Ice detail on frozen lake, Mackenzie Basin, South Canterbury

During the winter months in the Mackenzie Basin, when temperatures may fall below minus 20 ºC, lakes and sometimes small streams will frequently freeze over for days or even weeks at a time before a thaw sets in. In this time the ice covering can be altered in texture and consistency by changing weather conditions, each change leaving a characteristic imprint on the ice surface for as long as it remains frozen.

When snow falls on ice it gradually compacts down to form a new ice layer, the air spaces between the snowflakes forming a milky mosaic of tiny air bubbles that remain trapped in the ice. These bubbles will often coalesce when the ice partly thaws, slowly rising towards the surface through tiny cracks and spreading out into large rounded blobs just below the surface if the surface happens to re-freeze before the bubbles can escape.

The ice photographed here was on the surface of a small pond which had been frozen for several weeks, during which time several light snowfalls, partial thaws and numerous frosts had left their marks. During the brief warmth of the previous afternoon I’d noticed that the ice had a thin layer of meltwater on it, but a hard frost overnight quickly froze it again so that by the time I ventured out onto the ice at sunrise the next morning it was as hard as concrete. It was so cold by morning that in some places, such as in the lower left part of this photograph, moisture from the air had frozen directly onto the surface to form tiny flower-like ice crystals, a process called sublimation.