Box 11 History Of Ice On Earth

Ice has been part of the Earth's environmental system at several points throughout its 4.6 billion year history, and the passage of ancient ice sheets is recorded predominantly by glaciomarine sediments deposited and preserved in a variety of geological basins (Eyles, 2008). Much of this record can be interpreted by using our present understanding of glacial processes and products to interpret the past. In essence this is the application of the fundamental geological principle of uniformitarianism - the present is the key to the past. However, the Earth's Neoproterozoic glacial record challenges this idea. During the late 1990s the 'Snowball Earth' concept emerged (Hoffman and Schrag, 2002). The 'Snowball Earth' hypothesis envisages a series of cataclysmic global glaciations in which glacial ice reached tropical latitudes and the

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd ho

Glacio-epoch

Paleo-Mesoproterozoic mostly non-glacial interval1

Paleo-Mesoproterozoic mostly non-glacial

interval1

Supercontinent UR i—

KENORLAND -1-

ARCHEAN

NENA-COLUMBIA

RODINIA -1-

PALEOPROTEROZOIC

MESOPROTEROZOIC

500 Ma PALEOZOIC

CENOZOIC

NEOPROTEROZOIC

formed during collisional phase on active margins formed during rifting and on margins passive margins passive

Tectonic Phase

Localized glaciation in areas of active orogenesis?

MESOZOIC

Earth's hydrological system almost completely shut down. The glacial processes involved in this global freeze would have been very different from those of the Cenozoic Ice Age. This hypothesis has become the source of considerable controversy over recent years and a number of alternative ideas have been advanced to explain the presence of glacial sediments at low latitudes at this time. One of these hypotheses, the Zipper Rift model, is based on the idea of adiabatic or uplift-driven glaciation associated with the progressive rifting of the Rodinia supercontinent. The debate surrounding 'Snowball Earth' continues to generate controversy as geologists attempt to decipher the record contained within the glacial rocks of the Neoproterozoic. The key to these debates lies in our ability to read the clues within these ancient glacial records and, in particular, in a rigorous understanding of contemporary glacial processes and products.

Sources: Eyles, N. (2008) Glacio-epochs and the supercontinental cycle after 3.0 Ga: tectonic boundary conditions for glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 258, 89-129. Hoffman, P.F. and Schrag., D.P. (2002) The Snowball Earth hypothesis: testing the limits of global change. Terra Nova, 14,129-55. Reproduced with permission from: Eyles, N. (2008) Palaeogeography, Palaeoclimatology, Palaeoecology, 258, figure 1, p. 9].

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