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  • Print publication year: 2009
  • Online publication date: February 2010

3 - Microstructure of natural ice features



The microstructure of a natural ice feature is a direct result of its thermal-mechanical history. Glaciers, for instance, form from snow through the processes of sintering and densification. At the surface, sintering of dry snow is driven by a reduction in surface and grain boundary energy. Below a depth of 2–3 m, density increases under the combined effects of particle rearrangement and sintering. Snow gradually changes into firn, a form of porous ice of relative density of about 0.6. The transition marks the end of particle packing as the dominant densification process. Firn densifies mainly through creep. Closed pores progressively form, leading to bubbly ice. In polar ice sheets, depending upon the snow accumulation rate and temperature, the transition from firn to bubbly ice occurs at a relative density between 0.82 and 0.84 (Arnaud, 1997). At the close-off density, ice contains cylindrical and spherical pores. The density of bubbly ice increases with depth, and the pressure within bubbles progressively increases. In polar sheets, bubbles transform into hydrate crystals below a depth of 500 m (Miller, 1973). The evolution of the microstructure with depth depends on temperature, strain rate and impurities, and depends as well on grain growth and recrystallization. The grain size of the ice that eventually forms is typically between a millimeter and several centimeters (Chapter 6).

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