An Ocean Surrounded by Land
In the Arctic, permanent sea-ice formation occurred when the Arctic Ocean became land locked by the position of the continents. The North Pole, situated in the middle of the Arctic Ocean, is surrounded by the landmasses of Asia, America and Europe. They allow little mixing of warm water currents from the south, resulting in an ice-covered Arctic Ocean.
In terms of recent geologic history, there have been several major glacial episodes in the Northern Hemisphere, with the most recent one beginning about 80,000 years ago and ending 10,000 years ago. The ice sheet that resulted advanced and retreated several times during that period. During the interval of greatest ice formation, most of the northern United States, and all of Canada and Greenland were covered in ice up to 2,000 m (6,600 ft) thick. In Eurasia ice covered most of Europe and areas eastward to the Bering Strait.
The Antarctic Ice Sheet accounts for 90% of the world's permanent ice, the Greenland Ice Sheet for about 9% and alpine glaciers the remainder.