Ice Ice Baby

Antarctica is pretty icy. But it turns out, not all ice is the same! There are ice sheets, glaciers, ice shelves, icebergs, sea ice … and they’re all slightly different.

An ice sheet is a massive layer of ice that forms when snow that falls on land compacts and recrystallises. Ice sheets are huge—there are two that cover most of the Antarctic continent. Currently only around 2 per cent of the continent isn’t covered by ice. Ice sheets can be thousands of metres thick, formed over hundreds of thousands of years as layer after layer of snow accumulates.

Super-strictly speaking, ice sheets are a type of glacier. More commonly, though, we think of glaciers as frozen rivers that slowly flow from areas of higher ground down to sea level. In Antarctica, there are specific glaciers within the massive ice sheets, but you also find glaciers in alpine regions like the Swiss Alps, or the Southern Alps of New Zealand. The Australian territory of Heard Island has glaciers, so we can lay claim to some too!

Ice sheets also flow downhill until they meet the ocean. When glaciers or other areas of an ice sheet meets the ocean and extend out over the water without breaking off, that section of floating ice is called an ice shelf. A chunk of ice that breaks off the ice shelf give you an iceberg, and the process that produces them is called ‘calving’. It can be pretty dramatic—complete with enormous cracking noises as the ice splinters off the ice sheet. Over time, as they float away in the ocean at the whim of the ocean currents and waves, these icebergs form beautiful and spectacular natural sculptures.

Also on the ocean, and not to be confused with icebergs, is sea ice. Sea ice is formed in very cold oceans where the temperature is so low that ice forms straight from the ocean water. Sea ice that floats around in large masses (called floes) is called pack ice, and there’s also fast ice, which forms when a continuous sheet of sea ice becomes stuck to a land mass or icebergs. It can also form structures stuck to the sea floor, though this isn’t common in Antarctica where the ocean is pretty deep. It's more common in the Arctic, where they are called stamukhi.

The extent of sea ice varies with the season—there’s more in winter, and it melts and decreases in extent during the summer months. As only ‘fresh’ water freezes, the salt from the ocean water becomes concentrated in the surrounding water, making regions of much denser water that then sinks to the bottom of the ocean. This can be an important driver of ocean circulation, currents and global climate systems.

So in the heat of summer, as those ice cubes clink in your glass, remember those cubes are just one of many very different types of ice.