Transcript Slide 1

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 preexisting 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.
INPUTS
Zone of accumulation
(accumulation exceeds melting)
Line of equilibrium
Zone of ablation
(melting exceeds accumulation)
OUTPUTS
Public domain
image
Mass balance: the annual budget of a glacier describes the input/output
relationships 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.
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.
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
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