And now, for something completely different.
When speaking of glaciers, ice fields, and mountain snow packs, penitentes or nieves penitentes are spikes of ice, looking somewhat like jagged stalagmites. They are very cool, but they are a little complicated. Bear with me.
First of all, I lied. This isn't really something completely different. Nieves penitentes literally translates as 'penitent snow'. This comes from the hunched, bowed form of the snow hillocks, which look like white-robed penitents huddled in the snow. This image of snow hillocks as hunched penitents is also referred to in the German term for these formations, Büsserschnee. Probably both the German and the Spanish terms have been around for centuries.
Penitentes are formed in various ways. The traditional 'hunched snow' forms are probably mostly a simple matter of wind and water erosion. They form particularly in areas with a strong wind blowing consistently from a single direction, and regular melting and freezing cycles are also common in penitentited areas. These forces form hillocks of snow that are curved ('hunched') on the windward side and drop off sharply in the leeward side. These sorts of penitentes are often found in Europe.
But, there is a second form of penitent formation, which is both more interesting and often more impressive. Found particularly in the mountains of South America, these penitentes grow in sharp spikes, often covering large snowfields. They can grow to be 4-5 meters tall, and their sharp, jagged forms show little evidence of directional erosion. They are formed by a process known as sublimation.
Sublimation is just like evaporation, except that it involves no liquid form. The solid ice never reaches melting temperature, but the heat energy from the sun and the circulation of air plucks up ice molecules directly from the surface of the icepack. At first the sublimation is random across the surface of the ice, but as some areas sublimate more quickly than others the surface becomes more uneven. Eventually pockets develop that concentrate reflected light, making the sublimation go faster and digging the pockets deeper faster. In the lab penitentes have been grown to the height of up to two inches in a matter of hours. Over decades of sublimation natural penitentes grow into truly amazing formations.
There is some speculation that penitentes may protect ice fields from the effects of global warming. Sunlight is spread over a greater surface area, and this may buffer the melting of snowpacks. Increased energy from the sun may even speed the growth of penitentes, making a nice negative feedback loop. A light layer of soot has also been shown to speed penitent growth, at least in the lab. An increase in ambient temperature could still do a lot of damage to a field of penitentes, however.
Pictures:
http://pixdaus.com/single.php?id=55098
http://www.summitpost.org/view_object.php?object_id=180982&confirm_post=5&page=1
References:
http://www.eurekalert.org/pub_releases/2007-03/uoca-mli030507.php
http://focus.aps.org/story/v17/st7