Annual ice typically reaches a thickness of about 2 metres over the winter. This type of pack ice begins to form by the middle of September, and by the end of October the ice field is almost solid along the coasts. As winter progresses, the annual ice extends offshore, connecting some islands with a solid sheet of sea ice. The thickness of the ice increases until May at which point it begins to thaw, with shallow blue pools forming on the ice surface. Rising summer temperatures cause the ice to melt further and eventually it breaks up. Cracks form that soon widen into channels or leads that separate the ice pack into chunks called floes. Floes sometimes collide with each other, and break into many pieces. Ice breakup occurs in early July in the low Arctic, and at the end of the month in the high Arctic. The ice density declines until late August, but by mid-September the freezing cycle begins again.
Segments of the ice pack that are firmly frozen to the shore are known as landfast ice. Offshore winds push ice away from the coast creating a gap of open water between the landfast ice and the pack ice - a shore lead. The winds also push the pack ice further out to sea, causing the drifting floes to collide and ride up over each other, or hummock. The result is a distinct ridge of jumbled ice blocks.
The loose, floating pieces of ice present in the summer scour the shorelines, preventing most plants and animals from settling in shallow water. This makes the intertidal zone, the area between the shoreline and the near shore bottom, much more barren than it is in other marine ecosystems.