The width (and the presence) of this ice zone is usually seasonal and depends on ice thickness, topography of the sea floor and islands. In Arctic seas the fast ice extends up to the depths of 20m, while in the Subarctic seas, the zone extends to depths of about 10m. Grounded icebergs may also act as anchors for fast ice, in particular in Antarctica. In some coastal areas with abrupt shelf and no islands, e.g., in the Sea of Okhotsk off Hokkaidō, tides prevent the formation of any fast ice. Smaller ocean basins may contain only the fast ice zone with no pack ice (e.g. McMurdo Sound in Antarctica).
The topography of the fast ice vary from smooth and level to rugged. Fast ice may either grow in place from the sea water or by freezing pieces of drifting ice to the shore or other anchor sites.