3.3.6 Pressure ridges
The ice that covers the Arctic Ocean ranges in thickness from 1-2m for first-year ice to 3-4m for multi-year ice. Relative to the area it covers, then, the sea ice is extremely thin. The sea ice drifts and is carried by ocean currents, tides and wind. The shearing forces create cracks and fractures in the sea ice, causing open water to appear. These fractures can grow larger, opening up leads, but the fractures can also close up in a short time. These leads are often covered by a thin layer of ice, but when they close up the thin ice layer is crushed and rafted, finally piling up on the old ice. This heavy piled-up ice creates new factures parallel to the original fractures, but these new fractures soon follow a similar crushing and piling-up process. Finally a range of piled-up ice is formed along the first fracture; this formation is called a pressure ridge. The underwater portion of the pressure ridge is called a keel, and the surface portion is called the sail. The average sail height of pressure ridges in multi-year ice is 2-3m, but sometimes they can tower as high as 8m or more.
In multi-year ice, the ratio of the sail height of pressure ridges to the ice thickness ranges from 0.9 to 1.5. Although the sea ice is very thick and the sails of pressure ridges are relatively low in the ice massifs in the center of the Arctic Ocean, at the margins of the southwest Kara Sea and the Chukchi Sea the sails of ice ridges are often higher than the thickness of the sea ice. When the distribution of the height of pressure ridges on multi-year ice in the Arctic Ocean is plotted as shown below, pressure ridges on the Canadian coast are relatively high at 3m, while those of the NSR are relatively low at 1-1.5m.