Department of Physics,
University of Toronto, 60 St. George St., Toronto, Ontario, Canada M5S 1A7.
Columnar jointing is a fracture pattern, best known from locations such as the Giant's
Causeway, or Fingal's Cave, in which cracks self-organize into a nearly hexagonal
arrangement, leaving behind an ordered colonnade. In this thesis observations of columnar
jointing are reported from both a controlled laboratory setting, and in cooled lava
flows. Experiments were performed in slurries of corn starch and water, which form
columnar joints when dried. This drying process is examined in detail, and it is shown
how desiccation leads to the propagation of a sharp shrinkage front. In general, but
with some signicant exceptions, the size of columnar joints is inversely dependent on
the speed of this shrinkage front during their formation. The exceptions, which include
sudden jumps in column scale, show that hysteresis is also important in choosing the
column scale. Novel observations of the 3D structure of joints in starch show that columnar
joints do not settle down to a perfect hexagonal pattern, but rather mature into a
continuously evolving dynamic pattern. This pattern is scale invariant, and the same
statistical distribution of column shapes applies equally to joints in both starch and lava.
Field work was performed to study columnar jointing in the basalts of the Columbia
River Basalt Group and the island of Staffa, and the more heterogeneous lava
flows of Southwestern British Columbia. The widths of columns and the heights of striae (chisel-like
markings that record details of cooling) were examined in detail, and these length
scales are shown to be inversely proportional to each other. An additional length scale,
that of wavy columns, is also first reported here. Based on these measurements, empirical
advective-diffusive models are developed to describe the transport of water in a drying
starch-cake, and the transport of heat in a cooling lava
flow. These models have only a single scaling parameter, the Péclet number,
which relates the fracture front velocity
times the column size to the (thermal or hydraulic) diusivity. In both cases, the formation
of columnar joints occurs at a Péclet number of about 0.2. This model explains the
hundred-fold differences in scale between columnar joints in starches and lavas, and can
be used as a tool for the interpretation of joint patterns in the field.
The Experimental Nonlinear Physics Group / Dept. of Physics / University of Toronto / 60 St. George St. Toronto, Ontario, Canada, M5S 1A7. Phone (416) 978 - 6810