Sea ice is formed by the freezing of sea water (Parkinson et al., 1987). When salt water freezes to form sea ice, most of the salt is left behind in the unfrozen sea water, forming a cold, salty, dense brine that rapidly sinks. However, some salt crystals are also frozen into the sea ice (Parkinson et al., 1987). Thus, newly formed sea ice is not frozen fresh water, like lake ice, glacial ice, or icebergs, which form from calving of glaciers when they meet the ocean. Because sea ice is not frozen fresh water, it behaves differently from lake ice or icebergs.
There are many names and classifications given to sea ice. Here we'll only be concerned with distinguishing the following three ice types.
Frazil Ice -- the fine, individual ice crystals, spicules (needlelike structures), or plates of ice suspended in water when sea water first begins to freeze
First Year Sea Ice -- sea ice formed in a given winter that is at least 0.30 m thick. This ice will trap some of the salty brine from the sea water between the ice crystals (mean salinity of first year sea ice is approximately 10%). This gives the ice a degree of flexibility. It will bend slightly before it breaks. Sea ice formed in a given winter remains classified as first year sea ice until it either melts during summer or survives one summer without completely melting.
Multiyear Sea Ice -- sea ice that has survived at least one summer without completely melting. Unlike first year ice, multiyear ice is almost completely frozen fresh water. Since salty water cannot completely freeze, small unfrozen pockets of salty brine exist within the first year ice. Over time, this salty brine either works its way down through the ice because it is denser than the ice, or is flushed from the ice by surface melt water (from melting snow) in summer (Maykut, 1985). Since the salt in the ice is lost, this ice is entirely frozen fresh water, and tends to be very hard with no flexibility.