An ice floe is a flat, expansive segment of sea ice floating freely on the ocean surface, representing a fundamental component of the polar marine environment. This type of ice is distinct from fixed ice sheets, often called fast ice, that remain attached to the coastline. Ice floes are found within the Arctic and Antarctic oceans, forming part of the larger, dynamic system known as pack ice. The study of ice floes is important for understanding climate patterns and ensuring safe navigation in polar waters.
Defining an Ice Floe
An ice floe is technically defined as any relatively flat piece of sea ice measuring at least 20 meters (66 feet) across at its widest point. Unlike icebergs, which are chunks of compressed freshwater glacial ice that have broken off from land-based glaciers or ice shelves, ice floes are formed from the freezing of saltwater. They are significantly less massive below the water line compared to icebergs, which typically have about 90% of their mass submerged.
The ice that forms a floe originates from the crystallization of seawater at temperatures around -1.8°C (28.8°F). This process expels much of the salt, which is why sea ice is less salty than the surrounding ocean water. A floe is a free-moving piece of ice, meaning it drifts with the currents and winds rather than being fixed to the shore or seafloor.
The Formation Process
Ice floes originate when continuous sheets of sea ice, whether from pack ice or fast ice, fracture and separate into distinct pieces. The initial formation of sea ice begins with tiny ice crystals called frazil, which gather to form a slushy layer on the water’s surface. As these sheets thicken, they eventually consolidate into thin sheets that become vulnerable to external forces that cause them to break apart.
The primary physical mechanisms responsible for this fracturing are strong ocean currents, tidal movements, and powerful winds. These forces apply stress to the ice sheet, exploiting existing weaknesses until large sections break away as individual floes. When floes collide due to these forces, they can buckle and overlap, a process called rafting or ridging, which creates complex pressure ridges on the surface.
Classifying Ice Floes by Size
Navigators and scientists classify ice floes based on their horizontal dimensions to standardize reporting and assess operational risks. The classification begins when a piece of sea ice exceeds 20 meters in width, moving beyond the designation of ice cake.
The size categories include:
- Small ice floes range from 20 to 100 meters (66 to 330 feet) across.
- Medium ice floes measure between 100 and 500 meters (330 to 1,640 feet).
- Big ice floes span from 500 meters up to 2 kilometers (1.2 miles) in width.
- Vast ice floes measure from 2 to 10 kilometers (1.2 to 6.2 miles).
- Giant ice floes are any flat piece of sea ice exceeding 10 kilometers across.
The floe’s size directly influences its stability and drift pattern. Larger floes are more stable but pose a greater navigational hazard due to their mass and momentum.