Glaciers and glacial erosion

Detail

What a glacier is

• A glacier is a large mass of ice that moves slowly downhill under its own weight (gravity).
• Speed: from a few centimetres to a few metres a day. This is far slower than a river.
• How it forms:
• Snow falls in cold places, and more falls each winter than melts each summer.
• Each new layer presses down on the snow below. The old snow packs into dense, grainy ice.

• When the ice is thick and heavy enough, it starts to flow downhill.

• Glaciers are agents of gradation (agents that wear down high land and build up low land). They erode, carry and deposit material, just as rivers, wind and waves do.

Types of glaciers

Type	What it is	Where
Continental glacier / ice sheet	A huge sheet of ice that covers a whole landmass. It buries hills and valleys alike (it covers the land whatever the relief below).	Antarctica, Greenland (today)
Piedmont glacier	Ice that comes out of mountain valleys and spreads over the plains at the foot of the mountains ("piedmont" = foot of the mountain)	Foot of high ranges, e.g. Alaska
Mountain / valley glacier	A long, narrow tongue of ice that flows down a mountain valley	Himalaya of Uttarakhand, Himachal Pradesh, Jammu and Kashmir (and Ladakh); the Alps

Indian glaciers and river origins

• Gangotri glacier (Uttarkashi district, Uttarakhand):
• The Bhagirathi rises at Gaumukh ("cow's mouth"). Gaumukh is the snout (lowest end) of the Gangotri glacier.
• The glacier is about 30.2 km long and 0.5–2.5 km wide. It is one of the largest glaciers in the Himalaya [4].
• It has been retreating since 1780. The retreat became faster after 1971 [4].

• It retreated by more than 850 m in the 25 years up to 2001, including 76 m in 1996–1999 alone [4].

• The Alaknanda is fed by the Alkapuri glacier (Satopanth–Bhagirath Kharak area, Chamoli district, Uttarakhand).

• The Bhagirathi and the Alaknanda meet at Devprayag (Tehri Garhwal district, Uttarakhand) to form the Ganga.
• Siachen glacier (Karakoram Range, Ladakh):
• It is about 75 km long. That makes it the second-longest non-polar glacier in the world, after the Fedchenko glacier in Tajikistan [5]. (NCERT-derived figure: about 76 km.)

• It is a source of the Nubra river. The Nubra joins the Shyok, which joins the Indus.

• Zemu glacier (Sikkim) lies at the foot of Kangchenjunga. It is the largest glacier in the Eastern Himalaya and feeds the Teesta.

• Baltoro glacier lies in the Karakoram of Pakistan-administered territory, near K2.

Scale of Himalayan glaciers today

• The Himalaya–Karakoram inventory lists 20,812 glaciers covering 43,178 km² [2][3].
• Mean retreat rate of Hindu Kush Himalayan glaciers is 14.9 ± 15.1 m a year. The rate differs by river basin [2][3]:

River basin	Mean retreat (m/year)
Indus	12.7 ± 13.2
Ganga	15.5 ± 14.4
Brahmaputra	20.2 ± 19.7 (fastest)
Karakoram (exception)	−1.37 ± 22.8 (almost no change)

• The near-stable Karakoram glaciers are called the "Karakoram anomaly".
• Field measurements show a cumulative mass loss of about −26 m w.e. for Indian Himalayan glaciers over 1975–2023 [2][3]. m w.e. means "metres water equivalent": the depth of water the lost ice would make if melted.
• Chandra basin (Lahaul, Himachal Pradesh; 2,437 km²) [2][3]:
• NCPOR (National Centre for Polar and Ocean Research, under the Ministry of Earth Sciences) has monitored six glaciers here since 2013.
• Its field station "Himansh" has worked since 2016.
• The basin has lost about 6% of its glacier area in the last 20 years.
• Its glaciers retreated by 13–33 m a year over the last decade.

How glaciers erode

• Glacial erosion is very strong because of the sheer weight of the ice. Moving ice presses down on rock with huge force.
• Plucking (quarrying):
• Meltwater seeps into joints (cracks) in the bedrock and freezes onto the rock.
• The ice moves on and pulls out angular blocks of rock.

• The result is rough, broken rock surfaces, mostly on the downstream side of rock bumps.

• Abrasion:

• The plucked blocks stay frozen in the base and sides of the glacier.
• They act like sandpaper and scrape the valley floor and walls.

• The result is smooth, polished rock with scratches called glacial striations (parallel grooves that show which way the ice moved).

• Long-term effect: glaciers can wear high mountains down into low hills and plains. Meltwater spreads the eroded debris as outwash plains.

Erosional landforms

Cirque, tarn, horn and arete

• Cirque:
• A deep, armchair-shaped basin at the head of a glacial valley.
• Its head wall and side walls are steep, ranging from concave to vertical.
• It is the most common landform in glaciated mountains.

• How it forms: snow collects in a hollow, freezes and turns to ice. Plucking cuts back the back wall, and abrasion deepens the floor.

• Tarn:

• A cirque lake. It forms in the cirque basin after the ice melts.

• Several cirques can lie in a stepped chain, each with its own tarn.

• Horn (pyramidal peak):

• A sharp, pointed peak.
• How it forms: three or more cirques cut back into the mountain from different sides until their back walls meet.

• Examples: the Matterhorn (Alps, on the Switzerland–Italy border, 4,478 m). NCERT also cites Mount Everest.

• Arete:

• A narrow, saw-toothed (serrated) ridge between two cirques that cut back side by side.
• Both sides of the ridge are steep.

Glacial valleys and related forms

• U-shaped valley (glacial trough):
• A valley with a broad, flat floor and steep, smooth sides.
• How it forms: a thick glacier fills the whole valley. It widens and deepens the valley evenly, removing the spurs that stick out.
• Glacial debris often lies across the floor.

Feature	U-shaped (glacial) valley	V-shaped (river) valley
Agent	Glacier fills the whole valley	River cuts downward in a narrow line
Cross-profile	Broad flat floor, steep straight sides	Narrow floor, sloping sides
Course	Fairly straight; spurs cut off	Winding, with interlocking spurs
Main process	Plucking + abrasion over the full width	Vertical erosion by running water
Example	Himalayan valleys of Himachal, Kashmir; Yosemite (USA)	Young Himalayan river gorges

• Hanging valley:
• A side (tributary) valley whose floor is left high above the floor of the main trough.
• Why: the small side glacier erodes less deeply than the big main glacier.

• When the ice melts, the side stream falls into the main valley as a waterfall.

• Truncated spur:

• The spurs (ridges) between side valleys are cut back by the main glacier.

• This leaves steep, triangular faces (also called triangular facets).

• Fjord:

• A very deep glacial trough drowned by the sea. It forms where glaciers once reached the coast and sea level later rose.
• Fjords are found in high latitudes:
  • Norway: the Sognefjord, the longest and deepest in Norway, at about 61°N
  • Greenland
  • southern Chile
  • New Zealand's Fiordland (South Island)
  • Alaska and British Columbia
• Fjords have deep, sheltered water, so they make good natural harbours (NCERT Class 9).

Other erosional forms

• Roche moutonnée:
• A rock hump with two different sides.
• The up-ice side is smooth and gentle because abrasion polished it.

• The down-ice side is steep and jagged because plucking broke it.

• Crag and tail:

• A hard rock knob (the crag) shields softer rock behind it from the ice. That softer rock is left as a long, gentle tail.

• Example: Edinburgh Castle Rock in Scotland.

• Glacial striations: the scratches left by abrasion, which record the direction the ice moved.

Glaciers in the cycle of erosion

• Young glaciated mountains have cirques, horns and aretes that stand out sharply.
• Over a long time, plucking and abrasion lower the peaks and widen the troughs. The relief becomes subdued.
• The final stage is low hills and outwash plains.

Retreat as a climate signal

• Most Himalayan glaciers are retreating and losing mass. This is a clear sign of a warming climate [2][3][4]. See climate-change.
• Effects of retreat:
• Snouts move upstream, and new glacial lakes form behind loose moraine dams (ridges of rock debris left by the glacier).
• These lakes can burst and cause GLOFs (Glacial Lake Outburst Floods: sudden floods released when a moraine or ice dam breaks). See natural-hazards-disasters.
• Over the long term, summer flow in glacier-fed rivers such as the Indus, Ganga and Brahmaputra is at risk.

Prelims Hooks

• Gaumukh is the snout of the Gangotri glacier and the source of the Bhagirathi. The Alaknanda is fed by the Alkapuri glacier. The two join at Devprayag to form the Ganga.
• Siachen (Karakoram, about 75 km) is the second-longest non-polar glacier. The longest is the Fedchenko in Tajikistan [5].
• Horn: three or more cirques meeting (Matterhorn; NCERT also cites Everest). Arete: a saw-toothed ridge between two cirques. Tarn: a lake in a cirque.
• The cirque is the most common landform of glaciated mountains.
• Trap: a U-shaped valley comes from glaciers, a V-shaped valley from rivers.
• Hanging valley → waterfall. Truncated spur → triangular facet.
• Fjords and their countries: Sognefjord–Norway, Fiordland–New Zealand, also southern Chile, Greenland, Alaska and British Columbia.
• Plucking pulls out blocks along joints. Abrasion scrapes and polishes, leaving striations.
• Retreat by basin, fastest to slowest: Brahmaputra (20.2 m/yr) > Ganga (15.5) > Indus (12.7). The Karakoram is nearly stable [2][3].
• Himansh is NCPOR's glacier research station in the Chandra basin, Himachal Pradesh, operating since 2016 [2][3].

Mains Points

• Glaciers as India's "water towers":
• Glacier melt keeps the Indus, Ganga and Brahmaputra flowing in the dry pre-monsoon season.

• Retreat (14.9 m/yr on average) and a mass loss of about −26 m w.e. over 1975–2023 put irrigation, hydropower and drinking water for the northern plains at risk [2][3].

• Uneven retreat across the Himalaya:

• The eastern Himalaya (Brahmaputra basin) is retreating fastest, while the Karakoram is nearly stable [2][3].

• Water planning in each basin therefore needs local data, not one Himalaya-wide average.

• Rising disaster risk (GS-III):

• Retreating glaciers leave moraine-dammed lakes that can burst as GLOFs.

• Glacial lakes need monitoring, early warning systems, and planning of hydropower and roads in valleys at risk.

• How glacial landforms affect people (GS-I):

• U-shaped valleys give flat land for settlements, farming and roads in the Himalaya and the Alps.
• Hanging valleys provide hydropower sites.
• Fjords make deep natural harbours, as in Norway.
• Steep cirque walls and aretes, on the other hand, are prone to avalanches.