How does glacier form?

Glacier

The snow that has accumulated on the ground (snow) gradually becomes thicker and becomes ice, and it is called a glacier when it becomes fluid due to gravity.

How does glacier form?

Let us see how does glacier form. The density of fresh snow (fresh snow) is about 0.05-0.15g/cm ³ and the space between the snow crystals is filled with air. However, when the snow is thick, the gap from the top is crushed (consolidation), and the air escapes to become hard (snow). Also, if the snow melts during the day and freezes again at night, coarse-grained rough snow will form. In this way, the snow cover gradually increases in density, and when the density reaches 0.5 g/cm ³ or more, it is called firn.  When the firn is further consolidated and the density becomes about 0.83 g/cm ³ or more, the gap between the crystals is disconnected and the air trapped as bubbles. This is glacier ice or glacial ice. The change from fresh snow to glacier ice is called metamorphosis, and it progresses relatively quickly when snow accompanies melting (warm metamorphosis), but when it does not melt due to low temperatures such as Antarctica (cold metamorphosis), the transformation slowly progresses over hundreds of years.

Glacier mass balance

A glacier is divided into a recharge area and consumption area. The recharge area is, in which the amount of snow and ice (recharge) that accumulates on, it is greater than the amount of snow and ice that melts (amount of consumption), and a consumption area in which the amount of recharge is less than the amount of consumption. The higher the temperature, the greater the amount of consumption, so the upstream side of the glacier is the recharge area and the downstream side is the consumption area. At the boundary between the two, the amount of recharge and the amount of consumption are equal. This line is called the balance line (balanced line). Because the amount of snow in the recharge area exceeds the amount of snowmelt, the surface of the glacier is always covered with fresh snow or snow that fell in the previous year, that is firn, and glacier ice appears from a certain depth. On the other hand, in the consumable area, the amount of snowmelt exceeds the amount of snow cover, so glacier ice is always exposed except after snowfall. Therefore, in a glacier with snow in winter, the snow melts from the downstream side in summer, and at the end of summer, the only firn remains in the recharge area. The line connecting the lower limits of the firn is called the filling line, and approximately indicates the position of the balance line. The balance between recharge and consumption of glaciers is called the mass balance of glaciers. For glaciers such as Antarctica, where the temperature is low and the entire area is a rechargeable area, the glaciers drain into the sea and become icebergs (iceberg separation, carving calving), which consumes the glaciers. , Keeps its mass balance.

Glacier flow

Since the amount of snow in the recharge area exceeds the amount of melting, the glacier will become thicker and thicker, while the amount of melting will be larger in the consumable area, the glacier should become thinner. However, in stable glaciers, the shape and thickness of glaciers hardly change year by year. This is because the amount accumulated in the recharge area compensates for the amount melted in the consumption area, which is the flow of glaciers. However, the question of why ice (solid) glaciers flow has long been a problem for physicists. J. Forbes, who studied alpine glaciers in the 19th century, published a viscous theory that glaciers flow like candy, while J. Tyndall discovered glacier flow by Faraday. Explained by the theory of recovery ice. At present, it is believed that plastic deformation of ice crystals and the bottom slip of glaciers cause flow. Since the ice crystal is near the melting point, it is plastically deformed when the metal that is heated strongly receives a force to easily deform it. The ice crystals have a structure just like stacked playing cards, so if gravity is applied from above, the crystals will shift and deform like the collapsing cards.

The study of plastic deformation of ice has helped to develop rheology. The flow due to plastic deformation plays a major role in the flow of glaciers in Antarctica, such as glaciers in which ice is always frozen on the bedrock of the bottom (cold glaciers). On the other hand, in glaciers that melt due to the pressure of ice (pressure melting) and there is a thin water film on the bottom of the glacier (warming glacier), the glacier slides on this water film. Bottom slip is the main cause of flow. Cold glaciers that flow only by plastic deformation have a slow flow rate of only a few meters to tens of meters per year, but in warm glaciers where bottom slip is actively occurring, tens to hundreds of meters per year. In the valley glacier, the flow velocity is larger in the central part like the river and becomes smaller due to friction near the valley wall. When the ice is stretched or crushed due to such a difference in flow velocity and a sudden change in the slope of the bottom surface, crevices are formed in the glacier. When a glacier passes through a particularly steep slope, many crevasses break the ice glacier into small pieces of ice called shellac seracs. These places look like glacier waterfalls and are called icefalls. Downstream of the icefall, a stripe pattern with a convex surface on the downstream side may be seen on the surface of the glacier. These are called ogives, which consist of blue ice without bubbles and white ice with bubbles. Blue ice is glacier ice that passed through the icefall in the summer, and it repeatedly melts and freezes on the surface, becoming blue due to the inclusion of fine dust, and the white ice remains frozen in winter. It corresponds to the ice that passed through the fall. Therefore, the interval of the ogives represents the one-year drift distance of the glacier.

Climate change and glaciers

Glaciers expand and contract as the climate changes. If the temperature drops or the amount of snowfall increases, the amount of recharge increases and the amount of consumption decreases, so the position of the equilibrium line (snow line) decreases and the glacier advances (expands). Conversely, glaciers recede (shrink) if there is a rise in temperature or a decrease in snowfall. The glacial period is when the glaciers moved forward and expanded due to the cold climate, creating a large ice sheet in the northern hemisphere.