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Geology 370: Glacial Geology

GLACIAL MELTWATER

A. Introduction

• Water sources:

  • surface: rain, melted snow/firn/ice, runoff from surrounding unglaciated slopes

  • englacial: melting within moulins, crevasses, tunnels due to heat brought in with surficial water; melting due to frictional heat of flowing water

  • subglacial: melting due to frictional heat, geothermal heat, pressure

• Majority of meltwater comes from surface sources

• Creates 3-D drainage network

• primary permeability

• secondary permeability

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B. Supraglacial Meltwater

• Low ice permeability:

  • lakes

  • stream channels

• Moulins & crevasses may disrupt surface drainage pattern

• Surface meltwater may be "hidden" beneath snow

• Most well-developed in ablation zone

Image credit: Scott McGee (2006) The Crevasse Zone http://crevassezone.org/Photos/photos_frameset.htm

Image credit: Scott McGee (2006) The Crevasse Zone http://crevassezone.org/Photos/photos_frameset.htm

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C. Englacial Meltwater

• Occurs in glaciers with a warm or mixed thermal regime

• Opening & closing of crevasses impacts englacial flow

  • fed primarily by moulins & crevasses

  • moulins: upper part straight & vertical; lower parts tilt down-glacier

• Englacial tunnels or small passageways (mm)

• glacier flow tends to close tunnels

• frictional heat from water flow helps keep them open

• size changes depending on water flow

• Flow direction depends on water pressure gradient

D. Subglacial Meltwater Affects: glacier flow velocity sediment erosion, transportation & deposition Discrete systems meltwater occurs in one or a few discrete tunnels or conduits number of channels decreases towards terminus

Image credit: Peter Knight http://www.physicalgeography.org.uk/photos/PhotosGlaciers.html Meltwater portal, Solheimajokul

• R-channels (Rothlisberger channels): cut up into ice

  • eskers: R-channels that have been filled with sediment

• N-channels (Nye channels): cut down into underlying bedrock

Distributed systems

• meltwater flow occurs over a large portion of the glacier bed

• thin water film:

  • minor component of subglacial meltwater

  • locally generated by regelation

• linked subglacial cavities:

• braided network

• porewater flow

Subglacial water pressure

• effective normal pressure

• controls over basal water pressure

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E. Discharge Fluctuations

• Diurnal fluctuations

• Seasonal fluctuations

• Catastrophic outbursts: jokulhlaups

F. Summary