664 SEDIMENTARY PROCESSES/Glaciers Figure The present day distribution of glaciers; locations are based in part on Satellite Image Atlas of Glaciers of the World (US Geological Survey, (2002) Satellite Image Atlas of Glaciers of the World) contribute to sea-level rise over the next 100 years as the climate warms Published figures for the potential contribution of glaciers to sea-level rise have large errors, but Table gives an indication of the relative importance of the different ice-masses The polar icesheets will continue to grow and decay in different places at the same time, until the increase in temperature exceeds the threshold at which they can remain in equilibrium Table Potential contribution of the world’s ice masses to sea level rise (US Geological Survey website, 2001); note that there are considerable uncertainties in the accuracy of these estimates Ice masses Sea level rise (m) West Antarctic Ice Sheet East Antarctic Ice Sheet Antarctic Peninsula Greenland Ice Sheet Mountain glaciers 65 0.5 6.5 0.5 Characteristics of Glaciers and Ice-Sheets Glacier Morphology Glaciers range in size from the great ice-sheets of Greenland and Antarctica to tiny ice-masses just a few hundred metres across The generally accepted definition of an ice-sheet is an ice-mass exceeding 50 000 km2 in area The Greenland Ice Sheet forms an elongated dome, with mountains projecting through it near the periphery, whilst the Antarctic Ice Sheet consists of three dynamically separate regions, of which the dome of East Antarctica has by far the greatest volume (Figure 2) Ice-caps are also dome-like masses, but measure less than 50 000 km2 (Figure 3) Both icesheets and ice-caps discharge the bulk of their ice via ice-streams – zones of accelerated flow with slowmoving ice on either side As ice-streams enter the sea, coalescence leads to the formation of ice-shelves – floating slabs of ice several hundred metres thick Like ice-caps, highland ice-fields also bury much of the topography, but mountains project through them as ‘nunataks’ Valley glaciers are typical of mountain regions, such as the Alps, and are the best studied of all ice-masses (Figure 4) Above them are upland amphitheatre-like basins where cirque (or corrie) glaciers accumulate Lastly, hanging glaciers are perched on steep mountain rock faces and are so prone to avalanches that below them are regenerated glaciers