GLACIAL ENVIRONMENTS 1

• Background

• Accumulation and ablation

• Ice movement and flow rates

About every 200 million years the Earth experiences a major period of ice activity – a glaciation. The most recent of these started about 2 million years ago and finished (*) about 10,000 years ago.

A glaciation consists of glacials (cold periods ) separated by interglacials (warmer periods). * Some people believe we are still in an interglacial!

About 30% of the world was covered by glacial ice when the glaciation was at its maximum. The UK was covered by ice between 1-3kms thick as far south as a line from London to Bristol.

Source: Image adapted from a file licensed under the Creative Commons Attribution Non-Commercial ShareAlike License

There are many theories as to the cause of glaciations:

• Milankovitch cycle – changes in incoming radiation due to changes in orbit, tilt and position in space.

• Variations in sunspot activity

• Changes in the amount of carbon dioxide in the atmosphere

• Changes in the movement of the ocean currents

• Periods of extreme volcanic activity which put huge amounts of ash into the atmosphere

Image source: NASA (public domain)

During the onset of a glaciation, more and more precipitation falls as snow. In addition, less and less snow melts each summer so that successive layers of snow gradually build up until there is a year-round snow cover in more and more places.

Fresh snowflakes trap much air and have a low density. As snow becomes more compacted, the air is driven out and density increases. Eventually, this process forms neve or firn (compacted snow).

After 20-40 years the firn will turn into glacial ice which contains little air and has a density of about 0.9. Glacial ice can begin to flow downhill under the influence of gravity as a glacier.

public domain image

Glaciers can be classified according to their size and shape. The main types are:

• Corrie glacier (also called a cirque glaciers or a cwm) – these occupy small hollows mainly on the sheltered north-facing slopes of mountains

• Valley glacier – these are linear masses of ice which move along pre-existing river valleys in the mountains

Corrie glaciers

Valley glacier

Source: Photographer Dirk Beyer - file licensed under the Creative Commons Attribution ShareAlike licence

• Piedmont glacier – these form when valley glaciers spread out on to low-lying areas and merge to form a single ice mass.

The Malaspina Glacier, Alaska is a classic example of a piedmont glacier lying along the foot of a mountain range. The main source of ice for the glacier is provided by the Seward Ice Field to the north which flows through three narrow outlets onto the coastal plain.

• Ice cap or ice sheet – these are extremely large ice masses which cover whole mountain ranges or even whole continents e.g. Antarctica.

Vatnajokull is Iceland's largest ice cap. It covers an area of 8,100 km² and is up to 1000 metres thick.

Photo sources: USGS and NASA (public domain)

Glaciers, like rivers, behave as a system with inputs, outputs, stores and transfers. The main input for glaciers is falling snow but avalanches can add considerable volumes of ice and snow. The glacier itself is the store in the form of frozen water. The outputs include evaporation, calving (where ice breaks off into water) and melting.

The upper part of a glacier where inputs exceed outputs is called the zone of accumulation. The lower part of a glacier where outputs exceed inputs is called the zone of ablation. In between the two is the line of equilibrium which is the same as the snow line.

Zone of ablation

(melting exceeds accumulation)

Zone of accumulation

(accumulation exceeds melting)

Line of equilibrium

INPUTS

OUTPUTS

Public domain image

Mass balance: the annual budget of a glacier describes the input/outputrelationships of ice, firn and snow and is measured in water equivalent (i.e. the amount of water involved if all the ice melted). In other words, it is the balance between accumulation, ablation and the stored ice. The difference between accumulation and ablation for a whole glacier over a year is called the net balance (usually measured over a year between periods of minimal loss). For most temperate glaciers the winter balance is positive and the summer balance is negative.

Diagram source: unknown

Amount and type of glacier movement depends on whether the glacier is classified as warm (temperate) or cold (polar). ‘Warm ice’ glaciers in summer release large amounts of meltwater which facilitates ice movement by acting as a lubricant. ‘Cold ice’ glaciers remain frozen to the bedrock for most of the year.

In temperate areas glacier ice moves by one of four processes:

Basal sliding – movement increases pressure at base and thus amount of meltwater which further lubricates the ice. Ice may move 2-3 metres per day and erosion is active through abrasion.

Creep – occurs when ice meets an obstacle and leads to pressure melting on upstream side which aids flow. Meltwater refreezes on downstream side.

Extending-compressing flow – where the gradient is steep, the glacier moves faster and thins (extending flow) leading to reduced erosion but where the gradient is gentle, the glacier moves slowly and thickens (compressing flow) leading to increased erosive power.

Surges – glacier moves forward very rapidly (maybe 300 metres in a day) due to the build up of sub-glacial meltwater or perhaps large rock avalanches near the source. Surges may happen every 50 – 100 years.

Glacier flow is fastest in the centre and near the surface where friction is least effective.

Side view of glacier Birds-eye view of glacier

Diagrams source: http://www.physicalgeography.net/fundamentals/10ae.html

In polar areas, glacier ice movement is mainly by internal deformation in which ice crystals deform or slide over each other. Ice movement is very slow, only 1-2 cm per day, and there is minimal erosion as glacier remains frozen to bedrock. Temperate glaciers in winter exhibit similar behaviour.

Summary of key points:

• an ice age consists of cold glacial periods and warmer interglacial periods

• the causes of the ice ages could be due to reductions in incoming solar radiation e.g. sunspot activity or blockage of incoming solar radiation by such things as volcanic eruptions

• glacial ice forms after many years from compacted snow which turns to firn or neve, then glacial ice as more and more air is squeezed out

• glaciers can be classified into four types: corrie (cirque or cwm) glacier, valley glacier, piedmont glacier, ice sheet (ice cap)

• Glaciers, like rivers, behave as a system with inputs, outputs, stores and transfers

• The upper part of a glacier where inputs exceed outputs is called the zone of accumulation. The lower part of a glacier where outputs exceed inputs is called the zone of ablation

• the annual budget of a glacier describes the input/output relationships of ice, firn and snow

• Amount and type of glacier movement depends on whether the glacier is classified as warm (temperate) or cold (polar)

• glaciers can move by any of four processes: basal sliding, creep, extending-compressing flow and internal deformation