The Muldrow Glacier, on the north side of Denali in Alaska, is
undergoing a rare surge. In the past few months the 63km-long river of ice has
been moving as much as 27m a day, 100 times its usual speed.
اضافة اعلان
The event has excited glaciologists, who have rushed to study it
using satellite imaging, specialized aerial photography and Global Positioning
System devices delicately placed on the shifting ice.
Surges often last only a few months. Most of them occur on
remote glaciers and are detected only after they’ve ended — when, for example,
satellite images show that a glacier front has rapidly advanced. But the
Muldrow is within Denali National Park and Preserve. Planes regularly fly over
the glacier carrying sightseers and climbers eager to ascend North America’s
highest mountain.
In early March, the pilot of just such a flight near the Muldrow
Glacier noticed large numbers of new crevasses as well as changes to lateral
moraines, areas of rocky debris that build up on the edges of glaciers.
“They looked all torn up,” said Chris Palm, the pilot, with K2
Aviation.
He took photographs with his phone, which were quickly shared
with researchers, including some from the National Park Service who have been
studying the glacier for years. Satellite data soon confirmed that the ice was
moving much faster than the speed it had averaged over recent decades, less
than 30cm a day.
The stress and strain from rapid movement of so much ice — the
glacier is up to about 457m thick and a 2km across — is causing all the
deformation and fracturing.
“The whole glacier is so cracked up,” said
Chad Hults, the
Alaska regional geologist with the Park Service, who landed on the Muldrow in
late March in a helicopter to set up equipment to measure the speed and other
characteristics of the surge.
Hults was involved in a study of the Muldrow two decades ago,
when the glacier was quiet and calm and relatively easy to walk across. This
time, he said, the ice was so shattered it was hard to find a place to land the
helicopter. And he could hear loud crashes and booms from breaking and falling
ice even over the noise of the aircraft’s engine.
The Muldrow was the route used by the first climbers to ascend
Denali, in 1913, and still some climbers choose that way up the mountain. But
with the climbing season about to begin, the route may be impassable, Hults
said.
Surges occur on only about 1 percent of glaciers worldwide. And
on a given glacier, decades may pass between events. Because of that relative
rarity, scientists haven’t been able to study them enough to have a complete
understanding of why they happen, or to gauge how climate change, which is
rapidly melting glaciers in Alaska and elsewhere, may be affecting them.
Mark Fahnestock, a glaciologist at the University of Alaska
Fairbanks, said changes in the mass balance between the top and bottom of the
glacier played a critical role in surges. Over time, ice accumulates in the
higher, colder stretches and is lost from the lower, warmer ones.
“The upper parts thicken and the lower parts melt back,”
Fahnestock said.
The surge restores the balance, rapidly shifting mass down to
the lower parts.
Since global warming is causing less ice accumulation and more
melting, Fahnestock said it was likely to have an impact.
“There will be effects, especially in Alaska because the mass
loss is so high,” he said.
The Muldrow last surged in 1956-57, and research from the region
suggested that it has done so about every 50 years. So scientists were
expecting the glacier to surge again at some point.
But what actually triggers a surge is not fully understood.
For the Muldrow and many other surging glaciers, meltwater that
becomes trapped at the base of the glacier by sediment or other rocky debris,
called till, may be part of what sets a surge off, said Martin Truffer, a
glaciologist at the University of Alaska Fairbanks.
This meltwater, created from the heat of friction between the
ice and bedrock and from heat within the Earth itself, builds up in the trapped
area. At some point the water pressure becomes so high that the friction
between the ice and bedrock is reduced, and the glacier picks up speed.
The Muldrow lies along a major fault, the Denali, and the steep,
rugged terrain has been broken up by earthquakes and suffers severe erosion. So
the glacier, like many other glaciers that surge, has a lot of debris
underneath it that could trap water.
“You need lots of till,” Truffer said. “But that alone is not
enough.”
Surges usually begin in the winter and end in the summer, when
water from surface melting greatly increases the flow through the glacier, to
the point where any bottlenecks are broken up. That lowers the water pressure
and increases the friction, slowing the ice.
Some surges start again the following winter when meltwater flow
decreases. But Truffer and others think the Muldrow is moving so fast that the
shift in mass balance will play itself out in the next few months, and it will
stop until the next surge, decades from now.
Read more
Americas
.jpg)
.jpg)
