  Volume alteration in solid materials is a common cause of material failure.
Here we investigate the crack formation in thin elastic layers attached to a
substrate. We show that small variations in the volume contraction and
substrate restraint can produce widely different crack patterns ranging from
spirals to complex hierarchical networks. The networks are formed when there is
no prevailing gradient in material contraction whereas spirals are formed in
the presence of a radial gradient in the contraction of a thin elastic layer.
