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'The largest iceberg in decades broke free from a North American glacier – and no one noticed.'

By Pakalolo

Tuesday Oct 25, 2016 · 9:28 AM EST

Porcupine Glacier is a 12 and a half mile long outlet glacier of an icefield in the Hoodoo Mountains in northern British Columbia. During late August of 2016 it calved a large iceberg but it was just recently discovered via satellite images. It took us almost two months to notice what’s been described as “the biggest calving event in North America” in recent memory. The ice chunk is described as “the largest single iceberg (by area) to calve from a North American glacier in recent decades”

The American Geophysical Union notes the significance of this discovery.

“During 2016 the glacier had a 1.2 square kilometer iceberg break off, leading to a retreat of 1.7 km in one year. This is an unusually large iceberg to calve off in a proglacial lake, the largest ever seen in British Columbia or Alaska… The retreat of this glacier is similar to a number of other glaciers in the area: Great Glacier, Chickamin Glacier, South Sawyer Glacier and Bromley Glacier. The retreat is driven by an increase in snowline/equilibrium line elevations which in 2016 is at 1700 m, similar to that on South Sawyer Glacier in 2016.”

Until recently, massive glacier fractures like what just happened at Porcupine didn’t really happen in North America. Unfortunately, over the past several decades, they’ve been increasing in frequency.

The Landsat 8 satellite passed over Porcupine Glacier on August 27, 2016, and observed the large, new iceberg (top). The second image shows the glacier as it appeared to Landsat 8 on August 27, 2015. The false-color images show the landscape in shortwave infrared bands at 30-meter resolution, a view that provides better distinction between ice, snow, and water.

As glacial ice thins, it melts from above and below, becoming more susceptible to rifts; eventually icebergs break off along those cracks. In the case of Porcupine, the iceberg broke off from a floating “ice tongue.” Such ice formations float on a small amount of water, lacking the structural support of a grounded terminus tongue, which is held up by the earth and rock on the seafloor or riverbed beneath it.

The iceberg from Porcupine comes from an ice tongue measuring 0.74 square miles (1.2 square kilometers). Tongues of this size typically occur in massive iced-over areas like the Larsen Ice Shelf, but are rare in relatively small Alaskan glaciers.

Unlike smaller chunks that fall into the water, this iceberg likely didn’t make much of a splash when it parted from the glacier, Pelto said. “It would have been more like if you’re pushing off from the shore in a canoe. It didn’t break off and fall in.”

In late August 2016, a deep rift widened and an iceberg heaved away from the Porcupine Glacier in northern British Columbia. Glaciologist Mauri Pelto, who has been analyzing satellite imagery of glaciers since the 1980s, called it “the biggest calving event in North America” that he has ever seen.
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