Question

Explain how glaciers can change the landscape and physical characteristics of continents.

Answer 1

Glaciers are powerful agents of both erosion and deposition. Over thousands to millions of years they can rearrange huge volumes of rock and sediment, carve new landforms and change continental coastlines, drainage systems, soils and groundwater. Key ways glaciers change continents:

1. Types of glaciers
- Alpine (mountain) glaciers: confined to valleys, carve sharp mountain scenery.
- Continental ice sheets (e.g., former Laurentide, present Greenland/Antarctica): cover large areas and radically reshape broad regions.

2. Erosion processes and landforms
- Abrasion: rock debris frozen in the ice grinds bedrock like sandpaper, producing polished surfaces and striations (scratches) that record ice flow direction.
- Plucking (quarrying): meltwater freezes into bedrock joints and pulls blocks away as the ice moves.
- Resulting landforms:
- U-shaped (glacial) valleys and truncated spurs (replacing previous V-shaped river valleys).
- Cirques (amphitheater hollows), arêtes (sharp ridges), horns (pointed peaks) where headward erosion isolates peaks.
- Hanging valleys (small tributary valleys left suspended above main valley) and waterfalls.
- Roche moutonnée (asymmetrical bedrock knobs shaped by abrasion on one side and plucking on the lee side).
- Fjords: U-shaped valleys flooded by the sea after glacial retreat.

3. Deposition processes and landforms
- Till: unsorted material dropped directly by melting ice; forms moraines (lateral, medial, terminal, ground moraines).
- Glaciofluvial deposits: meltwater sorts sediments into sand and gravel forming outwash plains.
- Distinct deposited landforms:
- Eskers: sinuous ridges of sediment from subglacial streams.
- Drumlins: streamlined hills of till oriented in the direction of ice movement.
- Kames: mounds or hummocks of stratified drift.
- Kettle holes and kettle lakes: depressions left by melting ice blocks.
- Windblown glacial silt (glacial flour) can form thick loess deposits downwind, creating very fertile soils.

4. Hydrology, lakes and drainage changes
- Glaciers can dam rivers (by moraines or ice), creating large lakes; breaching dams causes catastrophic outburst floods.
- Carved basins become long, deep lakes (e.g., many of the North American Great Lakes are glacier-carved).
- Glacial deposits redirect drainage divides and river courses.
- Outwash and sorted sediments create important aquifers (sand & gravel layers).

5. Isostatic and sea-level effects
- Ice sheets depress the crust; when ice melts, the crust rebounds (isostatic uplift), slowly altering coastlines and river gradients for thousands of years.
- Large glaciations trap water in ice and lower global sea level, exposing continental shelves (e.g., Bering land bridge); post-glacial sea-level rise can flood glacial valleys to form fjords.

6. Soils, ecosystems and human impacts
- Glacial deposits produce variable soils—till often poorly sorted and rocky; loess-derived soils can be very fertile.
- Redistribution of sediments creates new habitats and affects vegetation patterns and species migrations.
- Glaciation also concentrates some mineral resources and leaves sand/gravel resources valuable for construction.

7. Continental-scale consequences (examples)
- Creation of lake districts (Great Lakes), fjord coasts (Norway), drumlin fields (parts of New York, Ireland), thick loess belts (U.S. Midwest).
- Smoothing and lowering of relief in places under ice sheets, steep rugged topography in formerly glaciated mountain ranges.
- Long-term modification of drainage networks and groundwater systems.

Summary: Through powerful erosion (abrasion, plucking) and extensive deposition (till, moraines, eskers, outwash, loess), plus isostatic uplift and sea-level changes, glaciers reshape mountains and continents—carving valleys and basins, creating lakes and depositional landforms, altering soils, drainage and coastlines, and leaving a lasting imprint on the physical character of large regions.