Where Alpine Glaciers Form: The Recipe for Mountain Ice

1. Cold Temperatures: Mean annual air temperature **≤ 0°C (32°F)** at the equilibrium line altitude (ELA). Summer melt < winter accumulation.
2. Abundant Precipitation (Snow): **> 1,000 mm/yr** (often 2,000–5,000 mm) — mostly snow. Orographic lift on windward slopes maximizes deposition.
3. Topographic Traps: **Cirques (bowl-shaped hollows), valley steps, plateau basins** — where snow accumulates, compresses to firn, then ice, and exceeds the threshold to flow.

Process

1. Snow falls in cirque → survives summer → **firn** (granular, >1 yr old).
2. Burial pressure → **glacial ice** (interlocking crystals, <15% air).
3. Thickness > ~15–30 m + slope → **basal shear stress** exceeds yield strength → **creep flow** (internal deformation + basal sliding).
4. **Glacier born** — moves downslope, carving U-valleys, arêtes, horns.

Key Zones

• Accumulation zone (above ELA): Net gain.
• Ablation zone (below ELA): Net loss (melt, calving, sublimation).
• Equilibrium Line Altitude (ELA): Balance line — shifts with climate.

Global Examples

• **Alps** (Aletsch, Mer de Glace)
• **Andes** (Quelccaya, Patagonian Ice Fields)
• **Himalaya** (Khumbu, Gangotri — technically "valley glaciers" but alpine origin)
• **Rockies** (Columbia Icefield, Glacier NP)
• **NZ Southern Alps** (Tasman, Fox, Franz Josef)
• **Scandinavia** (Jostedalsbreen)

Climate Change

**ELA rising ~3–5 m/yr** globally. Alpine glaciers losing 1–2% volume/year. Many <1 km² will vanish by 2100. They're **sensitive thermometers** — and freshwater reservoirs for millions.