WELCOME TO THE MODULE

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Nov 16, 2013 (3 years and 8 months ago)

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A2.3GQ3

Glacial and Quaternary Geology


Mike Paul


WELCOME TO THE MODULE

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Module Aims


To describe glacial processes, sediments and
geomorphology from a range of modern settings


To study analogous settings at various places around
the former West Highland glacier complex of Loch
Lomond age


To attempt a glaciological reconstruction of the
WHGC in order to explain the contrasts in behaviour
seen at the positions studied.

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Weekly Module Topics


1. Glacier dynamics and introduction to the WHGC.


2. Study of highland glacial settings.
Field visit to Glen Roy


3. Study of meltwater deposits and tidewater glacial settings


4. Study of lowland glacial settings.
Field visit to Menteith area


5.

Further study of lowland glacial settings


6.
No class
-

university holiday


7. Synthesis and reconstruction of the WHGC


A2.2GQ3

Glacial and Quaternary Geology


LECTURE 1


BEHAVIOUR OF

GLACIERS AND ICE SHEETS

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OVERVIEW


Mass balance


Mechanics of glacier flow


Basal regime


Thermal regime



Patterns of glacier flow


Ice streams


Glacier surges



Mass Balance

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The
mass balance

of a glacier is the net gain or loss
of snow and ice during the
balance year
.


Clearly the balance will be positive on the upper parts
of a glacier and negative on the lower parts.


The area of positive balance is known as the
accumulation area

and the area of negative balance
as the
ablation area
.

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The boundary between the two is called the
equilibrium line
. Its height is the equilibrium line
altitude (ELA) and is determined by several factors:


the relative sizes of the accumulation and ablation areas;


the annual snowfall vs annual melting rate


the annual temperature and temperature gradient


Determining the ELA provides an important parameter
for the reconstruction of an ancient glacier.

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If any of these factors change then the ELA will
change in response. The most likely change is in
snowfall and/or melting rate:


higher snowfall lowers the ELA since the ablation area
must expand to balance the higher imput


lower snowfall raises the ELA since the ablation area
will contract until melting once again balances input.


Analogous changes occur if the melting rate
decreases or increases.

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The equilibrium line is difficult to measure in the field.
Thus many workers use the
firn line,
the boundary
between old compacted snow (firn) and glacier ice.


The firn line is the altitudinal limit of surface melting
and corresponds closely with the equilibrium line.



Glacier ice is darker than firn
-

thus the firn line is
usually visible on aerial photographs.

Mechanics of Glacier Flow

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Three basic mechanisms exist by which ice is able to
flow relative to its bed. These are:


internal plastic flow;


basal sliding;


subglacial bed deformation
.

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The operation of, or relative importance of, any
particular mechanism(s) depends largely on basal
conditions.


They are not mutually exclusive: many ice bodies flow
by more than one mechanism and they may switch in
importance both spatially and temporally.

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Internal deformation is described by Glen’s law of ice
flow (with various modifications).





This is a temperature dependant flow law that, on a rigid
bed, determines the long profile of a glacier or ice sheet.

n
A
dt
d



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Basal sliding is described by various models, including
those of Weertman, Lliboutry, Kamb and Nye.


All involve regelation and invoke the concept of a
controlling size of bedrock obstacles.


The differences lie in the mathematical model used to
decribe the obstacles and the ice flow around them.


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Bed deformation is described by Boulton and others’
model of a deforming subglacial layer.


Subglacial pore water pressure is a key feature of this
model.

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The basic shape of an ice sheet is a dome. Mountain
glaciers theoretically approximate to a tilted dome,
with the thickest point in the centre.


The exact form depends on several factors:


The rheology of the ice (controlled by temperature)



The topography of the bed


The shear stress at the bed (controlled by water
pressure and the strength of the bed itself).

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Profiles of an ice sheet on a rigid bed

(field evidence from Antarctica)

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Sliding models produce lowered surface profiles
compared with rigid bed models.


Rapid ice streams may be explained by deforming bed
models and this mechanism has been validated by field
observations.

Thermal Regime

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The

thermal regime

of the body, usually considered in
terms of the vertical temperature profile from bed to surface:


temperate (isothermal)



the ice is at the pressure melting
point throughout the ice body;


warm
-
based (basal melting)



the basal ice is at the pressure
melting point, although higher layers may be below the
pressure melting point;


cold
-
based (basal freezing)



the basal ice is below the
pressure melting point (as therefore must be the whole of the
ice body).

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More realistic models allow the thermal regime to vary
across the ice sheet. These regimes are termed
polythermal
.


In such a regime some parts of the basal ice are at the
melting point, other parts are below.


Models of varying complexity can be built, depending on
the pattern of surface temperature and the lateral
gain/loss of heat and mass.

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Basal Regime

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The
basal regime

of the ice bodyis the consequence
of the thermal regime and/or the nature of the bed:


sliding bed



some component of movement is derived
from relative motion between the basal ice and the bed,
colloquially as a result of ‘sliding’;


frozen bed



there is little or no relative movement
between the basal ice and the bed, the ice being
presumed frozen to the bed;


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deforming bed

-

some component of movement is
derived from relative motion within the bed below the
basal ice, as a result of internal deformation within the
substrate;


the opposite of a deforming bed is a
rigid bed
. The
distinction may be both geological and glaciological
causes.


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Patterns of Glacier Flow

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A parcel of ice a within a glacier follows a flow
-
line
dictated by the gain and loss of mass, and by the
longitudinal velocity gradient.

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In the accumulation area, above the equilibrium line, the
ice accelerates, thins and the flow
-
lines descend into the
body of the glacier.


This is termed
extending flow
.

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At the equilibrium line, there is no longitudinal velocity
change and the flow
-
lines are parallel to the bed.


In the ablation area, below the equilibrium line, the ice
decelerates, thickens and the flow lines move towards the
surface.


This is termed
compressing flow
.

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Compressing flow may be associated with more extreme
shortening by thrusting and/or folding, that causes
stacking of the frontal ice.


Compressing flow is a basic mechanism by which basal
ice and its associated debris is brought to the glacier
surface and so creates conditions of supraglacial
deposition.

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Basal thrust,

Breidamerkurjökull




Photo: M.A.Paul

Ice Streams

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Large ice sheets usually have some areas that drain
by quasi
-
static flow and other areas that drain via
rapid ice streams.


An ice stream is a narrow zone of ice that flows at
about 5
-
10 times the rate of the surrounding quasi
-
static area.


They are often located over areas of soft sediment or
in areas into which large volumes of basal meltwater
are channelled.


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Glacier surges

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Surging glaciers

undergo periodic increases in
discharge, perhaps by an order of magnitude.


Several hundred present
-
day
surge
-
type

glaciers
have been identified, either from direct observation or
from geological evidence.


They appear to be particularly common in certain
geographical areas, including Alaska, Spitsbergen
and Iceland.


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Comfortlessbreen: Spitsbergen

Norsk Polarinstitutt photo

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Comfortlessbreen

Spitsbergen

Photo: J.D.Peacock

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During a surge the glacier snout may advance by
several kilometres in a few years.


This is ~100x faster than quasi
-
static flow.



This rapid movement may be the result of large
scale detachment of the ice from its bed, possibly
due to the creation of a thick water film that
submerges the controlling obstacles.

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The surge causes extreme deformation to both the
glacial ice and to the deposits around the glacial
margin.


It also produces very large volumes of meltwater.


Following the surge the glacier enters a quiescent
phase, during which the ice wastes back to around its
previous position.


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Hilmstrombreen: Spitsbergen

Norsk Polarinstitutt photo

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Only temperate
-
based or subpolar (thermally
composite) glaciers are known to surge.


No instances are known of surging in entirely cold
-
based glaciers and theoretical glacier dynamics
suggests that this type of glacier cannot surge.

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OVERVIEW


Mass balance


Thermal regime


Basal regime



Mechanics of glacier flow



Patterns of glacier flow


Ice streams


Glacier surges



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THE END

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