Snow to Firn: Fresh snow has a low density and high air content; over time, the weight of overlying layers compresses the snow into firn (or névé), which is intermediate between snow and ice.
Glacial Ice Development: Continued pressure squeezes out remaining air bubbles, increasing density until the material becomes impermeable to water, forming crystalline glacial ice.
Optical Properties: Glacial ice often appears blue because the dense crystal structure absorbs long-wave red light while scattering short-wave blue light.
Temporal Variations: The transformation process is highly variable, taking as little as 30-100 years in temperate regions with high precipitation, but up to several millennia in arid polar regions like Antarctica.
Pressure Melting Point (PMP): This is the temperature at which ice begins to melt under a specific pressure; as pressure increases with ice depth, the melting point temperature actually decreases below .
Warm-based (Temperate) Glaciers: These glaciers exist in regions where temperatures allow the ice to reach the PMP throughout its depth, resulting in significant basal meltwater that facilitates rapid movement.
Cold-based (Polar) Glaciers: Found in extreme polar environments, these glaciers remain below the PMP even at their base, causing them to be frozen to the bedrock and move primarily through internal deformation.
Polythermal Glaciers: Some large ice masses exhibit characteristics of both regimes, with cold-based margins and warm-based interiors, creating complex flow patterns.
Internal Deformation: This occurs when individual ice crystals slide over one another or change shape under the stress of gravity, allowing the glacier to flow like a highly viscous fluid.
Basal Sliding: In warm-based glaciers, meltwater at the base acts as a lubricant, reducing friction and allowing the entire ice mass to slide over the bedrock at much higher velocities.
Shear Stress and Gravity: The rate of movement is determined by the weight of the ice and the slope angle, which together increase the shear stress acting on the glacier's base.
Velocity Profiles: Glaciers typically move fastest at the surface and in the center, where friction from the valley floor and sides is minimized.
| Feature | Warm-based Glacier | Cold-based Glacier |
|---|---|---|
| Location | Temperate latitudes (e.g., Alps) | Polar latitudes (e.g., Antarctica) |
| Basal Temperature | At or above PMP | Well below PMP |
| Primary Movement | Basal sliding + Internal deformation | Internal deformation only |
| Erosional Power | High (due to sliding and meltwater) | Low (ice is frozen to bedrock) |
| Velocity | Rapid (meters per day) | Slow (meters per year) |
Distinguish Retreat from Flow: Always remember that even if a glacier is 'retreating' (the snout is moving up-valley), the ice within the glacier is still flowing downslope due to gravity.
Analyze ELA Shifts: If an exam question asks about climate change, link rising temperatures to a higher ELA and a negative mass balance, leading to glacial retreat.
PMP Logic: Be prepared to explain why ice melts at at high pressure; emphasize that pressure lowers the melting point, which is counter-intuitive to many students.
Feedback Loops: Identify positive feedback loops, such as melting ice exposing dark rock (lowering albedo), which leads to further heat absorption and accelerated melting.
