Courant glaciaire

An ice stream is a region of fast-moving ice within an ice sheet. It is a type of glacier, a body of ice that moves under its own weight. They can move upwards of a year, and can be up to in width, and hundreds of kilometers in length. They tend to be about deep at the thickest, and constitute the majority of the ice that leaves the sheet. In Antarctica, the ice streams account for approximately 90% of the sheet's mass loss per year, and approximately 50% of the mass loss in Greenland. The shear forces cause deformation and recrystallization that drive the movement, this movement then causes topographic lows and valleys to form after all of the material in the ice sheet has been discharged. Sediment also plays an important role in flow velocity, the softer and more easily deformed the sediment present, the easier it is for flow velocity to be higher. Most ice streams contain a layer of water at the bottom, which lubricates flow and acts to increase speed. Ice streams are typically found in areas of low topography, surrounded by slower moving, higher topography ice sheets. The low topography arises as a result of various factors, the most prominent being that water accumulates at topographic lows. As water accumulates, its presence increases basal sliding and therefore velocity, which causes an increase in sheet discharge. Another factor causing ice streams to be found in low regions is that thicker ice results in faster velocity. As the thicker an ice stream is, the greater the driving stress at the bed, and thus the greater the velocity. In addition to driving stress, ice streams have better insulation as the thickness of ice increases, due to it retaining higher temperatures better, it can increase the rate of deformation, as well as basal sliding. As a substance's volume increases, it requires more energy per unit volume to raise its temperature, one of the reasons why it is so difficult for oceans to freeze or evaporate, water is also a poor conductor of heat, so increased thickness will not only increase the amount of heat that can be retained, but also make more energy required for heat to be lost.