Key Terms — Glacial Processes

📖 Key Terms — Glacial Processes

Accumulation

The addition of snow and ice to a glacier, primarily in the upper (accumulation) zone. Sources include snowfall, avalanches from valley sides, and the freezing of meltwater.

Ablation

The loss of ice from a glacier through melting, sublimation (direct conversion of ice to water vapour), and calving (chunks breaking off at the snout into water bodies). Dominates in the lower ablation zone.

Glacial budget (mass balance)

The annual difference between total accumulation and total ablation across a glacier. A positive budget means the glacier advances; a negative budget means it retreats; a zero budget means the snout remains stationary.

Firn (névé)

Granular, partially compacted snow that forms the intermediate stage between fresh snowfall and solid glacier ice. Has a density of 500–800 kg/m³.

Basal sliding

The primary movement mechanism in temperate (warm-based) glaciers. Pressure from the overlying ice melts the basal ice, creating a thin meltwater film that lubricates the glacier base and allows it to slide over bedrock.

Freeze-thaw weathering (frost shattering)

A weathering process in which water enters rock cracks, freezes (expanding ~9%), and exerts sufficient pressure to shatter the rock. Produces angular debris used as tools in glacial abrasion.

Plucking (quarrying)

A glacial erosion process in which meltwater refreezes in rock joints beneath/beside the glacier, bonding ice to rock, and the moving glacier tears out blocks. Produces an irregular, jagged bedrock surface and angular debris.

Abrasion

A glacial erosion process in which rocks embedded in the base of the glacier are dragged over bedrock, grinding and scratching it. Produces striations, glacial grooves, polished surfaces, and rock flour.

Striation

A scratch or groove carved into bedrock by a rock embedded in the moving glacier base. Striations are parallel and indicate the direction of past ice movement.

Rock flour

Very fine-grained rock particles produced by abrasion. Carried in suspension in glacial meltwater, giving it a milky blue or grey colour.

Supraglacial

On top of the glacier. Material in supraglacial transport has not been ground by the glacier and retains its angular shape from freeze-thaw weathering.

Englacial

Within the body of the glacier. Material can become englacial by falling into crevasses or being buried under snowfall in the accumulation zone.

Subglacial

At the base of the glacier, between ice and bedrock. Subglacial debris experiences continuous grinding and becomes progressively rounded and striated.

Till (boulder clay)

Sediment deposited directly by ice (not by meltwater). Characteristically unsorted — all particle sizes mixed together — and unstratified. May contain erratic boulders and striated clasts.

Erratic

A boulder or rock that has been transported far from its source by a glacier and is now entirely different from the local bedrock. Erratics are powerful evidence of past ice extent and direction of flow.

Fluvioglacial (glaciofluvial)

Relating to meltwater from glaciers. Fluvioglacial sediments are sorted and stratified by water (unlike till), and form landforms such as eskers, kames, and outwash plains.

Moraine

An accumulation or ridge of till deposited by a glacier. Types include terminal, recessional, lateral, medial, and ground moraine — each forming in a different position relative to the glacier.

Terminal moraine

A ridge of till deposited at the snout of a glacier at its furthest point of advance. Marks the maximum extent of glaciation.

Lateral moraine

A ridge of till deposited along the sides of a valley glacier, composed of freeze-thaw debris that fell from the valley walls above the ice. Left high on valley sides after the glacier retreats.