FEW CM TO M/DAY
PLUCKING (LEE)
SORTED OUTWASH
Glaciers & Glacial Processes
Continental vs. Valley Glaciers · Gravity Flow & Regelation · Plucking & Abrasion · Unsorted Till vs. Outwash (28 Micro-Realms)
The Solid Cryospheric Bulldozer (NCERT Chapter 7)
Masses of ice moving as sheets over the land (Continental Glaciers or Piedmont Glaciers) or as linear flows down the slopes of mountains in broad trough-like valleys (Mountain and Valley Glaciers) are called Glaciers. Moving slowly under gravity at a few centimetres to a few metres a day, glaciers cause tremendous erosion because of friction caused by the sheer weight of the ice—operating via Plucking and Abrasion, and depositing unassorted Glacial Till alongside water-sorted Outwash.
I. Cryospheric Genesis, Glacier Types & Velocity (NCERT Core)
What is a Glacier? (NCERT)
(PLAINS)
(STREAMS)
- Masses of ice moving as sheets over land
- Or as linear flows down mountain slopes
- In broad trough-like valleys (NCERT)
Snow → Firn → Glacial Ice
- Fluffy snow compacts under its own weight
- Air squeezed out $\to$ granular Névé/Firn
- Recrystallizes into blue Glacial Ice
Continental & Piedmont
- Continental: Vast ice sheets (Antarctica/Greenland)
- Piedmont Glacier: Valley glaciers spread
- Over plains at foot of mountains (Malaspina)
Mountain & Valley Glaciers
- Confined within high alpine rock walls
- Fed by cirque snowfields at head
- Himalayan, Alpine & Alaskan glaciers
Snowline & ELA Balance
- Above ELA: Snow gain > melt (Accumulation)
- Below ELA: Melt/calving > gain (Ablation)
- Snout advances or retreats by mass balance
Gravity Driving Force
- Unlike water, ice movement is not rapid
- Moves mainly because of the
- Force of gravity (NCERT verbatim)
Rate of Movement (NCERT)
A FEW METRES A DAY
- Movement of glaciers is slow
- A few cm to a few metres a day
- Or even less/more in surges (NCERT)
II. Glacial Movement Mechanics: Plastic Flow, Regelation & Crevasses
Brittle vs. Ductile Zone
- Upper 50 m: Rigid & brittle (cracks)
- Below 50 m: High overburden pressure
- Ice deforms plastically (Glen's Flow Law)
Basal Slip & Regelation
- High pressure on stoss side of bump
- Lowers melting point $\to$ ice melts
- Refreezes on low-pressure lee (Regelation)
Warm vs. Cold-Based Ice
WET SLIP
FROZEN BED
- Warm-based (Himalayas/Alps): Meltwater slip
- High basal erosion & plucking
- Cold-based (Antarctica): Frozen to bedrock
Differential Flow Profile
- Valley walls & floor exert friction drag
- Ice moves fastest at top-center axis
- Bends straight marker stakes into parabolas
Crevasses (Tension Cracks)
- Gaping fissures in upper 50 m brittle zone
- Transverse (convex steps) & Chevron (margins)
- Radial crevasses at spreading piedmont lobes
Bergschrund & Randkluft
- Bergschrund: Deep crevasse at cirque head
- Where moving ice pulls away from stagnant ice
- Randkluft: Gap between ice and warm rock wall
Icefalls, Seracs & Surges
- Icefall: Glacier cascades over steep cliff
- Seracs: Intersecting crevasse ice towers
- Khumbu Icefall (Everest) & Karakoram surges
III. Glacial Erosion Mechanics: Plucking & Abrasion (NCERT Core)
Sheer Weight Friction Power
- Erosion by glaciers is tremendous
- Because of friction caused by
- Sheer weight of the ice (NCERT)
1. Plucking (Quarrying)
- Meltwater freezes into bedrock joints
- Moving glacier plucks & tears out
- Large angular blocks from valley floor
2. Abrasion (Scraping)
- Plucked material dragged along floor
- Acts as coarse sandpaper under heavy ice
- Scrapes, polishes & scours bedrock (NCERT)
Striations & Grooves
- Parallel scratches carved by sharp clasts
- Record exact paleoflow direction
- Talchir (Odisha) Gondwana striations!
Rock Flour & Glacial Milk
- Intense basal abrasion grinds rock
- Into micron-sized silt (Rock Flour)
- Gives meltwater streams milky turquoise hue
Roche Moutonnée ("Sheepback")
- Asymmetric bedrock mound in glacial trough
- Stoss (up-ice) side: Smooth & abraded
- Lee (down-ice) side: Steep & plucked!
Crag and Tail
- Opposite asymmetry to Roche Moutonnée!
- Steep resistant volcanic plug faces up-ice
- Protects gentle tapering leeward till tail
IV. Glacial Transport, Unsorted Till vs. Outwash & Indian Cryosphere
Infinite Competence
- Solid ice does not settle clasts by weight
- Carries 1,000-ton boulders & fine clay
- Simultaneously at the exact same speed
3 Transport Zones
- Supraglacial: Angular frost-shattered surface debris
- Englacial: Buried inside crevasses
- Subglacial: Abraded & striated basal load
Unsorted Till vs. Outwash
UNASSORTED
STRATIFIED
- Glacial Till: Direct ice melt (unassorted coarse + fine)
- Outwash: Meltwater stream (assorted & stratified)
- Core NCERT Chapter 7 distinction!
Glacial Erratics
- Boulders dropped far from source outcrop
- Rest on completely different bedrock type
- Traces ancient Ice Age sheet pathways
Long-Run Peneplanation
- Continuous glacier action lowers high ridges
- Leaves low hills & vast outwash plains
- Canadian & Baltic shield lake plains (NCERT)
Indian Himalayan Glaciers
GANGOTRI · ZEMU · MILAM
- Karakoram: Siachen (76 km), Hispar, Biafo, Baltoro
- Uttarakhand: Gangotri, Milam, Pindari, Satopanth
- Himachal: Bara Shigri; Sikkim: Zemu
GLOF Hazards (Himalayas)
- Glacial Lake Outburst Flood (GLOF)
- Avalanche/melt breaches terminal moraine dam
- Chorabari (Kedarnath 2013) & South Lhonak (2023)
Core Distinctions
• Glacial Till: Unassorted coarse and fine debris dropped directly by melting glaciers; rock fragments are angular to sub-angular and show no stratification or sorting.
• Outwash Deposits: Deposited by meltwater streams issuing from the glacier snout; unlike till, outwash deposits are roughly stratified and assorted with rounded edges.
• Roche Moutonnée: Gentle smooth stoss (up-ice) slope polished by abrasion, and a steep jagged lee (down-ice) slope roughened by plucking.
• Crag and Tail: Steep resistant crag faces up-ice, protecting a gentle tapering tail of soft rock/till on the lee (down-ice) side.
Although glaciers move very slowly (only a few centimetres to a few metres a day under gravity), their erosion is tremendous because of the extreme friction caused by the sheer weight of the ice dragging plucked boulders across the valley floor.
Glacial Processes Matrix
NCERT Chapter 7 & Himalayan Key