(Science Wednesday is posted early due to some upcoming travel):
On the north side of Denali, North America’s highest peak, the Muldrow Glacier has been moving unusually fast: as much as 90 feet/day! What is causing this? It’s called a glacial surge which is a short-lived cyclical event where ice within a glacier advances suddenly and substantially, sometimes moving at speeds 10 – 100 times faster than normal. The Muldrow glacier is commonly used by those climbing Denali via a northern route. But this recent surge activity may make that impossible, as it has opened up many fractures (crevasses) along the surface now, making it extremely dangerous. The last time this 39-mile glacier surged was in 1956-57, in which it advanced a stunning 4 miles! Evidence for surging was seen in the early 1900s as well, showing that this particular glacier surges approximately every 50 years.
Only about 1% of the world’s glaciers surge, with Denali containing several of these types of glaciers flowing down its flanks. How does it happen? It has to do with ice building up at higher elevations over time, and meltwater being present at the base of the glacier. When the glacier is in its quiet phase, ice accumulates and thickens on the upper part of the glacier with only a slow flow or transfer of ice to the bottom (lower) part of the glacier. Eventually, the glacier thickness reaches a threshold that disrupts the internal hydrology. Meltwater that normally drains to the bottom of the glacier and out the terminus (end) of the glacier gets trapped instead. That lubricates the boundary between the glacier ice and the rock over which it’s flowing, leading to less friction and faster glacial flow: it’s like a slippery glacial waterslide but with tons of fast-flowing ice. After most of the meltwater eventually bursts out and finds an escape, the glacier returns to its quiet phase, steadily gaining mass at its upper elevations, starting the process all over again. It will take decades to build up enough mass to surge again.
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photo: crevassed glacier in Peru while on expedition in July 2013 (no options for Muldrow Glacier photos due to copyright issues).