An iceberg 50 per cent bigger than the city of Toronto is expected to break off from an Antarctic ice shelf later this year or early in 2012.

Antarctica's Pine Island ice shelf is being split by a giant crack that was 29 kilometres long, 73 metres wide and 58 metres deep in some places as of Oct. 26, NASA reported. Scientists said it is widening by 1.8 metres each day.

The iceberg expected to calve off will have an area of about 900 square kilometres — quite a bit larger than the entire city of Toronto, which covers 641 square kilometres.

The crack was found during NASA's third annual airborne survey to measure and map changes in Antarctic glaciers this past October and November. It's believed to be the first detailed airborne measurement of a crack that is actively calving off an iceberg.

In this case, the process is considered unrelated to climate change — the Pine Island ice shelf is known to periodically calve off large icebergs, most recently in 2007.

However, the larger glacier that the ice shelf is part of shows what NASA calls "one of the most significant climate change response trends that scientists see worldwide."

The glacier was measured to have lost 46 gigatonnes of mass in 2010 or the equivalent of 46 trillion litres of water. That's a huge increase in its melt rate — in 2005, it lost just six gigatonnes.

With files from The Associated Press