termination as the cold stage
ends.
3
Regional climate and topography drive glacier mass and energy balances; each glacier has a distinct
thermodynamic character. This determines flow rates and mechanisms and geomorphic activity. Cold, polar
ice sheets are stable, covering large areas with generally slow-moving ice except near their margins, where
ice flow replicates the behaviour of temperate, alpine glaciers. The latter are unstable, fast-moving ice
streams with considerable geomorphic impact. Floating shelf margins of ice sheets are metastable and
hold the key to ice sheet response to global warming.
4
Alpine and polar glaciers are respectively warm- and cold-based, which influences the deformation zone
between moving ice and sediment, water and bedrock at the glacier bed. Deformation can move from one
material to another, determining the nature and location of glacial geomorphic processes. Contrasting
glacier styles create their own geomorphic landsystems. Alpine glaciers are constrained in their valleys,
while ice sheets bury huge land areas, placing different emphases on scales of operation, spatial variability,
supraglacial and subglacial environments. Their residual landsystems enable us to reconstruct Late
Pleistocene ice sheets.
5
Cold-stage climates may be so severe and dry as to prevent glacier growth over large areas, and terrestrial
landscapes experience permafrost instead. Surface and underground water is perennially frozen to depths
dependent on the severity and duration of the cold stage. The exception is an active layer of seasonal
surface melting which houses almost all geomorphic activity, driven by freeze-thaw cycles and the impact
of interstitial water over an impermeable substrate. Apart from cryofracture, most processes merely rework
and ornament the landscape.
FURTHER READING
Benn, D. I. and Evans, D. J. A. (1998) Glaciers and Glaciation, London and New York: Arnold. Still one of the outstanding
textbooks, covering all aspects of glacial processes and landsystems, from local sub-glacier to planetary and even
extraterrestrial scales. This account is superbly illustrated from the authors' own first-hand research.
French, H. M. and Williams, P. (2007) The Periglacial Environment, third edition, Chichester: Wiley. The latest edition of
the classic text on all aspects of periglaciation. Very accessible in its treatment, with excellent illustrations in figures
and plates.
Hambrey, M. (1994) Glacial Environments, London: UCL Press. A comprehensive and integrative review of glaciological,
geological and geomorphic processes uninterrupted by any mathematics, this lavishly illustrated book is supported by
an extensive glossary.
