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Evolving crack patterns : mud cracks, columnar joints, and polygonal terrain
Contraction cracks can form captivating patterns, such as the artistic
craquelure sometimes found in pottery glazes, to the cracks in dried mud,
or the polygonal networks covering the polar regions of Earth and Mars.
Two types are frequently encountered : those with irregular rectilinear
patterns, such as that formed by an homogeneous slurry when dried (or
cooled) uniformly, and more regular hexagonal patterns, such as those
typified by columnar joints. Once cracks start to form in a thin
contracting layer, they will sequentially break the layer into smaller and
smaller pieces. A rectilinear crack pattern encodes information about the
order of cracks, as later cracks tend to intersect with earlier cracks at
right angles. In this manner they relieve the stresses perpendicular to
the pre-existing crack. In a hexagonal pattern, in contrast, the angles
between all cracks at a vertex are near 120°. In this presentation it
will be shown how both types of pattern can arise from identical forces,
and that a rectilinear, T-junction dominated pattern will develop into to
a hexagonal pattern, with Y-junctions, if allowed to. The ordering of
crack patterns are seen in a number of systems : columnar joints in starch
and lava ; desiccation cracks in clays that are repeatedly wetted and
dried ; cracks in eroding gypsum-cemented sand layers ; and the cracks in
permafrost known as polygonal terrain. These patterns will each be briefly
explored, in turn, and shown to obey the above principles of crack pattern
evolution.