Short answer
Sea ice forms when the ocean freezes and remains afloat. An ice sheet builds on land from accumulated snow that compresses into glacial ice and flows under its own weight. The Arctic Ocean has sea ice; Greenland and Antarctica have ice sheets. 1 2
Both are large icy features, but one forms from seawater and floats, while the other accumulates from snow on land.
On this page
At a glance
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| Question | Sea ice | Ice sheet |
|---|---|---|
| Origin | Freezing seawater | Accumulated snow on land |
| Support | Floats on ocean | Rests on land, though outlets may reach the sea |
| Typical change | Seasonal growth and melt | Long-term mass gain and loss plus flowing outlets |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Sea ice. Frozen seawater that forms, grows and melts in the ocean. 1
Ice sheet. A vast mass of land-origin glacial ice flowing outward over a continental-scale area. 2
Key differences
The key distinction is where the ice came from and what supports it. Sea ice can cover large stretches of ocean but is still ocean-origin ice. An ice sheet spans continental-scale land and can feed outlet glaciers and floating ice shelves at its edges. 1 2
How to tell them apart
Ask whether freezing happened in seawater or snow accumulated on land. A polar map’s white patch is not enough: inspect whether it lies over ocean or land. A floating iceberg came from land ice, so floating does not by itself make ice “sea ice.” 1 2
Where they overlap
Both occur in polar regions and influence climate by reflecting sunlight. Sea ice may border an ice sheet’s coastal outlets, and satellite images can show both together. 1 2
Edge cases
An ice shelf floats but remains attached to land ice, whereas sea ice formed in the ocean. A calved iceberg can drift through sea ice. The surface of an ice sheet may melt temporarily without changing its land-ice identity. 1 2
Why the distinction exists
Cryosphere science separates ocean freezing from the land-ice reservoir because their formation, motion and sea-level implications differ. 1 2
Common misconceptions
The Greenland Ice Sheet is not frozen seawater, and a broad Arctic sea-ice cover is not a land ice sheet. 1 2
Examples
Winter ocean freezing makes Arctic sea ice. Snow compacted over many years forms the Greenland Ice Sheet, which can send glaciers toward the coast. 1 2
Sources
Sources checked October 3, 2026.
- National Snow and Ice Data Center — Quick Facts About Sea Ice. What is sea ice?.
- National Snow and Ice Data Center — Ice Sheet Science. Ice sheet formation and movement.