Characteristics

Ice formation

 

 

As the ocean water begins to freeze, small needle-like ice crystals called frazil form. These crystals are typically 3 to 4 millimeters (0.12 to 0.16 inches) in diameter. Because salt doesn't freeze, the crystals expel salt into the water, and frazil crystals consist of nearly pure fresh water. See also Salinity and Brine.

Sheets of sea ice form when frazil crystals float to the surface, accummulate and bond together. Depending upon the climatic conditions, sheets can develop from grease and congelation ice, or from pancake ice. These processes are described below.

In calm waters, frazil crystals form a smooth, thin form of ice, called grease ice for its resemblance to an oil slick. Grease ice develops into a continuous, thin sheet of ice called nilas. Initially, the sheet is very thin and dark (called dark nilas), becoming lighter as it thickens. Currents or light winds often push the nilas around so that they slide over each other, a process known as rafting. Eventually, the ice thickens into a more stable sheet with a smooth bottom surface, called congelation ice. Frazil ice cannot form in the relatively still waters under sea ice, so only congelation ice developing under the ice sheet can contribute to the continued growth of a congelation ice sheet. Congelation ice crystals are long and vertical because they grow much slower than frazil ice.

If the ocean is rough, the frazil crystals accummulate into slushy circular disks, called pancakes or pancake ice, because of their shape. A signature feature of pancake ice is raised edges or ridges on the perimeter, caused by the pancakes bumping into each other from the ocean waves. If the motion is strong enough, rafting occurs. If the ice is thick enough, ridging occurs, where the sea ice bends or fractures and piles on top of itself, forming lines of ridges on the surface. Each ridge has a corresponding structure, called a keel, that forms on the underside of the ice. Particularly in the Arctic, ridges up to 20 meters (60 feet) thick can form when thick ice deforms. Eventually, the pancakes cement together and consolidate into a coherent ice sheet. Unlike the congelation process, sheet ice formed from consolidated pancakes has a rough bottom surface.

 

Multiyear ice

 

Multiyear ice has distinct properties that distinguish it from first-year ice, based on processes that occur during the summer melt. Multiyear ice contains much less brine and more air pockets than first-year ice. Less brine means "stiffer" ice that is more difficult for icebreakers to navigate and clear.

Hummocks of multiyear ice that are several years old are fresh enough that someone could drink their melted water. In fact, multiyear ice often supplies the fresh water needed for polar expeditions.

First-year and multiyear ice have different electromagnetic properties that satellite sensors can detect, allowing scientists to distinguish the two. For more information, see Remote Sensing in the Studying section.

Multiyear ice is much more common in the Arctic than in the Antarctic. This is because ocean currents and atmospheric circulation move sea ice around Antarctica, causing most of the ice to melt in the summer as it moves into warmer waters, or as the upper ocean heats up due to absorption of solar heat by open water areas. Most of the multiyear ice that does occur in the Antarctic persists because of a circulating current in the Weddell Sea, on the eastern side of the Antarctic Peninsula. The Arctic Ocean, in contrast, is relatively land-locked, allowing extensive multiyear ice to form.

Multiyear ice. Image courtesy of Ted Maksym, United States Naval Academy.  

Difference between sea ice, icebergs, glaciers and lake ice

 

The most basic difference is that sea ice forms from salty ocean water, whereas icebergs, glaciers, and lake ice form from fresh water or snow. Sea ice grows, forms, and melts strictly in the ocean. Glaciers are considered land ice, and icebergs are chunks of ice that break off of glaciers and fall into the ocean. Lake ice is made from fresh water and freezes as a smooth layer, unlike sea ice, which develops into various forms and shapes because of the constant turbulence of ocean water.

The process by which sea ice forms is also different from that of lake or river ice. Fresh water is unlike most substances because it becomes less dense as it nears the freezing point. This difference in density explains why ice cubes float in a glass of water. Very cold, low-density fresh water stays at the surface of lakes and rivers, forming an ice layer on the top.

In contrast to fresh water, the salt in ocean water causes the density of the water to increase as it nears the freezing point, and very cold ocean water tends to sink. As a result, sea ice forms slowly, compared to freshwater ice, because salt water sinks away from the cold surface before it cools enough to freeze. Furthermore, other factors cause the formation of sea ice to be a slow process. The freezing temperature of salt water is lower than fresh water; ocean temperatures must reach -1.8 degrees Celsius (28.8 degrees Fahrenheit) to freeze. Because oceans are so deep, it takes longer to reach the freezing point, and generally, the top 100 to 150 meters (300 to 450 feet) of water must be cooled to the freezing temperature for ice to form.