Glaciers

This week’s guess the planet image shows a glacier on the island of Svalbard in the terrestrial arctic. The image I posted on Monday shows crevasses in the surface of the glacier. The larger image below, which is also from Google Earth, shows the “snout” of the glaciers, the end where they breaks up as it encounters the sea. 

Glaciers are very interesting features, because they are a great example of how seemingly solid substances can flow. They form in locations where ice accumulates from year to year, rather than melting away in the warmer months. In general this occurs on shadowed, pole facing slopes in mountainous areas, which remain very cold throughout the year. More and more snow accumulates in such areas and is gradually compacted into ice under its own weight. 

Once a large enough mass of ice forms it will begin to flow, as the weight of accumulating ice forces material down slope. Once a glacier gets moving it will flow through several mechanisms. You can see in the image above that this glacier has numerous fractures in its surface. These are perpendicular to the direction of flow, and indicate that the upper layer of the glacier is brittle. However deeper regions of the ice do not behave in the same way. Once a glacier reaches 30-50 meters deep it begins to deform in a “plastic” manner. This means that it behaves more like a Newtonian fluid.

However the glacier doesn’t stay solid all the way through. A massive slab of ice moving across the uneven surface of the ground inevitably causes friction. Glaciers have a vast amount of erosive potential and can scour out huge valleys as the move down hill. This means that there is often water below the ice, which has thawed as a result of the energy the glacier releases. This lubricates the lower surface of the glacier and lets it slide over the surface more efficiently. One of the main ways glaciers are classified is as “warm based” and “cold based”, depending on whether they have liquid water helping them along. Massive volumes of rock and soil can be eroded by the glacier and displaced to the sides of its path, or carried to its snout where they are deposited as “Moraines”, ridges of clay and boulders mixed up in their journey down the glacier. 

The depositional zone is defined by the overall balance of accumulation and melting across the glacier as a whole. The zone of accumulation is generally in high areas where a large volume of snow and ice is deposited. The zone of ablation at the snout of the glacier is where it is actively melting. In this image you can see that the end of the glaciers is calving into icebergs as they reach the sea. The accumulation zone is off the edge of the image, towards the interior of Svalbard. 

The line between the zones of accumulation and ablation is very dependent on climate. If it is near the snout then there will be lots of accumulation and the glacier will continue to grow and advance. The warmer the climate becomes the further this divide moves back towards the source. The glacier begins to retreat, exposing the u-shaped valley it has spent at the very least thousands of years carving. It leaves behind moraines and deposits of boulder clay, as well as “eratics” huge blocks of rock carried far from the sites where they were eroded. 

At present our warming climate means that the world’s glaciers are retreating at an unprecedented rate. We need to take action in order to check climate change and ensure that these magnificent landforms don’t vanish altogether. This isn’t just an academic concern. The seasonal melting that takes place at the snout of an upland glacier produces a vast amount of fresh water. In upland areas they are an important source of drinking water, relied upon by thousands of communities worldwide. 

Image Credits: Google Earth