HR: 14:00h
AN: C12B-02 INVITED     [PDF]
TI: Borehole Observations of Cracks in a Polythermal Glacier
AU: * Fountain, A G
EM: andrew@pdx.edu
AF: Department of Geology, Portland State University, Portland, OR 97207 United States
AU: Schlichting, R B
AF: Science Department, Cleveland High School, Portland, OR 97212 United States
AU: Jacobel, R W
AF: Department of Physics, St Olaf College, Northfield, MN 55057 United States
AU: Jansson, P
AF: Department of Physical Geography, Stockholm University, Stockholm, 5 Sweden
AU: Nyman, S F
AF: Department of Geology, Portland State University, Portland, OR 97207 United States
AB: We drilled 53 boreholes in the ablation zone of Storglaciaren, a polythermal glacier in northern Sweden, to characterize the englacial hydraulic system. Eighty percent of the holes connected to an englacial system, as indicated by a drop in water level within the borehole during drilling and subsequent diurnal fluctuations in water level, suggesting input from surface melt. Our expectation was that these connections would be Rothlisberger-type conduits with elliptical cross-sections. To our surprise all englacial connections but one were made to cracks in the ice. The one exception was a Rothlisberger-type conduit intercepted by a borehole drilled 11 m away from a moulin. Video images from a camera lowered into the boreholes revealed that the cracks varied in width from about 1 cm to several cm. However, we believe the warm drill water escaping from the borehole into the cracks during the initial connection melt-enlarged the openings. The plunge of the cracks is quite steep, typically near vertical, but the lateral extent is unknown. Cracks were observed at depths between 20 and 158 m below the glacier surface, with a few relatively close to the glacier bed. The location of each crack was associated with a band of clear ice, suggesting some refreezing during its lifetime. The clear ice also made observations difficult at times. Orientation of the shallow cracks (less than 50 m depth) was parallel to the observed surface crevasses, while deep cracks were generally oriented perpendicular. Most cracks exhibited some natural water movement, speeds of 0.1- 1 cm/s (laminar). Water pressures were generally close to the total overburden pressure of the ice, but were less than the local overburden estimated relative to crack depth. These results suggest that alpine glaciers are highly fractured with cracks existing at all depths. Whether the cracks are formed in situ or are advected from surface areas higher on the glacier is unclear. The cracks are the main conveyor of englacial water suggesting that models of flow through a network of fractures may be more appropriate for glaciers than the classic Rothlisberger approach.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1827 Glaciology (1863)
DE: 1829 Groundwater hydrology
DE: 1863 Snow and ice (1827)
SC: Cryosphere [C]
MN: 2003 Fall Meeting