The ice pack is the layer of ice which forms at the surface of the ocean when the temperature of the sea water descends to below -1.8°C. It thickens until there is no more heat exchange between the cold exterior and the sea water. The ice is today between 2 and 3m thick at the North Pole; it has lost 1.4 m since the 1960s-70s.  

Under the effect of tensions built up during its drift, the ice pack fractures, opening water channels which re-freeze. Elsewhere the collisions between the plates create compression peaks, walls of ice blocks several kilometres long which rise up to 5 metres above the sea.

Underwater the compressions form ice keels, which descend down to 30m below sea level. 		 
Due to the effect of ocean currents and wind, the ice pack is continually moving, in two main directions: the transpolar drift current which crosses the Bering Strait towards Greenland via the North Pole, and the Beaufort Gyre eddy current off the Canadian north-west coast and Alaska.
The “tectonics” of the ice plates makes the pack thickness very non-uniform. The age of the ice also affects the thickness: an ice pack which has lasted several winters is thicker than first-year ice which is generally about one metre thick. This unevenness complicates the average thickness estimate, which therefore requires measurements over a large ocean area.
 
Map of sea ice drift currents and oceanic deep trenches.
Compression peak
Compression in formation