Glaciers move slowly due to their viscosity, which makes them resemble a thick liquid. Despite their enormous weight, the ice flows gently because of the pressure, but it resists movement due to its crystalline structure.
Ice, despite its very solid appearance, has a particular crystalline structure that facilitates its movement. In reality, it is made up of many crystals, grains of ice glued together. When a glacier moves, it is partly because these small crystals slowly slide against each other. This is called internal deformation. The larger and better aligned these crystals are, the less easily they slide, and thus, the slower the glacier moves. Conversely, ice with smaller or more disordered crystals slides better. This phenomenon makes the glacier somewhat like a very thick dough: it seems solid, but over long periods, it gradually deforms under its own weight.
Under a glacier, the ice comes into direct contact with the rock below. This glacier-rock contact causes significant friction: it's a bit like dragging a very heavy piece of furniture across a rough floor, it rubs and slows down. The rougher this contact is, the harder it will be for the glacier to move quickly, despite the immense power of its weight. If the rock is smooth or covered in meltwater, a thin film forms between the ice and the rock, reducing this friction. Conversely, if the ground is full of bumps or irregular hollows, the ice grips more and significantly slows down its movement. This friction seriously limits glacial movement, largely explaining why a glacier, despite its enormous mass, advances at such a slow pace.
Under their immense weight, glaciers undergo significant internal mechanical stresses. In simple terms, their own mass creates forces that compress them from within and stretch them at the surface. These stresses cause the ice to respond with a slow deformation: it flows very slowly, somewhat like a very viscous dough that deforms under pressure. This phenomenon of internal creep is possible thanks to the ice crystals that slide and gradually change position, driven by the accumulation of internal stresses. As a result, even if it is not visible at first glance, the glacier is constantly moving, slowly but surely, propelled by these subtle interplay of force and pressure within its thick mass of ice.
The warmer the ice is (well, let’s say less cold), the more malleable it becomes and can easily deform. In contrast, when the temperature is very cold, the ice becomes rigid and brittle. Near the surface, the ice is often very cold and brittle, while deeper down, it is warmed by pressure and natural heat from the ground: as a result, it deforms slowly like very hard modeling clay. This temperature difference changes the overall speed of glacier movement: when they are warmer at the base, they slide and deform better, gradually accelerating their movement.
When a glacier descends a slope, it often encounters natural obstacles: rocky blocks, steep cliffs, or deep crevasses. Facing this terrain, the glacier slows down, deforms, or slowly bypasses these barriers. Sometimes, the obstacles even lead to crevasses: the ice stretches, cracks, and separates, like when you gently bend a piece of chocolate that eventually breaks. The rugged terrain also forces the ice to change its trajectory, which further slows its movement. Ultimately, each bump or hole acts like a speed bump, gently (but surely!) slowing the immense mass of ice in its descent toward the valley.
Under very high pressures, such as those encountered beneath glaciers, ice does not necessarily melt immediately. On the contrary, it can deform plastically, somewhat like an extremely viscous fluid. This largely explains why glaciers flow slowly, rather than sliding quickly over their substrate.
In some cold and dry regions like Antarctica, certain glaciers can reach an internal temperature close to -40°C, making them extremely rigid and significantly slowing their movement, sometimes to such low speeds that they are almost imperceptible on a human scale.
Despite their slowness, glaciers have such abrasive power that they can carve immense valleys and steep rock faces over thousands of years. The famous U-shaped valley, typical of mountain landscapes, is directly shaped by these masses of ice that move slowly yet steadily.
The average speed of glaciers is often measured using GPS and very precise satellite images. Today, these modern techniques allow us to track their movements with an accuracy of up to one centimeter!
Crevasses occur due to the mechanical stresses generated by the slow movement of the glacier. When the ice flows over a steeper slope or encounters an obstacle, it fractures under tension, thus forming the visible crevasses.
Sure, here's the translation: "Yes, that is quite possible. A glacier always advances under the effect of gravity, but it can retreat if the melting of its front face is faster than its natural advance. This is referred to as glacial retreat related to the net loss of ice at its front."
These sounds generally result from internal movements or fractures in the ice. When mechanical stresses accumulate, they can generate cracks, thereby creating audible creaking or rumbling noises that can sometimes be quite impressive.
Yes, a glacier can accelerate its movement if meltwater lubricates its base or when temperatures rise. This reduces friction with the bedrock, thus facilitating faster movement.
The speed of a glacier generally ranges from a few centimeters to several meters per day, depending on the slope, temperature, and conditions of the underlying rock. In general, glaciers move very slowly, barely noticeable to direct human observation.
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