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Glacier holes you can crawl into

Holes in glaciers are of interest because of the work they do, transferring meltwater through the glacier. They are also interesting in their own right, although crawling into them is an acquired taste. They are dark and cold, you don’t know where you are going, and you do know that the journey will get harder and even more dangerous the further you go. But this is just what attracts one group of people, the cavers or speleologists.

Two recent papers illustrate the (to me) doubtful pleasures of glacier speleology, while also offering valuable insights into how the meltwater finds its way into the hole and, once there, keeps on doing its job (which is to obey the forces of gravity and pressure, while also conforming to the constraints of thermodynamics).

Doug Benn and co-authors describe caving in three glaciers of diverse type, in Svalbard, Nepal and Alaska, while Jason Gulley has more detail about the ice caves of Benn’s Alaskan glacier, Matanuska Glacier.

Even cavers draw the line, usually, at plunging into water-filled holes, but some of these caves are only partly full. Others are dry. The most favourable time for exploring ice caves is after the summer meltwater has gone, but before the ice has had time to squeeze them shut.

The photographs alone are worth it. Some of the dry caves show evidence of former water levels in the shape of sills. These are shelves of ice protruding from the cave wall. They record episodes in which the water began to refreeze at its contact with the overlying air, but then drained away.

Others have keyhole-shaped cross-sections, recording a transition from enlargement of a water-filled hole, by net melting all around the cross-section, to deepening of just the cave floor by a lesser flow of water.

Two of the caves have scalloped walls. The scallops are dish-shaped, and roughly dish-sized, indentations covering all surfaces around the cave wall. Their size reflects the vigour of interaction between the turbulent water flow and the wall, which is vulnerable to erosion by melting. Scallops are common features in many caves in limestone, where the wall is vulnerable to erosion because the rock is slightly soluble in water.

Veins of clear ice are common, running through the milky-white ice making up most of the cave wall. The veins record the refreezing of meltwater in a now-vanished crack. Refreezing yields clear ice. The milkiness of the milky-white ice derives from the abundant tiny bubbles that get left over because you can never squeeze all of the air out as you turn snow into ice.

Probably the most significant item of photographic evidence is that every one of these holes preserves some evidence of its origin as a much narrower crack. Sometimes you can see the crack in the cave roof, sometimes in the floor.

So the unifying theme of this work is hydrofracturing. Suppose your glacier already has a crack in it. If the crack is big enough we call it a crevasse. Whatever its size, it tells us that the body of the glacier is under a tensile stress exceeding the fracture toughness of ice. Whether the crack grows depends on the balance of forces at its tip, the point (in a two-dimensional diagram; in the glacier it is a line) where its width becomes negligible. This is where meltwater comes in. It is a thousand times denser than air, and may well be under the pressure of still more meltwater arriving from the glacier surface. The force balance is very different when the matter pushing against the wall of the crack is water instead of air.

It is now widely accepted that hydrofracturing — crack enlargement facilitated by meltwater — is a crucial piece of several glaciological puzzles, notably the disintegration of ice shelves.

I am glad to have these studies of holes because they increase my understanding of how surface meltwater gets to the bed of the glacier, where it spells trouble, or at least complexity. But it takes all sorts to make a world. No doubt my caving glaciological colleagues, in addition to being curious in general about glacier meltwater, are also glad to have the holes just so they can crawl into them.

Posted by Graham Cogley on March 29, 2010 3:00 PM | Permalink