4 STAGES OF PRESSURE RIDGING

The pressure ridging process is simulated using a two-dimensional part icle model. Blocks are broken from an intact sheet of relatively thin lead ice pushed against a thick, multiyear flee at a constant speed. T he blocks of ice rubble accumulate to form the ridge sail and keel. Du ring the simulations the energy consumed in ridge growth, including di ssipation, is explicitly calculated. On the basis of the results of si mulations performed with the model, the ridging process can be divided into four distinct stages. The first stage begins with an intact shee t of lead ice impacting a flee and ends when the sail reaches its maxi mum height. In the second stage the ridge keel deepens and widens. The stage ends when the maximum keel draft is reached. In the third stage the direction of growth is leadward creating a rubble field of more o r less uniform thickness. The third stage ends when the supply of thin ice is exhausted. In the fourth stage the rubble field is compressed between converging flees. The results of simulations establish the dep endence of ridging energetics in the first and second stages on the th ickness of the ice sheet and the amount of ice pushed into the ridge. The average profiles of the simulated ridges delineate the growth proc ess in the first, second, and third stages. The energetics and profile s of the fourth stage were described by Hopkins et al. [1991]. Lead ic e extents of up to 1300 m are pushed into ridges to determine maximum sail heights, keel drafts, and ridging forces.