Glacial till has a lower albedo than the surrounding white snow and thus absorbs more solar radiation. Debris on the ice can both insulate and melt. A thick blanket of sand and rock will prevent any sunlight from contacting the ice, keeping the glacier surface in the dark and frozen. On the other hand, any exposed snow next to a rock face or surrounding a small pebble will melt from the warmth given off by the absorbing body. A large rock face, for example, will melt the nearby snow creating a large and undercut moat. The transition from snow to a rock outcrop may not look hazardous but moats have a reputation for swallowing climbers just like crevasses.

The solar radiation absorbed by a small pebble or grain of sand will have a much different effect. A small pebble will melt the snow around its perimeter and this water will eventually wash it and other fine dust particles, or cryoconite, into small clots. These larger aggregations of rock debris absorb more solar energy and gradually bore holes into the ice known as cryoconite holes or dust wells that can reach bathtub proportions.

If you were to kick these dirt ridges with your boot you would find that you could not scatter them like piles of dirt. Rather, your toes would come in contact with a much more painful debris covered cone of ice. A dirt cone, as they are called, is actually an inverted crevasse, puddle or moulin. These ones are from an old crevasse—notice their linear alignment. They form when debris accumulates in a dust well or cryoconite hole in a zone of high ablation. Gradually, as more and more material accumulates and the surrounding ice melts away, the two to three centimeter thick coating of dirt insulates the ice underneath. The top of the cone marks the bottom of the original depression. Dirt cones demonstrate the astounding ability of debris to both insulate and melt ice.

Glacier tables are formed when large chunks of rock become perched on the ice surface. As it slowly rides down the valley with the ice, the rock protects the snow beneath it from solar radiation. Gradually the surrounding snow melts away leaving the rock perched on a pillar of snow. In the northern latitudes the tables tilt towards the south where ablation is the highest. Eventually the table will slide off its base and begin the process again.

Some rock bits are ground so extremely fine that they can remain suspended in water. These particles are nicknamed glacial milk as it colors the water a milky gray. The Hoh River that transports the Blue Glacier’s melt water is chalky gray from all the ground rock.



intro | terminology | accumulation | firn | blue | ablation | water | equilibrium | massbalance | movement | crevasse | structure | algae | moraine | debris | erosion
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Benjamin Drummond 2002