Pfeffer, W.T., 1988

Publication Details

  • Title:

    Structure and deformation in a propagating surge front
  • Authors:

    Pfeffer, W.T.
  • Publication Date:

    1988
  • Publisher:

    University of Washington, Seattle 
  • Ordering Info:

    Not available
  • Quadrangle(s):

    Mount Saint Elias; Yakutat

Bibliographic Reference

Pfeffer, W.T., 1988, Structure and deformation in a propagating surge front: University of Washington, Seattle, Ph.D. dissertation, 134 p.

Abstract

Detailed daily measurements of geometry and velocity made at the terminus of Variegated Glacier, Alaska, during the latter part of the 1982/83 surge showed that thick (~100 m), rapidly sliding (~30 m d-1) ice entered thin (~50 m) stationary ice, producing high longitudinal strain rates (~-0.1 d-1). The associated total strain reached a maximum of 0.75 (logarithmic). Finite element calculations showed that horizontal velocities were essentially constant with depth. Discrepancies between measured and calculated vertical velocities indicate that volume was not conserved in the surge front, and that void formation caused an increase in glacier volume of about 12%. Calculated basal stresses show anomalously high basal shear at the leading edge of the front, and anomalously low basal shear in the central part of the front. Basal normal stresses are essentially overburden. Simplified analytical modelling based on depth independent velocity and continuity provides simple estimates of finite strain versus depth and indicates that a front may propagate through ice at a steady speed and with unchanged shape, provided variables such as initial thickness and upstream velocity do not vary. The observed variations in shape and speed of the front are consistent with changes in these variables. A structural study made after the surge showed that macroscopic structures in the terminus include recognizable primary layering, and two sets of foliation developed during the surge: a transverse bubble foliation, which was well developed in all ice experiencing compressive strains of more than about 0.1, and a pervasive set of longitudinal cracks and associated bubble traces caused by transverse extension. C-axis fabrics in the ice affected by the surge and containing the transverse foliation resemble fabrics seen in ice subjected to plane strain compression near 0oC in other glaciers and experiments, and have symmetry consistent with the transverse bubble foliation. Neither foliation is explained by passive reorientation of preexisting inhomogeneities by large compressive strains, which is commonly supposed to be the cause of foliation in glacier ice. These foliations were developed rapidly by high strain rate.

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