Posted by Julie
Wind Speed: Gusting between 20 – 40 knots
Temp with wind chill: - 36 to -41
Ever since the spectacular ice breakout in February, the ice forming over the water in front of Scott Base has steadily been growing thicker. Last week the ice was judged thick enough to walk on. Troy, our base leader, Jane, and I set out to mark a safe route for walking through some nearby pressure ridges.
Troy is an experienced glacier guide, and so we got the bonus of getting Troy to talk about the snow and ice. In a crack in the ice we discovered some spectacular, large, faceted ice crystals. This is sometimes known as crevasse hoar. Troy explained that these ice crystals form and grow in glacial crevasses and in other cavities where a large cooled space is formed and in which water vapor can accumulate under calm, still conditions with a large temperature gradient. The vapour then attaches itself straight to the ice crystal forming a hollow hexagonal shape.
Faceted ice crystal. © AHT/Julie
We also found impressively large icicles. Icicles require liquid water and so they are notable: the temperature rarely goes above freezing, and certainly has not been above 0 degrees for many months now.
Jane impersonating a radioactive rhinoceros by shining her headlamp through an icicle. © AHT/Julie
Most spectacular are the pressure ridges themselves. Pressure ridges form because even very thick sheets of sea ice are mobile. As the sea underneath moves, or when the temperatures fluctuate, the ice shrinks and expands, cracks and shifts. At Scott Base, the sea ice is bordered by Ross Island on one side and the permanent, immobile ice shelf (80 km thick) on the other. The relatively thin sea ice between has nowhere to move except upwards, and so plates of ice are very slowly pushed up vertically along cracks, eventually developing into spectacular ice formations. These pressure ridges survived the recent sea ice breakout in February, meaning they are at least 14 years old, the date of the last ice breakout.