Iceberg

a large lump of ice which has broken off from a GLACIER or ice sheet and is floating in the sea. Icebergs can be carried long distances before they finally melt.

Taken from Dictionary of Science

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An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice. Alternatively, it may come to rest on the seabed in shallower water, causing ice scour (also known as ice gouging) or becoming an ice island.

Taken from Wikipedia