HR: 1340h
AN: V33A-1463    [Abstracts]
TI: Length Scales in Columnar Jointing of the Columbia Basalt Group
AU: * Goehring, L
EM: goehring@physics.utoronto.ca
AF: University of Toronto, Department of Physics 60 St. George Street, Toronto, ON M5S1A7 Canada
AU: Morris, S W
EM: smorris@physics.utoronto.ca
AF: University of Toronto, Department of Physics 60 St. George Street, Toronto, ON M5S1A7 Canada
AB: We report a set of observations and interpretations of the pattern of columnar jointing as displayed in the Columbia Basalt Group of Washington and Oregon. This pattern of nearly hexagonal prismatic columns of rock originates when lava pools, and slowly cools from its surfaces. Fractures initiate at the cooling surfaces, and advance periodically into the flow. The length scale of each fracture advance is believed to be related to the height of the elastic layer between the already fractured rock, and the still compliant interior of the flow. This sequential advance leaves behind alternating bands of rough and smooth rock, called striae, on the sides of the columns. Field observations were made along the Grand Coulee, and in the Snake and Columbia River valleys, investigating the relationship between striae height and the face widths of individual columns. We confirm that, on average, there is a significant correlation between the striae height and the column width. However, this correlation can be misleading, as striae heights can vary by over a factor of three on some columns -- without corresponding changes in column widths. Observations made near the edges of flow units show a tendency of the striae width to increase away from the flow margins. We suggest that, although both striae and face length scales may be linked to the cooling rate of the flow, the striae length scale can more readily adapt to changing cooling conditions than can the column width. This hypothesis is supported by recent experiments in laboratory analog systems. We also report tentative observations of a zig-zag modulation visible on some column faces. The height of each zig (or zag) presents a length scale 3 to 5 times larger than that of corresponding striae heights, and is about half the size of the column widths.
DE: 8429 Lava rheology and morphology
DE: 5104 Fracture and flow
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting