Pancakes Floating in the Ocean by Lauren

We see a lot of ice on S04P, but how does it freeze? Icebergs are made of mostly fresh water. Precipitation (rain, sleet, and snow) fall on Antarctica and compact to form glaciers and ice shelves. I say ‘mostly’ because seawater can crystallize onto the underside of these structures and salty sea breeze can freeze onto their sides. You can see the horizontal layering here on this tabular iceberg, which was probably once part of an ice shelf.

  While this ice is firmly planted on the continental shelf, the glacier or ice shelf is said to be “grounded.” Sections of the glacier or ice shelf can break away, or “calve off.” These freely floating chunks have now become “icebergs.” These are carried around by ocean currents and wind. Because the underside of an iceberg often melts more quickly than the top (like ice cubes in a glass of lemonade), they can become top-heavy and flip over. Below is an iceberg that may have flipped. The top is smooth from being melted and reshaped by moving ocean water. If you look carefully, you may be able to spot the previous water line!

What happens to these icebergs? Many of them become eroded by Antarctica’s intense “katabatic” winds or flow northward and melt. Here is a photograph of a nearly melted iceberg (with a bonus ice shelf in the background). When icebergs melt, they inject freshwater into the ocean.

Contrary to icebergs, sea ice is formed directly from the freezing of the ocean surface. Since this water is salty, it has a slightly colder freezing point that a lake or pond. As the ice freezes, the molecular structure squeezes out salts in a process called “brine rejection,” injecting saline water into the underlying ocean. Sea ice freezes in stages. Depending on water temperature, air temperature, and water salinity, one of the first stages can be “grease” ice.

Another stage is “pancake” ice.

 

These pancakes can collide and merge with newly frozen seawater to form a solid sheet of “first year” ice.

Some of this ice survives the summer melt to become “multi-year” ice, which is mixed with first year ice in this photograph.