How icebergs form


In November 2013, the ice shelf attached to Antarctica’s Pine Island Glacier shed an iceberg the size of Singapore. Gigantic ice shelves fringe 30 per cent of the Antarctic coast, and thousands of huge blocks of ice break off each year. Icebergs can be streaked green by algae growing beneath them or bluish if they’re made of compacted ice, which scatters lots of blue light. They float because ice is less dense than the surrounding seawater. As water freezes, the molecules spread out into ice crystals. The crystals fill more space than seawater, but have identical mass, making them lighter.

Iceberg in the making

1. Snow buildup
Snow builds up in Antarctica’s continental interior until it slides downhill under its own weight, forming an ice stream.

2. Glacier on the move
When the glacier reaches the coast, it won’t stop there, but extends into the sea to form a floating platform called an ice shelf.

3. Ice shelf weakens
The ocean’s tidal motion raises and lowers the ice shelf, causing cracks. Warm seawater melts and weakens the shelf bottom from below.

4. Calving
Chunks of ice break off the shelf and crash into the ocean, creating huge waves. Icebergs are born.