Dynamic Planet - Glaciers

Overview

Glaciers form where snowfall persists and compacts into ice, then flow under their own weight to sculpt landscapes through erosion and deposition. To work problems quickly, connect mass balance and equilibrium‑line altitude to climate, tie thermal regime to motion and geomorphic signature, and read landforms and stake data into consistent histories of advance and retreat.

Learning objectives

• Describe firnification from snow to ice and define the equilibrium line altitude (ELA)
• Distinguish warm‑based (temperate) versus cold‑based (polar) glaciers and their geomorphic signatures
• Explain glacier motion mechanisms (internal deformation, basal sliding, subglacial till deformation)
• Interpret budgets, calculate simple mass balance and velocity from stake data, and estimate ELA from hypsometry
• Identify erosional and depositional glacial landforms on maps and imagery

Formation, thermal regimes, and motion

Accumulation and ablation set net balance: snowfall, drift, and rime add mass while melt, sublimation, and calving remove it. Snow compacts to firn (~0.4–0.8 g/cm³) and recrystallizes into glacier ice (>~0.83 g/cm³). The ELA marks where annual balance is zero and divides accumulation from ablation areas. Thermal regime governs motion and geomorphology: temperate glaciers at the pressure‑melting point generate abundant meltwater, slide efficiently, and erode strongly; polar glaciers remain frozen to their beds, deform internally, and erode less. Internal deformation (creep) rises with ice thickness and surface slope, basal sliding accelerates over lubricated beds and obstacles (regelation), and soft‑bedded systems can deform subglacial till. Velocity peaks near the surface and center, with friction slowing margins and basal ice; crevasses open where strain exceeds strength—transverse in extending flow, longitudinal in compressing flow, chevron in shear—and bergschrunds form near headwalls.

Mass balance, hydrology, and landforms

Specific balance b = c − a integrates over the glacier to yield net change; hypsometry (area–elevation distribution) supports ELA estimates via the accumulation‑area ratio method (commonly 0.5–0.7 for valley glaciers, region dependent). Response times run from years–decades (small glaciers) to centuries (ice sheets). Drainage evolves seasonally from distributed linked cavities to efficient channelized systems (R‑channels), modulating sliding speeds and hazards such as jökulhlaups. Erosional landforms include cirques, arêtes, horns, U‑shaped valleys, hanging valleys, fjords, roche moutonnée with stoss/lee asymmetry, and striations indicating flow direction; depositional forms include moraines (lateral, medial, terminal, recessional), drumlins, eskers, kames/kettles, outwash plains, varves, and erratics. Postglacial isostatic rebound lifts crust and tilts shorelines.

Calculations and quick checks

• Stake velocity: 36 m in 90 days is 0.4 m/day ≈ 146 m/yr
• Annual mass balance: accumulation 1.2 m w.e. minus ablation 1.6 m w.e. gives b = −0.4 m w.e.
• ELA via AAR: if accumulation area is 12 km² of a 20 km² glacier, AAR = 0.6; choose the elevation where cumulative area above equals ~60%

Advanced topics (qualitative)

Surging glaciers exhibit short‑lived speedups linked to subglacial hydrology and thermal conditions, leaving looped moraines and contorted flowlines. Debris‑covered glaciers suppress melt under thick debris while enhancing it on thin debris and at ice cliffs; kettle formation follows. Tidewater glaciers cycle through calving‑dominated stability modes where geometry and climate interact.

Map and image skills

Identify landforms on shaded relief and aerial images and infer paleo‑flow directions from drumlin elongation and striations. Delineate moraines and reconstruct retreat sequences from recessional positions and varve counts. Use cross‑sections to separate U‑shaped glacial from V‑shaped fluvial valleys.

Practice prompts

• Given an area–elevation histogram and seasonal snow/wind data, estimate the ELA and justify the AAR you used
• A glacier shifts from distributed to channelized drainage in late spring; predict pressure and sliding changes
• Separate drumlins from roche moutonnée in the field based on shape and stoss/lee asymmetry

Glossary

• AAR (Accumulation Area Ratio): fraction of glacier area above the ELA
• Ablation/Accumulation: loss/gain processes for glacier mass
• Bergschrund: crevasse separating moving glacier ice from near‑stationary headwall ice
• ELA: equilibrium line altitude where net balance is zero
• Jökulhlaup: catastrophic glacial outburst flood

References

• USGS Glaciers and Climate Change (primers and monitoring methods)
• Benn & Evans, Glaciers and Glaciation (advanced reference; consult summaries)