The Scaling of Columnar Joints in Basalt Journal of Geophysical Research, vol. 113, B10203 (2008). doi:10.1029/2007JB005018 . Lucas Goehring and Stephen W. Morris Department of Physics, University of Toronto, 60 St. George St., Toronto, Ontario, Canada M5S 1A7.

We describe field work, analysis and modeling of columnar joints from the Columbia River Basalt Group. This work is focused on the regions around the Grand Coulee, Snake River, and Columbia Gorge, which form parts of this unusually homogeneous and very large sample of columnar basalt. We examine in detail the scaling relationship between the column width and the size of the striae, and relate these quantitatively to thermal and fracture models. We found that the column radius and striae size are proportional to each other, and inversely proportional to the cooling rate of the lava. Near a flow margin, our results put observational constraints on diffusive thermal models of joint formation. Deeper than a few meters into a colonnade, our measurements are consistent with a simple advection-diffusion model of 2-phase convective cooling within the joints, regardless of the direction of cooling. This model allows an accurate comparison of igneous columnar jointing and joints due to desiccation in laboratory analog systems. We also identify a new length scale in which wavy columns can appear in some colonnades. The mechanisms leading to the wavy columns are likely related to those underlying similar wavy cracks in 2D analog systems.

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