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Geology 370: Glacial Geology

GLACIER FLOW

 

A. Introduction

Ice temperature characteristic

• PMP: pressure melting point of water

• heat sources:

  • atmospheric heat flux

  • geothermal heat flux

  • frictional heat flux

 

• temperature increases with depth

• thermal classification of glaciers

• warm-based

• cold-based

• sub-polar

Flow mechanisms

Flow patterns

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B. Internal Deformation

Stress

• measure of how hard material is being pushed or pulled by external forces

• force per unit area; measured in Pascals or bars

1 Pa = 1 N/m2 1 kPa = 1000 Pa 1 bar = 100 kPa

1 N = 1 kg m/sec2 or, the force that would impart an acceleration of 1 m/sec/sec to a mass of 1 kg.

• shear stress: acts parallel to surface

= density of ice (~900kg/m3)

g = acceleration due to gravity (9.81 m/s/s)

h = height or thickness of glacial ice

= surface slope

predicting ice sheet profiles

Strain measure of the amount of deformation that occurs due to stress Internal deformation creep (flow): Glen's Flow Law = strain rate A = constant related to ice temperature = shear stress n = exponent with an average value of 3 folding & fracturing

Image credit: Knight, P.G. http://www.petergknight.com/photos/greenland.html Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

C. Basal Sliding Enhanced basal creep Regelation (pressure melting) Factors controlling sliding adhesion bed roughness quantity & distribution of water at bed amount debris embedded in base of glacier

Image credit: Knight, P.G. http://www.petergknight.com/photos/greenland.html

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D. Subglacial Bed Deformation

Deforming layer confined to upper-most part of bed in fine-grained sediments

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E. Flow Patterns

Continuous Movement

vertical dimension	transverse dimension
longitudinal dimension extending flow	compressive flow

Periodic Movement: surging glaciers

• cyclic behavior of ~4% of glaciers

flow velocities 10-100X normal extensive crevassing contorted moraines Bering Glacier, Alaska. 1993 surge terminus advance ~ 36 feet/day; 1995 surge terminus advance ~ 70 feet/day. Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank, Photo ID hgmzl3. http://www.earthscienceworld.org/images

Susitna Glacier Image credit: Austin S. Post (1970) Susitna Glacier: From the Online glacier photograph database. Boulder, Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology. Digital media.

• which glaciers surge?

• why glaciers surge?

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F. Summary