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.
Old icebergs may be hundreds of thousands of years old. Many years of falling snow, consisting of snow crystals by the countless billions, act like tiny mirrors and reflect the light.
Some icebergs are also formed by freezing ocean water instead of snow and those areas are full of tiny air bubbles.
Beautiful bluish streaks that appear in some icebergs are caused by the refreezing of meltwater that previously filled very old glacier ice crevasses.