[links, images and videos in on-line article]
'The largest iceberg in decades broke free from a North American glacier
– and no one noticed.'
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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