Glaciology

Types

Glaciers can be identified by their geometry and the relationship to the surrounding topography. There are two general categories of glaciation which glaciologists distinguish: alpine glaciation, accumulations or "rivers of ice" confined to valleys; and continental glaciation, unrestricted accumulations which once covered much of the northern continents.

• Alpine – ice flows down the valleys of mountainous areas and forms a tongue of ice moving towards the plains below. Alpine glaciers tend to make topography more rugged by adding and improving the scale of existing features. Various features include large ravines called cirques and arêtes, which are ridges where the rims of two cirques meet.
• Continental – an ice sheet found today, only in high latitudes (Greenland/Antarctica), thousands of square kilometers in area and thousands of meters thick. These tend to smooth out the landscapes.

Zones of glaciers

• Accumulation zone – where the formation of ice is faster than its removal.
• Ablation (or wastage) zone – when the sum of melting, calving, and evaporation (sublimation) is greater than the amount of snow added each year.

Glacier equilibrium line and ELA

The glacier equilibrium line is the line separating the glacial accumulation area above from the ablation area below. The equilibrium line altitude (ELA) and its change over the years is a key indicators of the health of a glacier. A long term monitoring of the ELA may be used as an indication of climate change.

Movement

When a glacier is experiencing an accumulation input by precipitation (snow or refreezing rain) that exceeds the output by ablation, the glacier shows a positive glacier mass balance and will advance. Conversely, if the loss of volume (from evaporation, sublimation, melting, and calving) exceeds the accumulation, the glacier shows a negative glacier mass balance and the glacier will melt back. During times in which the volume input to the glacier by precipitation is equivalent to the ice volume lost from calving, evaporation, and melting, the glacier has a steady-state condition.

Some glaciers show periods where the glacier is advancing at an extreme rate, that is typically 100 times faster than what is considered normal, it is referred to as a surging glacier. Surge periods may occur at an interval of 10 to 15 years, e.g. on Svalbard. This is caused mainly due to a long lasting accumulation period on subpolar glaciers frozen to the ground in the accumulation area. When the stress due to the additional volume in the accumulation area increases, the pressure melting point of the ice at its base may be reached, the basal glacier ice will melt, and the glacier will surge on a film of meltwater.

Rate of movement

The movement of glaciers is usually slow. Its velocity varies from a few centimeters to a few meters per day. The rate of movement depends upon the factors listed below:

• Temperature of the ice. A polar glacier shows cold ice with temperatures well below the freezing point from its surface to its base. It is frozen to its bed. A temperate glacier is at a melting point temperature throughout the year, from its surface to its base. This allows the glacier to slide on a thin layer of meltwater. Most glaciers in alpine regions are temperate glaciers.
• Gradient of the slope.
• Thickness of the glacier
• Subglacial water dynamics

Glacial Terminology

Ablation

Wastage of the glacier through sublimation, ice melting and iceberg calving.

Ablation zone

Area of a glacier in which the annual loss of ice through ablation exceeds the annual gain from precipitation.

Arête

An acute ridge of rock where two cirques meet.

Bergschrund

Crevasse formed near the head of a glacier, where the mass of ice has rotated, sheared and torn itself apart in the manner of a geological fault.

Cirque, Corrie or cwm

Bowl shaped depression excavated by the source of a glacier.

Creep

Adjustment to stress at a molecular level.

Flow

Movement (of ice) in a constant direction.

Fracture

Brittle failure (breaking of ice) under the stress raised when movement is too rapid to be accommodated by creep. It happens, for example, as the central part of a glacier moves faster than the edges.

Glacial landform

Collective name for the morphologic structures in/on/under/around a glacier.

Moraine

Accumulated debris that has been carried by a glacier and deposited at its sides (lateral moraine) or at its foot (terminal moraine).

Névé

Area at the top of a glacier (often a cirque) where snow accumulates and feeds the glacier.

Nunatak/Rognon/Glacial Island

Visible peak of a mountain otherwise covered by a glacier.

Horn

Spire of rock, also known as a pyramidal peak, formed by the headward erosion of three or more cirques around a single mountain. It is an extreme case of an arête.

Plucking/Quarrying

Where the adhesion of the ice to the rock is stronger than the cohesion of the rock, part of the rock leaves with the flowing ice.

Tarn

A post-glacial lake in a cirque.

Tunnel valley

The tunnel that is formed by hydraulic erosion of ice and rock below an ice sheet margin. The tunnel valley is what remains of it in the underlying rock when the ice sheet has melted.

Glacial deposits

Source:

Stratified

Outwash sand/gravel

From front of glaciers, found on a plain.

Kettles

When a lock of stagnant ice leaves a depression or pit.

Eskers

Steep sided ridges of gravel/sand, possibly caused by streams running under stagnant ice.

Kames

Stratified drift builds up low, steep hills.

Varves

Alternating thin sedimentary beds (coarse and fine) of a proglacial lake. Summer conditions deposit more and coarser material and those of the winter, less and finer.

Unstratified

Till-unsorted

(Glacial flour to boulders) deposited by receding/advancing glaciers, forming moraines, and drumlins.

Moraines

(Terminal) material deposited at the end; (ground) material deposited as glacier melts; (lateral) material deposited along the sides.

Drumlins

Smooth elongated hills composed of till.

Ribbed moraines

Large subglacial elongated hills transverse to former ice flow.