“The terminus of the glacier is an instructive place. Ceaselessly changing, and yet always the same, like the seashore. Ice streams becoming rivers, mountains wearing down into valleys. The transition zone between two worlds.”
Thomas Wharton, Icefields
At the terminus or toe of a glacier, ice crumbles. This zone marks the end of the life cycle for ice within any glacier, regardless of the glacier’s overall health. Death by natural causes, if you will. From the sheer wall of ice, elaborate sculptures emerge and fade as massive blocks of ice detach and fall in a process known as calving. (For spectacular footage of calving glaciers, nothing beats James Balog’s documentary – Chasing Ice.)
A healthy glacier is in a constant state of balance or dynamic equilibrium. Dynamic equilibrium is a simple and elegant idea that deserves at least a paragraph of its own. Let’s for a moment forget the ‘dynamic’ and start with good old-fashioned equilibrium. Imagine your favorite nephew has just knocked over a can of red paint into your bathtub. At first, the paint is localized to the area of the spill. Gradually, it begins to spread as it mixes with the water. In the process, the spill begins to fill a progressively larger space while the color becomes less saturated with time. What started as a bathtub of clear water and a can of bright red paint eventually becomes a faded red mixture. Once the water and paint have fully mixed, the system has reached equilibrium and won’t change any more with time.
Imagine now a different experiment, where we ignore the paint altogether and just look at the water level. First, we open the plug at the bottom of the bathtub and let the water slowly drain. At the same time, we continue adding water, so that the water level in the bathtub stays constant. Here, we’ve reached dynamic equilibrium. A single water molecule will eventually find its way down the drain and out of the system, but the bathtub as a whole remains in balance. Dynamic equilibrium can be thought of as a ‘balance of changes’, illustrated by the cartoon of the man on the escalator below.
A glacier functions in much the same way. Knowing a little bit about the local temperature allows us to draw a roughly horizontal line (as viewed from above) across the glacier marking the equilibrium line altitude (ELA). This line separates the zone of accumulation above from the ablation or melting zone below. Above the ELA, average temperatures remain below freezing year round, allowing snow and ice to accumulate. Below, the temperature spends at least part of the year above freezing, and part of the ice melts.
The equilibrium line altitude changes with climate; as the global climate has warmed in the past century, the ELAs of glaciers in many regions have moved higher, leaving more of the ice in the melting zone. When in equilibrium, a glacier will accumulate as much snow at its head as it loses at its toe over a year. If accumulation and melting are not equal, the glacier is said to be out of balance. If accumulation is greater, the glacier will advance. If melting is greater, the glacier will retreat.
Just like a river, ice flows downhill over time. If you parked your car on a glacier and left it for a few years (assuming the glacier was nice enough not to swallow it), you would find it downstream from where you left it. How far downstream it ends up will depend on the speed of ice flow within that particular glacier. The further ice moves below the ELA, the warmer the average temperature and the more melting occurs Eventually, at the terminus, the ice is weak enough that it falls apart into a jumble of blocks and fragments. As it does, it leaves behind moraines – ridges of rock and debris, formed from material caught and carried by the ice. A retreating glacier doesn’t move backwards; rather, the terminus creeps uphill over time, more and more of the ice melting in the process.
The divide between the accumulation and ablation zones can be clearly seen even on a photo based on the color of ice. While accumulating ice is clean, bright, and white, melting ice looks dark and dirty. You might notice the same effect looking at a pile of snow outside your window, which will gradually become more dirty as it melts away. (As a Canadian, I assume that everyone has piles of snow outside their window.)
And as always, be warned: you may have just sustained a lethal dose of mostly harmless science.
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Cover photo courtesy of wanderflechten, Flickr Creative Commons. Dynamic equilibrium cartoon from http://declanfleming.com/man-vs-escalator-equilibrium-model/. All other images from Wikimedia Commons.