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SUMMARY:Mechanisms of englacial conduit formation and their implications f
 or subglacial recharge - Speaker to be confirmed
DTSTART:20090422T153000Z
DTEND:20090422T163000Z
UID:TALK17850@talks.cam.ac.uk
CONTACT:Poul Christoffersen
DESCRIPTION:Ideas about the character and evolution of englacial drainage 
 systems have been deeply influenced by the theoretical model developed by 
 Shreve (1972). The Shreve model is based on three main assumptions: (1) en
 glacial drainage is in steady state\; (2) englacial water will flow along 
 the steepest hydraulic gradient within the glacier\; and (3) pressure head
  equals the pressure of the surrounding ice minus a small component due to
  melting of the walls. The Shreve model has been widely adopted as a funda
 mental component of englacial drainage theory.\n\nTo evaluate Shreve's the
 ory\, we used speleological techniques to directly survey englacial condui
 ts. We have explored more than 45 distinct englacial conduits to ice depth
 s of 110 m and mapped a total of 9.5 km of passage in 28 conduits in tempe
 rate\, polythermal\, cold-based and debris-covered glaciers between 2005 a
 nd 2009. New information reported here is supplemented by published data o
 n 40 other englacial conduits located worldwide and surveyed to ice depths
  of 203 m using speleological techniques. In all cases\, englacial drainag
 e systems consisted of a single unbranching conduit. Englacial conduit mor
 phologies were found to be intimately linked to the orientation of a glaci
 er's principal stresses or the presence of pre-existing lines of high hydr
 aulic conductivity. If a sufficient supply of water is available\, hydrofr
 acturing forms vertical conduits in zones of longitudinal extension and su
 bhorizontal conduits where longitudinal stresses are compressive. On unfra
 ctured glacier surfaces\, relatively shallow subhorizontal conduits with m
 igrating nickpoints form by cut-and-closure provided channel incision is s
 ignificantly faster than surface lowering. Conduits can also form along pe
 rmeable debris-filled crevasse traces that connect supraglacial lake basin
 s of different potential. Only conduits formed by extensional hydrofractur
 e were found to be connected to glacier beds. Our results suggest that a S
 hreve-type englacial drainage system probably does not exist and implies t
 hat englacial conduits can only penetrate through thick ice to recharge th
 e bed of the Greenland Ice Sheet where supraglacial water bodies either in
 tersect\, or are advected through\, zones of acceleration.
LOCATION:Scott Polar Research Institute\, LIBRARY (1ST FLOOR)
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