Icebergs are composed of freshwater. Their origin and formation process, distinct from the surrounding ocean, give them this unique characteristic. Understanding how these colossal ice formations come into being reveals why they are essentially frozen rivers of pure water adrift in the salty sea.
The Freshwater Identity of Icebergs
Icebergs are freshwater because they originate from glaciers and ice sheets, massive accumulations of compacted snow. Snow, formed from atmospheric water vapor, is inherently freshwater. As snow falls and accumulates over millennia, it transforms into dense glacial ice. This process effectively excludes salt, ensuring the resulting ice remains free of salinity.
The freshwater composition of icebergs contrasts with the surrounding marine environment. This highlights a fundamental difference between ice that forms on land and ice that forms directly from the freezing of seawater.
From Snow to Glacier to Iceberg
The journey of an iceberg begins as countless snowflakes accumulate in polar regions or high mountain valleys. Over time, successive layers of snow bury and compress older layers. This pressure forces the snow to recrystallize, first into a denser, granular form called firn, which contains less air than fresh snow.
As more snow accumulates, the firn is further compressed. Air pockets within it become smaller and eventually seal off, transforming into solid, dense glacial ice. Once the ice mass becomes sufficiently thick and heavy, gravity causes it to flow slowly downhill or spread outwards. When these flowing glaciers or ice sheets reach a coastline and extend over water, large chunks break off into the ocean, a process known as calving, creating icebergs.
Distinguishing Icebergs from Sea Ice
Icebergs and sea ice differ significantly in their formation and composition. Icebergs are detached pieces of glaciers or ice shelves, originating from freshwater on land. In contrast, sea ice forms directly from the freezing of ocean water.
During sea ice formation, most salt is expelled from ice crystals, though some brine pockets can remain trapped, especially in newer sea ice. Over time, particularly in multiyear sea ice, much of this trapped brine drains out, reducing its salinity. Freshly formed sea ice still contains some salt, making it distinct from virtually salt-free icebergs. Icebergs are also typically much larger and thicker than sea ice, which rarely exceeds a few meters in thickness.
The Fate of Melting Icebergs
When freshwater icebergs melt into the ocean, they introduce fresh water into the marine environment, leading to a localized freshening of the surrounding ocean water and reducing its salinity. Such changes in ocean salinity can influence ocean currents, as freshwater is less dense than saltwater.
The melting of icebergs, while contributing to the overall volume of water in the ocean, has a minor impact on global sea level rise compared to the melting of land-based glaciers and ice sheets. This is because floating icebergs displace a volume of water roughly equal to their mass. However, since freshwater is less dense than saltwater, the melting of freshwater ice in saltwater does cause a slight increase in sea level, estimated at 2.6% of the iceberg’s volume.