Glaciers & Glacial Landforms — Comprehensive Study Notes

Definition & Core Attributes of Glaciers

  • Glacier = mass of ice that persists year-to-year and moves downslope (movement can be subtle on continental sheets).

    • Requires perennial snow accumulation that exceeds seasonal melt.

    • Ice gradually compresses under its own weight; later snowfall drives further compaction.

  • Always contains embedded debris (rocks, sediment, ash) picked up or deposited on top.

  • General downslope motion; on continental sheets “downslope” = radial flow from thick interior toward thinner margins.

Snowfields & Surface Color

  • Snowfield = high-elevation or high-latitude source area where accumulation > melt; birthplace of glaciers.

  • Color palette

    • Dominant white (fresh snow & surface ice).

    • Black/gray streaks = transported debris or ash fall (e.g., Eyjafjallajökull eruption 02/2010 over south-coast Icelandic glaciers).

    • Distinctive blue ice:

    • Ice is highly compressed ➜ absorbs longer visible wavelengths (reds/oranges) and scatters short wavelengths (blues).

    • Most obvious near terminus where surface snow has been shed.

Crevasses

  • Vertical cracks produced by tensile stress during flow.

  • Factors

    • Faster motion of brittle upper ice vs. slower basal ice.

    • Tension at bends, steeper slopes, or differential velocities.

  • Occur mainly in the brittle zone (upper ~50 m).

Glaciers & Mountain Valleys

  • Alpine glaciers often occupy pre-existing stream valleys—advantages:

    • Flatter floor promotes accumulation.

    • Valley walls shade floor (especially at high latitude) ➜ reduced melt.

  • As glacier advances it erodes valley walls/floor, produces sediment that is ground finer during transport (“rock flour”).

Termination (Glacier Ends)

  • Possible settings

    • Proglacial lake / glacier lagoon (e.g., Jökulsárlón, Iceland).

    • Direct into ocean ➜ tidewater glaciers (e.g., Glacier Bay, Alaska).

    • Onto land ➜ meltwater creates braided streams & alluvial fans.

  • Calving = blocks fracture from terminus, forming icebergs.

Major Glacier Categories

  • Continental Ice Sheets – regionally contiguous, >50{,}000\;\text{km}^2.

    • Modern: Greenland interior & Antarctica.

  • Alpine (Mountain) Glaciers – restricted to high terrain; sub-types:

    • Cirque glacier – small, bowl-shaped (< valley length), birthplace for larger flows.

    • Valley glacier – long, narrow ribbon within a valley; accepts tributary glaciers; analog to river with tributaries.

    • Piedmont glacier – valley glacier exits confining valley, spreads like an ice fan (e.g., Malaspina, Alaska).

    • Tidewater glacier – ends in sea or near-sea lagoon; subject to tidal forces and calving (e.g., south-coast Iceland tidewater systems).

Glacial vs. Interglacial Periods

  • Glacial period – global cooling, glaciers advance/expand.

  • Interglacial period – warming, glaciers retreat (net melt).
    ➜ We are in an interglacial today.

  • Implications of ongoing retreat

    • Sea-level rise ➜ coastal flooding & storm-surge vulnerability.

    • Albedo feedback: less ice ➜ lower surface reflectivity ➜ higher absorption ➜ further warming ➜ more melt.

Global Distribution Highlights

  • Southern Hemisphere: Antarctica dominates; smaller alpine glaciers in Andes & South Island NZ.

  • Northern Hemisphere: Greenland ice sheet; alpine glaciers scattered in Coastal Ranges (BC/AK), Rockies, Cascades, Sierras, Alps, Pyrenees, Norway fjords, Iceland, Svalbard.

  • Equatorial glaciers exist on high peaks (e.g., Mt. Chimborazo, Ecuador) – temperature drops with elevation.

  • Elevation of the snowline declines poleward (lower‐altitude glaciers possible at high latitude).

Pleistocene North American Ice Sheets

  • Last maximum extent (~28{,}000 yr BP):

    • Laurentide Ice Sheet (east) & Cordilleran Ice Sheet (west).

    • Notably Alaska largely ice-free.

  • Driftless Area (SW Wisconsin + parts MN/IA/IL): untouched by Laurentide ice ➜ steep ridges, deeply incised rivers, absence of glacial drift.

  • Retreated by ~11{,}000 yr BP, leaving:

    • Great Lakes (deep scour basins), myriad small kettle & pothole lakes (e.g., Minnesota – “Land of 10{,}000 Lakes”).

From Snow to Glacial Ice

  • Vertical “core” sequence

    1. New snow – 90\% air, delicate flakes.

    2. Firn – granular, irregular “snow pellets”; volume ≈ 50\% air.

    3. Glacial ice (>50\,\text{m} burial):

    • Air squeezed to isolated bubbles.

    • Crystal lattice deformed ➜ ice becomes ductile (flows).

  • Ice cores trap paleo-air ➜ window into ancient atmospheres & climates.

Glacier Mass Balance Concepts

  • Zone of accumulation – input > losses (upper glacier).

  • Equilibrium line altitude (ELA) – boundary where net balance = 0.

  • Zone of ablation – melt, sublimation, calving exceed input (below ELA); surface often shows blue ice.

  • Glacier begins to flow once thickness ~50\,\text{m}.

Glacier Motion Mechanics

  • Differential velocity

    • Upper brittle zone faster; basal ice slower owing to friction with bedrock.

  • Basal sliding facilitated by pressure melting (thin film of water at ice–rock interface).

  • Internal deformation: ice crystals shear & realign under stress.

  • Crevasse formation tied to differential flow & surface brittleness.

Calving & Icebergs

  • When terminus meets deep water, buoyant ice toes fracture.

  • Produces icebergs (Titanic’s nemesis 1912); observed in

    • Jökulsárlón (Iceland) – bergs drift to “Diamond Beach”.

    • Ilulissat, Greenland – large calving events.

  • Air/sea travel: summer flights over S Greenland reveal iceberg fields.

Glacial Erosion Processes

  • Plucking – glacier freezes onto weathered blocks; pulls them away during flow.

    • Enabled by pressure melting–freeze cycle at base.

  • Abrasion – embedded debris grinds bedrock like sandpaper; produces:

    • Striations: linear scratches indicating flow direction.

  • Subglacial meltwater channels – pressurized streams under ice erode tunnels & move sediment.

Erosional Landforms

  • Roche moutonnée ("sheepback rock")

    • Asymmetric bedrock knob: gentle, polished stoss (up-ice) side; steep, plucked lee side.

    • Commonly striated on stoss.

  • Glacial erratic – isolated boulder dropped after ice melts; often lithologically different from local bedrock ("out-of-place").

Transport Pathways & Rock Flour

  • Englacial – debris encased within ice mass.

  • Supraglacial – debris riding on ice surface (e.g., volcanic ash, valley-wall rockfall).

  • Subglacial – debris dragged at ice–bed interface; key for abrasion.

  • Rock flour – ultra-fine silt/powder generated by grinding; gives proglacial streams/lakes milky blue-gray color.

Depositional Products

  • Glacial drift (umbrella term): any sediment of glacial origin.

    1. Till – unsorted, unstratified mix of all sizes; deposited directly from melting ice (ablation till, lodgment till on bed, etc.).

    2. Outwash ("glaciofluvial drift") – sorted & stratified by braided meltwater streams; forms outwash plains, sand & gravel bars.

    • Heavy clasts settle near ice margin; finer material carried farther.

Environmental & Societal Connections

  • Rising sea level from ice-sheet melt threatens low-lying coasts & magnifies storm-surge reach.

  • Ice-albedo feedback accelerates polar warming.

  • Ice-core climate archives underpin modern climate science & policy.

Cross-Topic Connections & Study Tips

  • Compare glacier drainage patterns to river systems (tributaries, braided channels, fans).

  • Link abrasion vs. plucking to stream hydraulic action vs. solution/abrasion.

  • Memorize alpine glacier sub-types via visual cues (cirque = "circle", valley = "river-like", piedmont = "fan", tidewater = "meets tide").

  • Practice drawing diagrams: roche moutonnée profile, mass-balance zones, till vs. outwash cross-sections.

  • Remember key thresholds: 50\,\text{m} thickness for flow, 90\% \to 50\% (\text{air} \to \text{firn}).