Constraints on formation of columnar joints in basaltic lava

Columnar joints form as a brittle relaxation response to tensile stresses within cooling lava flows and magma bodies, and are found in lavas that vary greatly in chemistry and outcrop geometry. However, columnar joints do not form in all cooling igneous rocks, and the specific conditions under which columnar joints form are unknown. In this study, outcrops containing columns in the Cheakamus Valley basalt flows near Whistler, BC are studied, and the size, orientation, and distribution of columns is recorded. Forward numerical models using the finite element method are created with Matlab using the Partial Differential Equation Toolbox to model the outcrops in the Whistler field area, and determine the cooling rates (∂T/∂t) and thermal gradients (∂T/∂x) experienced by the lava flows during their formation. High temperature experimentation involving basalt rock samples is then used to determine the cooling rates and thermal gradients present during the cooling of these samples under a variety of naturally occurring conditions. This study finds that noticeable differences in the distribution of columns within an outcrop occur only when there are large differences in cooling rates between the upper and lower outcrop surfaces. Modeling shows that the cooling rates must differ by approximately an order of magnitude. High temperature experiments show that extremely high cooling rates (especially in the small sample sizes used in this study) between approximately 700 to 800 ˚C are necessary for the formation of columnar joints.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat