There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to debunk a few of those misconceptions, specifically dealing with how glaciers work. There will be others. Find them here.
Misconception: Glacial ice moves backwards during glacial ‘retreats’
Misconception: Glacial ice is stationary during times when front is neither advancing or retreating.
Misconception: Glacial retreat and advance is just motion of whole ice cap back and forth (sort of like a bad toupee) with no significant change in total ice volume
Misconception: Glaciers are only moving ice masses
Misconception: Use of ‘bulldozer’ analogy for glacial movement and sediment transport
Each of these has to do with how glaciers move, and how they transport materials. It’s easy to understand where these misconceptions come from. It’s more difficult to straighten them out.
The first three deal with how glaciers move. We hear a lot these days about glacial advance and retreat. We’re particularly concerned about retreat, because this means that the glacier is melting. We also have an intuitive sense that when a glacier is ‘advancing’ that means that the end (technically called the toe) of it is moving further downhill, away from where it started. A retreat then is the opposite of an advance, meaning that the toe must be going uphill.
This is where the confusion arises. We know that glaciers usually flow downhill, just like rivers. But in a retreat, the toe is going up hill, so does that mean that the whole glacier is flowing up hill?
The answer is no.
Glaciers always flow down hill. They cannot flow up hill. But the toe does advance and retreat. This happens due to the balance between accumulation in the glacier (the addition of snow and ice) and ablation (the melting or evaporation of snow and ice). If there’s more accumulation than ablation, the glacier grows and advances. If there’s more melting than accumulation, then the glacier shrinks and retreats.
A glacier can be divided into two parts, the zone of accumulation (where there’s more accumulation than melting) and the zone of ablation (where there’s more melting than accumulation). Bear in mind that melting and accumulation both occur over the entire extent of the glacier, there’s just places where one is greater than the other.
Between the zones of accumulation and ablation is a line of equilibrium, where accumulation and ablation balance each other perfectly.
If climates warm, then melting will increase along the entire length of the glacier, causing the line of equilibrium to move uphill and the glacier to retreat. But the glacier is still moving down hill. It’s just melting more at the toe, causing the toe to shift uphill. A decrease in snow accumulation will also have the same effect, because the reduction in accumulation will effectively make melting more dominant.
If the amount of snowfall in an area increases, this will result in an advance of a glacier, because there will be an overall increase in accumulation over melting. Cooler temperatures can also result in the advance of glaciers, because the rate of melting will be reduced. In either case, the equilibrium line moves downhill and the glacier gets larger.
Glaciers are not simply moving masses of ice, however. Just like rivers, they are powerful agents of erosion, capable of picking up boulders and carrying them for thousands of miles.
As a glacier moves, it tears rocks from the surface over which it flows. These rocks are entrained in the ice and travel as far as the ice goes, all the way to the toe of the glacier. As the toe melts, the rocks and other debris are dumped into piles called moraines.
If the glacier does not advance or retreat for a while, the moraines can get very large.
Moraines can also mark the places where a glacier paused for a while during retreat. It’s through identifying and mapping these moraines that we can know things like the total extent of the ice sheets during the last glacial event that ended some 10,000 years ago.
In both photographs, the glacier is still flowing down hill and carrying debris as always. But, because there is now an increase in melting over accumulation, the toe is melting back uphill, and the glacier does not make it as far as it used to.