Tide crack in established fast ice
 - Sea Ice

Once fast ice (sea-ice frozen in situ and attached to the coast) has become established, the patterns of the earlier pieces disappears. The tide still rises and falls however meaning that the sea surface expands and shrinks slightly as it does so.

Tide cracks are a result of this movement (as ice is not known for its elastic properties) they are formed when the ice moves apart, they close again when the tide falls. A tide crack is often many miles long, in this case stretching for around 5 miles (8 kilometres), but never more than about 18", 45cm wide between Signy and Coronation Islands in the South Orkneys group.

Tide cracks are valuable resources for wild-life as they provide a region where birds such as snow petrels can fish through for krill and also as a breathing hole for crab eater and Weddell seals.