Rock Flows

Reverse faultWhen you compress rock – and by “you” I really mean vast tectonic forces, and not your literal thumb and forefinger – it will tend to deform in such a way as to reduce its size in the direction of force. There are two broad categories of deformation. In brittle deformation the rock breaks and moves along the resultant fault. The scale of this movement varies considerably: while not produced from compression, the Alpine Fault in the South Island displaces the rock on either side by hundreds of kilometres and presently moves at a rate of tens of millimetres a year, but these processes go as small as individual tiny fractures in a rock. While the scales might be impressive, brittle deformation is not all that alien to our experience – everyone knows that rock breaks.

FoldingAt the other end of the spectrum is ductile deformation, including the process of folding. In this case, when compression is applied the rock layer gains waves like the folds in a rug. The scale of these folds can range from mountains down to individual crystals. But this latter kind of deformation is more than a little counter-intuitive: in our experience, rock generally doesn’t bend.

John D. Morris, the ICR’s president and holder of a PhD in geology, writes a regular geological column in his organisation’s Acts & Facts magazine. Several of those articles have recently exploited this potential for incredulity to put forward an explanation of the observed folding of rock that is more consistent with his creationist beliefs. While faulting can potentially happen in an instant, folding takes time – something which young Earth creationists famously lack. Continue reading