Glaciers with a flotilla of 'ice sails'

Rare and somewhat esoteric. These are the huge pyramids of ice that stand proud of the surface on some glaciers.

To date, the phenomenon has only really been seen around the Karakoram mountain region of Pakistan.

The Baltoro glacier, which begins life at the very summit of K2, has some particularly fine examples.

Up to 25m in height and with widths of up to 90m, their triangular shapes when viewed from a distance give the impression of a flotilla of sail boats.

Now, scientists are getting a handle on how these giant "ice sails" form and wither over time, and how the processes involved depend on the special conditions that exist in the Karakoram region.

It all starts with the dirty surface at the head of a glacier.

Fragments of rock fall from the steep valley sides, darkening the top of the ice stream.

This makes it less reflective and able to absorb more solar radiation.

As a consequence, the glacier warms and begins to melt downwards; its whole surface drops - except where there are patches of clean ice.

These brilliant white areas, in contrast, continue to reflect the sun's rays. Their melt-rate is much slower and so they appear to rise as everything around them descends.

Thus, the sails are born, and they move down the valley with the progressing stream - like boats sailing down an estuary.

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