Physics of Glacier Flow

Physics of Glacier Flow

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How Glaciers Flow

- Glaciers flow under the force of gravity as snow accumulates on the upper parts of the glacier and wants moves down slope.

- The snow compresses to become ice and flows through the glacier into the ablation zone where it is lost.

- If the accumulation equals the ablation than the glacier is said to be in equilibrium and its position will not change. This does not mean that the ice will not flow!

Accumulation Zone
The area where inputs occur into a glacier system. This usually occurs near the top of the glacier or ice sheet and such inputs to the system include snowfall, wind blown snow, rain and avalanches.

Ablation Zone
The region in which more mass is lost than gained in a glacier system. This usually occurs at the end and sides of the glacier. Forms of losses include wind ablation, avalanching, iceberg calving and melting.



Glaciers flow through three different mechanisms: (1) by internal deformation; (2) by basal sliding; and (3) by subglacial deformation.

Glacier Flow Mechanisms

Internal Deformation
Ice deforms under its own weight due to gravity and the movements of tiny ice crystals. Thicker and warmer ice deforms more rapidly although the overall movement is very slow, only around tens of meters a year. There are two main processes of internal deformation; creep, which forms fold structures, and faulting, which occurs when ice cannot creep fast enough and forms superficial tensional fractures.

Basal Sliding
Enhanced Basal Creep - Stress concentrations around the upstream side of an obstacle result in locally high strain rates which causes ice to accelerate around the obstacle. The basal ice continually modifies its shape to allow a continued sliding. This process works best when the obstacle is over 1m in size.
Regelation - The process allows glacier ice to slide over rough beds by melting and refreezing on the downglacier side. It occurs as the most resistance to glacier movement is provided by the upstream side of obstacles. This results in locally high pressures and the consequent encouragement of ice melting immediately upglacier of the obstacle. The resulting meltwater migrates to the lower pressure area downstream where it refreezes. This process is most effective when objects are less than 1m in size.

Subglacial deformation
Sediment has a lower yield strength than rock and ice and so it is deformable. As the sediment deforms, it moves the ice sheet with it.



Ice Velocites

*
The surface velocities of a glacier can be measured quite easily using GPS.

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This was first demonstrated in 1874 by placing stakes in a straight line across the glacier and then coming back later and observing there change in position. This meathod is still used today, execpt that with GPS acurate to withing a few centimeters it can be done in a matter of days instead of years. The change in position over the change in time equals the velocity of the ice on the surface of the glacier.

* This method gives us the velocity of the ice on the surface of the glacier. If we want to know how much ice is flowing through a cross section of a glacier we need to know how fast the ice is flowing throughout the glacier. Through a serires of equations that relate ice velocities to density, gravity, slope of the glacier, and the temperature of the ice Paterson was able to show that the mean velocity of a cross-section is within a few per cent of the mean surface velocity.




Calculating Mass Flux

* The mass flux of a glacier is amount of ice that flows through a cross section in a given amount of time.

For instance on the Llewellyn glacier in British Columbia the measured the surface velocities are seen below. The average of these velocities is .24m/day. According to The Physics of Glaciers is is within a few per cent of the mean cross sectional velocity. Seismic reflection methods were then used to determine the depth and shape of the glacier and a cross sectional area could then be estimated.

* The cross sectional area was determined to be approximately 2060 square meters.
* Mass flux equals cross sectional area times mean cross sectional velocity.

Mass flux = 495 cubic meters per day. Or 180,000 cubic meters per year.



Power of a Glacier

*
Power is equal to energy divided by time.
* The energy of a glacier is its kinetic energy which is expressed as K = ½ mv^2 where m is the mass of the glacier and v is the average velocity.

Using the Llewellyn glacier as an example:

* m = volume times density of ice (918kg/meters cubed)
* m = 70,000,000m^3 x 918 = 64260000000kg
* K = (.5)(64260000000kg)(.00000277m/s)^2 = .2479joules
* P = K/t = .2479joules/1sec = .2479 watts

This can be compared to something more tangible such as a caterpillar D-10 dozer which puts out 580 Hp which is equivalent to .7775 watts.

(www.caterpillar.com)

* If a glacier take 100,000 years to create a large glacier valley this would be 781777440000 watts of power.
* It would take a single caterpillar D-10 31,885 years to produce this much energy.


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