Glaciers & Ice Erosion: Sixth Grade Science
Glaciers are massive, slow-moving rivers of recrystallized ice that alter continental and alpine landscapes through plucking, abrasion, and massive deposition.
What is a Glacier?
A glacier is a thick, perennial mass of crystalline ice that forms on land from the accumulation, compaction, and recrystallization of snowfall over centuries, displaying evidence of past or present downslope movement under the influence of gravity. Glaciers currently cover roughly 10 percent of Earth's land surface, storing roughly 69 percent of the world's freshwater:
- Alpine (Valley) Glaciers: Form in high mountain cirques and flow slowly down mountain valleys like frozen rivers (e.g., in the Alps, Alaska, and the Himalayas).
- Continental Glaciers (Ice Sheets): Vast, dome-shaped ice sheets that cover entire continental landmasses, spreading outward in all directions from central accumulation zones (e.g., the Antarctic and Greenland ice sheets).
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| ANATOMY OF A MOVING GLACIER |
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| ZONE OF ACCUMULATION (High Elevation) |
| Snowfall in winter exceeds summer melt; snow compacts into FIRN & ICE. |
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| v (Ice flows downslope under gravity) |
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| ZONE OF ABLATION (Low Elevation) |
| Summer melting (ablation), sublimation, and calving exceed snowfall. |
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| v |
| [ TERMINUS / SNOUT ] --> Melting ice drops unsorted TILL sediment! |
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The Conversion of Snow to Glacial Ice
Glaciers do not form from frozen rainwater. They form through the compaction of snow:
- Delicate, fluffy, hexagonal snowflakes (density ~0.1 g/cm^3) accumulate in deep layers.
- As more snow accumulates above, the weight presses down, shearing off delicate snowflake arms and compacting snow into granular, gravel-like ice pellets called firn (density ~0.4 to 0.8 g/cm^3).
- Over decades and centuries, firn is compressed into dense, solid, blue-tinted glacial ice (density ~0.9 g/cm^3) completely sealed from air.
The Two Primary Mechanisms of Glacial Erosion
As billions of tons of glacial ice creep downslope, they erode bedrock through two powerful mechanical processes: plucking and abrasion.
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| PLUCKING VERSUS ABRASION |
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| 1. Plucking (Quarrying): |
| Meltwater freezes in bedrock joints beneath glacier -> Ice bonds |
| to fractured rock -> Glacier moves forward, yanking blocks out! |
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| 2. Abrasion: |
| Plucked rocks embedded in glacier bottom act like giant sandpaper |
| -> Grinds bedrock, carving parallel GLACIAL STRIATIONS and fine |
| pulverized rock flour! |
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Alpine Glacial Erosional Landforms
Alpine glaciers transform rugged V-shaped river valleys into dramatic, sculpted glacial landscapes:
- U-Shaped Valleys (Glacial Troughs): Unlike rivers, which carve narrow, V-shaped valleys, a valley glacier fills the entire width of a canyon, eroding both the floor and vertical walls equally to produce a wide, steep-walled, flat-bottomed U-shaped valley.
- Hanging Valleys: Formed where small tributary glaciers join a massive main trunk glacier. Because the deeper main glacier carves down much further, when the ice melts, the tributary valley floor is left suspended high up on the cliff wall, producing spectacular sheer waterfalls (like Yosemite Falls).
- Cirque: A steep-sided, bowl-shaped amphitheater carved high on a mountainside at the head of a glacier.
- Arête: A sharp, knife-edged, razor-thin bedrock ridge separating two adjacent parallel glacial cirques.
- Horn: A sharp, pyramid-shaped mountain peak formed when three or more glacial cirques carve backward into the same central peak (e.g., the famous Matterhorn in Switzerland).
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| V-SHAPED VERSUS U-SHAPED VALLEYS |
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| RIVER VALLEY (V-Shaped): GLACIAL TROUGH (U-Shaped): |
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| \ / | | |
| \ / | | |
| \ / |____________| |
| \/ |
| Carved by water current Carved by massive ice sheet |
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Glacial Deposition: Till and Moraines
When a glacier retreats and melts, it unloads its massive cargo of rock and sediment:
- Glacial Till: Completely unsorted, unstratified sediment dropped directly by melting ice, containing everything from microscopic clay and silt up to house-sized boulders (glacial erratics) mixed randomly together. (This contrasts with water deposits, which sort sediment by size).
- Moraines: Ridges of deposited till:
- Lateral Moraines: Ridges of debris along the sides of a valley glacier.
- Medial Moraines: Formed when two lateral moraines merge as tributary glaciers join together.
- Terminal (End) Moraine: A massive ridge of till deposited at the farthest forward advance of a glacier's snout, marking its historical maximum reach (e.g., Long Island, New York, and Cape Cod, Massachusetts, are colossal terminal moraines from the last Ice Age!).
Exercises and Solutions
Exercise 1: Decoding Valley Profiles and Historical Glaciation. A hiker in the Rocky Mountains walks through a wide, flat-bottomed valley with vertical granite walls rising 500 meters straight up on both sides. High on the canyon wall, a stream emerges from a suspended canyon and plunges 300 meters into the valley as a sheer waterfall. Near the mouth of the valley, the hiker finds bedrock polished smooth with long, parallel gouges and scratches running down-valley. (a) Identify the geological agent that carved this landscape; (b) What are the scientific names for the U-shaped valley, the elevated waterfall canyon, and the parallel scratches? (c) Why does this evidence prove an ancient glacier was present?
Solution 1: (a) The landscape was carved by an ancient alpine valley glacier that has since melted away. (b) The wide, flat-bottomed, steep-walled valley is a glacial trough (U-shaped valley); the elevated waterfall canyon is a hanging valley; the parallel bedrock gouges are glacial striations. (c) Running water in rivers cannot carve wide, vertical U-shaped valleys (rivers carve narrow V-shaped cuts) and cannot suspend tributary valleys hundreds of meters in the air. The parallel striations prove that hard rock boulders embedded in the bottom of a heavy, moving ice mass acted like giant sandpaper, gouging the bedrock along the direction of ice flow.
Exercise 2: Differentiating Glacial Till from River Alluvium. A geologist analyzes two sediment samples: Sample A consists of rounded quartz sand grains of nearly identical size, with all pebbles and silt sorted into separate, stratified layers. Sample B is a chaotic, unlayered mixture of fine clay powder, sharp angular pebbles, medium sand, and a jagged 10-kilogram boulder jumbled together. Identify which sample is glacial till and which is river alluvium, explaining the transport mechanics behind the sorting.
Solution 2: Sample A is river alluvium; Sample B is glacial till. Running river water is a fluid whose transport capacity depends directly on water velocity. As river currents slow down, water drops heavier particles first and lighter particles later, naturally sorting sediments by size, weight, and density into stratified beds. In contrast, glacial ice is a rigid solid. A glacier acts like a giant conveyor belt that carries boulders, sand, and clay frozen together in fixed positions regardless of weight or size. When the glacier melts, it drops its entire load simultaneously in place, producing the chaotic, completely unsorted, unlayered mixture characteristic of glacial till.