Structure of Glacier
Structure of Glacier — the internal arrangement and layering of ice within a glacier, including features such as firn, solid ice, crevasses, and layers formed by seasonal accumulation and compression.
Main article: Glacier
A glacier’s structure is like a layered building or a multi-layered cake — each layer of snow and ice tells a story of past seasons, while cracks, crevasses, and moraines are like windows and walls shaped by the pressure and flow over time.
A glacier's structure is defined by its accumulation and ablation zones, separated by an equilibrium line. The accumulation zone, at higher elevations, gains mass from snow and ice, while the ablation zone, at lower elevations, loses mass through melting and evaporation. The constant flow of ice from accumulation to ablation, driven by gravity, forms internal and external features like crevasses, folds, and moraines.
Key zones and features
- Accumulation Zone: The upper part of the glacier where snowfall exceeds melting, contributing to the glacier's mass. Snow is compacted into "firn" (granular ice) and then into dense glacial ice.
- Ablation Zone: The lower part where the glacier loses mass through melting, evaporation, and calving (breaking off icebergs).
- Equilibrium Line: The boundary where the amount of ice added (accumulation) equals the amount of ice lost (ablation). Its position shifts seasonally.
Zones of ice flow
- Zone of Brittle Flow: The upper layer of ice, which is under less pressure and is more rigid. This zone can fracture, forming deep cracks known as crevasses.
- Zone of Plastic Flow: The lower layer of ice, which is under immense pressure from the weight above and flows more like a viscous fluid, conforming to the bedrock's shape.
Internal and external structures
- Moraines: Debris, rocks, and sediment carried by the glacier.
- Lateral Moraines: Ridges of debris along the sides of the glacier.
- Medial Moraines: Debris ridges formed when two lateral moraines from tributary glaciers merge in the middle of a larger glacier.
- Terminal Moraine: A ridge of debris pushed to the front of the glacier's terminus.
- Crevasses: Deep, wedge-shaped cracks that form in the brittle upper layer of ice as it flows over irregular terrain.
- Foliation: Layers of ice with different properties that form from shear stress as the glacier moves.
Glacier Head and Terminus
A glacier begins at its head — the high, snow-fed cradle where new ice accumulates — and ends at its foot, also called the snout or terminus, where the ice thins, melts, or breaks away.
Its surface is divided into zones shaped by the balance between snowfall and melting. The ablation zone is where the glacier loses mass; higher above lies the accumulation zone, where snow builds faster than it can melt. Between them stretches the equilibrium line, a shifting contour where gain and loss momentarily balance. Typically, the accumulation zone makes up most of a glacier’s surface, especially in glaciers that calve icebergs. Ice here is deep and heavy enough to grind the underlying rock, sculpting the land into bowls, basins, or sweeping amphitheater-like cirques that remain long after the ice withdraws.
Even the accumulation zone can be further divided by how snow behaves:
- The dry snow zone: a realm untouched by melt, where winter’s layers remain crisp and unchanged even through summer.
- The percolation zone: where modest surface melt trickles downward and refreezes into lenses and layers of clear ice, though the snowpack never fully reaches the melting point.
- The superimposed ice zone: near the equilibrium line, meltwater refreezes into a solid sheet, forming a continuous layer atop older glacier ice.
- The wet snow zone: where every snowflake deposited since the previous summer warms to 0 °C, creating a saturated, slushy mass.
A glacier’s overall well-being is tracked through its mass balance or by watching how its terminus behaves. Healthy glaciers retain broad accumulation zones and end the melt season with more than 60% of their surface still cloaked in snow, feeding a strong, active flow.
Since the end of the Little Ice Age around 1850, glaciers worldwide have withdrawn significantly. A brief mid-20th-century cooling allowed some alpine glaciers to advance between 1950 and 1985, but since then, retreat and mass loss have accelerated and spread almost everywhere.
Glacier Zones Overview
| Zone | Description |
|---|---|
| Accumulation Zone | Upper part where snowfall exceeds melting; snow compacts into firn and then dense ice. |
| Ablation Zone | Lower part where ice is lost through melting, evaporation, or calving. |
| Equilibrium Line | Boundary between accumulation and ablation; position shifts seasonally. |
