Environmental Engineering Reference
In-Depth Information
KEY POINTS
1
Earth experiences both icehouse and greenhouse extremes of global climate. Over long
geological time scales, icehouse conditions appear to coincide with fragmented
continent-ocean stages of the supercontinental cycle, with continents in more polar
locations. Each Ice Age lasts millions of years, comprising separate cold or glacial
stages interspersed with temperate interglacial stages. During the current Quaternary
Ice Age, longer cold stages (10
4-5
a) and shorter temperate stages (10
3-4
a) coincide
with regular orbital (Milankovich) cycles and contain 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 - but recede or disappear
altogether during temperate stages. The Antarctic Ice Sheet, Earth's largest ice mass,
has endured the past 40 Ma. Global changes in albedo and sea level accompany ice
sheet growth and initially intensify the cold stage but ice sheet-ocean-atmosphere
coupling is sensitive enough to cause rapid deglaciation as the cold stage ends.
3
Regional climate and topography drive glacier mass and energy balances, endowing
each glacier with a distinctive thermodynamic character. This determines flow rates
and mechanisms and geomorphic activity. Cold, polar ice sheets are stable and cover
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 are reflected in
their geomorphic landsystems. Alpine glaciers are constrained in their valleys, while
ice sheets inundate huge land areas, placing different emphases on their scales of
operation, spatial variability and supraglacial and subglacial environments. Our
knowledge of northern hemisphere late Pleistocene ice sheets is reconstructed from
their residual landsystems.
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.
An outstanding textbook covering all aspects of glacial processes and land-systems, from local
