Imagine what it would be like if your house or apartment grew and shrank every year. In the Antarctic winter, sea ice more than doubles the surface area of Antarctica, giving expanded habitat for animals like penguins, seals, and seabirds. The winter sea ice can extend over 7.2 million square miles (19 million square kilometers) of ocean surrounding Antarctica, that’s an area of sea ice more than two times the size of the United States. In the Antarctic summer, much of the sea ice disappears and then reforms the next winter.
Animals who live on the sea ice and humans who travel across and through it must be able to “read” the changing ice and choose safe places to travel and rest. As surface water cools to the freezing point, ice crystals start to form. If the weather and sea conditions are calm, the ice crystals join together and thicken into young ice, a fibrous and weak ice you would want to try to avoid if you were out for a walk. Even a slight ocean swell can break the young ice into pieces that knock against each other, forming flat circular slabs of thin ice that look like pancakes and are called pancake ice. If it’s cold enough the pancakes will eventually freeze together and then freeze fast to the shore, forming fast ice.
Animals like penguins, seals, whales, and sea birds rely on waves, wind, tides and currents to buckle and crack the fast ice to allow access between the ocean water and the ice surface and to create breathing holes. Humans traveling across the sea ice try to avoid these leads of open water.
During normal summers, the fast ice breaks apart and forms floes, pieces of ice anywhere from 65 feet (20 meters) to 6 miles (10 km) across. The floes drift on the currents until they pack tightly together forming pack ice. Modern ice breaking ships can move through sea ice, but pack ice gave early explorers a lot of trouble—in 1915 Ernest Shackleton’s wooden ship The Endurance was trapped when the pack ice closed around it, holding it for ten months before crushing it as the spring thaw allowed giant floes to grind the ship between them.