Annual snowfall averages from 85 cm to 250 cm at the same stations. In general, smaller islands receive less precipitation than larger islands. The ecoregion is composed of a chain of islands exposed along the crest of a submarine ridge. Most of the islands, formed of blockfaulted Tertiary sediments derived from earlier igneous processes, are surmounted by volcanoes of Tertiary and Quaternary Age. Regional elevations range from sea level to greater than 1,900 m.
Most high volcanoes have icecaps or small glaciers.
The islands have been intensely glaciated and evidence of glacial erosion is common. Cliff faces surrounding coastal fjords can rise more than 600 m above the ocean. The region is generally free from permafrost, but periglacial erosional processes are active because of the cold, wet climate. Streams are short and swift. Many enter the sea as waterfalls.