1. Stress and Strain:
* Upper Zone: The upper portion of a glacier is brittle and subject to tensile stress caused by the glacier's movement over uneven terrain. As the glacier flows over bumps and depressions, the upper layers are stretched and pulled apart, creating cracks called crevasses.
* Lower Zone: Below a depth of 50 meters, the pressure from the weight of the overlying ice becomes significant. This pressure acts as a compressive force, squeezing the ice together. This compressive force makes the ice more ductile, and it deforms rather than fractures.
2. Ice Flow Behavior:
* Upper Zone: The upper portion of a glacier behaves more rigidly due to lower pressure and temperatures. The ice is less likely to deform under stress, leading to fracturing.
* Lower Zone: As the ice moves deeper, it experiences higher pressure and temperatures. This causes the ice to behave more like a viscous fluid, allowing it to flow and deform under stress without fracturing.
3. Melting and Regelation:
* Upper Zone: Crevasses exposed to air and sunlight are subject to melting, which can further widen them.
* Lower Zone: The pressure at depth inhibits melting. Additionally, any meltwater that does form is often refrozen due to the pressure and cold temperatures. This process of melting and refreezing (regelation) helps to close up any cracks that may form.
4. Strain Rate:
* Upper Zone: The strain rate (how quickly the ice is deformed) is higher in the upper portion of the glacier, especially near the surface. This high strain rate contributes to the formation of cracks.
* Lower Zone: The strain rate decreases with depth, allowing the ice to deform more slowly and avoid fracturing.
In summary:
The combination of higher tensile stress, brittle behavior, melting, and a higher strain rate in the upper portion of the glacier leads to the formation of crevasses. The increased pressure, ductile behavior, pressure-induced refreezing, and lower strain rate at depths below 50 meters prevent the formation of crevasses.
