You’d think it would be simple to write a post defining “moulins” – an ice phenom that appears in glaciers – but instead I’ve spent the last hour completely distracted by the beauty of photographs like this one.
No matter how often I see photos of glacial ice formations, the colours and scale blow me away. Even more so now that we know these seemingly indestructible formations are being dramatically altered by climate change (the Antarctic Wilkins Ice Shelf shattered completely just this April, for example, news story here; post about ice shelves forthcoming).
But to get to the definition: Moulins are chutes or tunnels or channels that form on the surface of a glacier and carry meltwater in rushing channels down into the glacier’s interior.
Moulin streams usually come out at the glacier’s base, but sometimes will appear exiting an ice wall or other exposed glacier face.
Apparently scientists aren’t entirely sure how moulins form – is it simply because a pool of meltwater warmed by the sun becomes deep enough to bore a hole downward? Or are they caused by pressure vectors unrelated to a glacier’s surface temperatures?
But they agree that when a moulin carries water all the way to the base of a glacier, it lubricates the glacier’s movement. And it eventually helps split the glacier apart. Here’s a succinct description from the science website http://www.global-greenhouse-warming.com/moulin.html
The cascading water through the ice pushes down on the glacier at the same time water seeps through cracks to the underside. In this way, water becomes a lubricating fluid at the base of the glacier, enhancing glacial motion and speeding disintegration of the ice sheet. The melting water encourages further ice loss and accelerates the glacier’s flow to the sea, where large chunks break off to form icebergs.
And from Wikipedia, the further detail that:
Given an appropriate relationship between an ice sheet and the terrain, the head of water in a moulin can provide the power and medium with which a tunnel valley may be formed.
The beauty of all this can be deadly for hikers. The thought of tumbling into churning, freezing water that falls steeply into untraceable channels makes me quite content to respect any trail sign that states, “Stay back!”
The problem is, since glaciers are changing more rapidly than what we’ve been used to in the past few hundred years, moulins are also now appearing in new places. In Yosemite Park last October, Pete Devine, a veteran glacier observer who manages educational programs for the nonprofit Yosemite Association, reported seeing a “yawning death-trap of a moulin” at the base of the retreating, thinning Lyell glacier.
In locations where glaciers are melting into ice patches, it may become more and more dangerous to get close enough to see what’s going on, either for recreational or scientific reasons.