The making of beautiful giants

When we think of Antarctica or the Arctic, one of the first things that comes to mind are images of beautiful icebergs. Shimmering in striking hues of blue, white and silver, their fantastical shapes and sheer size lead to a general sense of awe when observing them from afar. However, close encounters are best avoided as the most famous iceberg of history, which caused the RMS Titanic to sink, teaches us. While we are generally aware that their origins lay within the Arctic and Antarctic Circles, the more intrinsic questions of their formation often eludes us.

Titanic_iceberg
The iceberg suspected of having sunk the RMS Titanic, photographed on the morning of April 15, 1912, just a few miles south of where the “Titanic” went down.

First, one must know that icebergs are not limited to the sea. Any glacier that terminates in water will likely produce icebergs, whether that water be the expanses of the southern ocean or a small pro-glacial lake.

Iceberg formation, generally referred to as calving, is intrinsically related to the physical properties of water and ice. A glacier is not a static, unmoving, accumulation of snow and ice. It moves downslope under the forces of gravity and its own weight. As the glacier makes its way downslope many cracks and crevasses form within it, caused by changes in the underlying topography and related change of speed in parts of the glacial system. While confined on land, these fractures within the ice have little impact on the integrity of the glacier but as it terminates in water the ice stream is no longer confined and can expand, widening the fissures and crevices.

As ice has a lower density then water it floats which leads to an uncoupling of the glacier from the bedrock, with the area of uncoupling known as ‘grounding line’. It is at this point that icebergs are generally formed.

Wave action in combination with increased subsurface melting, especially along the fissures and cracks, leads to weakening of the internal structure of the ice stream. Eventually, a stability threshold is reached and ice breaks off the front of the glacier along fracture lines.

In the scientific literature calving processes are classified according to a three tier system. Primary factors are longitudinal stretching related to friction at the glacier bed which leads to crevasse formation, this is tied to factors such as glacier geometry and water pressure at the bed. Calving usually occurs when the fracture fully separates a piece of ice from the main body of the glacier or ice shelf.

Second and third order processes generally influence the occurrence of individual calving events rather than the overall calving regime. Waterline melting, causing undercutting of sub-aerial ice and subsequent collapse is considered an important secondary factor. Third order processes are related to the ‘breaking off’ of underwater ice toes that were formed after sub-aerial parts of the glacier calved at an earlier stage.

Glacier-calving
Glacier calving event

The size of the icebergs produced is largely related to the size of the glacial system. Small terrestrial systems, terminating in pro-glacial lakes are obviously limited in the size of icebergs they produce. In Antarctica however, icebergs can reach truly staggering dimensions especially if they are produced by ice shelves. The largest iceberg recorded, Iceberg B-15, calved from the Ross Ice Shelf in March 2000 along pre-existing cracks. It was 295 km long and 37 km wide with a surface area of 11000 sqkm (by way of comparison, Counties Wexford, Waterford and Cork together are 11600 sqkm).

If, however, observing glacier calving and icebergs in the making is not enough to tingle your nerves, perhaps you should try glacier surfing … ride the wave created by the calving process.

Authored by Andrea Waitz, PhD student, TCD Geography

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