Glaciations

A glacier may be defined as a mass of ice that is formed from recrystallized snow and refrozen melt water, and that moves under the influence of gravity. Glaciers develop above the snowline, that is, in regions of the world that are cold enough to allow snow to remain on 

The commonest features produced by glacial abraston are striations on rock surfaces that were formed by rock fragments embedded in the base of the glacier. Many glaciated slopes formed of resistant rocks that are well Jointed display evidence of erosion in the form of ice-moulded 

Crag and tail, Castle Rock, Edinburgh, Scotland. Castle Rock probably represents an early phase of volcanic activity associated with the volcano at Arthur s Seat (Carboniferous). 

Corrie near Athabasca Glacier, Alberta, Canada. 

A stepped U -shaped valley, Yosemke National Park, California, 

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Figure 2b. Since the end of last glaciation, atmospheric CO2 concentration has remained around 280 until it began rising in the 18th century.

Quaternary 3 Myr Repeated glaciations. North and South America join mass extinctions of large mammals, evolution of Homo rise of civilizations, humans begin to modify biogeochemical cycles... 

Church, M. and Slaymaker, O. (1989). Disequilibrium of Holocene sediment yield in glaciated British Columbia. Nature 337,452 54. 

This chapter examines climatic and tectonic controls on erosion in the tropics and the implications of these observations regarding the composition of erosion products in general. The role of glaciations in continental denudation will then be examined and contrasted with tropical conditions. Finally, we will briefly examine human effects. 

Many important erosion-related phenomena are episodic and infrequent, such as flash floods, landslides, and glaciations, while others such as orogenesis and soil formation involve time scales that exceed those of major climate fluctuations. In either case, the time scale of human existence is too short to make adequate observations. Consequently, it is difficult to directly estimate the rates or characterize the effects of such phenomena on erosion products. The key to understanding weathering and erosion, on a continental scale, is to decipher the relationship between landforms, the processes that produce them, and the chemistry and discharge of river-borne materials. 

During the past 2.4 million years the Earth has gone through several episodes of continental glaciation (see Hughes, 1985). Each of the last seven of these episodes (600 000 years) shows a characteristic pattern of waxing and waning... 

Glaciers are powerful agents of physical erosion. In a detailed geomorphological study of the glaciated Canadian Shield, Sugden (1978) concluded that erosional style was related primarily to the basal thermal regime of the ice. From the center of divergence on an ice cap. 

Measurements of CO2 concentrations in air bubbles trapped in glacial ice (Berner et ah, 1980 Delmas et ah, 1980 Jouzel et ah, 1993 Raynaud et ah, 1993) show that atmospheric Pqo was about 200 ppmv toward the end of the last glaciation 20000 years ago (Fig. 11-19). 

Fig. 18-10 (a) Accumulation rate history for Greenland (Alley et ai, 1993 Guffey and Clow, 1997), with insets focusing on the Younger Dryas cold interval and the very rapid termination of the last glaciation, with substantial change that occurred in only 3 years noted (after Alley et ai, 1993). (b) Accumulation-rate history, normalized to the modem rate, for Antarctica (Raisbeck et ah, 1987), derived from °Be analysis. 

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