Bendy Rocks

Folded rocks in Glen Gairn, Scotland

Solid as a rock or rock solid are expressions that are all too frequently used, abused and misused. For example describing a premiership football teams back four as a rock solid defence may be stretching the bounds of metaphor to breaking point, but such metaphor is in frequent use on the evening footy shows. And besides, solid as a rock is a bit of a misconception anyway.

Contorted Man 'O' War Gneiss from the lizard Peninsula, Cornwall.  Sample 15cm

The idea of rock being able to bend or flow seems a little counterintuitive, but is it? Solid state flow is something that we have been aware of (even if we could not explain it) for centuries. It has long been noted that stained glass windows (Original ones that is) in ancient country churches are thicker at the bottom than they are at the top. The glass has flowed – admittedly at an extremely slow rate – under the influence of gravity. That example takes place over a period of hundreds of years – over thousands or millions of years, under the influence of gravity, pressure, heat, and other forces such as pulling, stretching and offset lateral pressure, rocks can do the seemingly impossible. The gigantic nappes, folds, and antiform/synform structures observed in the Grampian Highlands of Scotland are classic examples of just this.

An agonizingly contorted pebble of Migmatite from Nigg Bay, near Aberdeen.  Sample 6cm 

So why does rock bend? In short, it's a matter of Competence. The pressure and heat generated by the Grampian Phase (the mountain building episode that produced the original Grampian mountains) did several things including changing the chemical and mineralogical nature of the Dalradian sediments (the bedrock at that time). It also contorted those bedrocks in seemingly impossible ways. Rocks bend as a reaction to pressure because of competency contrast. Competency is a measure of how rocks behave under pressure. Competent rocks are more viscous, maintain their thickness when deforming and may fracture, incompetent rocks are more ductile and will flow more easily. The contrast between the competency of different rock layers dictates how a rock sequence will adjust to applied pressure. In the Grampian Highlands, evidence can be seen of folding on a scale of a few millimetres to metres to a scale of many kilometres.

Microfolds in Dalradian Metasediment from Glen Gairn, Scotland.  Sample 2cm across

At a far smaller scale though, why are even individual mineral grains able to be distorted by these pressures? This question is at the heart of solid state flow, which is in turn a vital part of what keeps the Earth a dynamic living planet. We have all seen pictures, film footage (or maybe even in real life) of molten rock – lava – pouring out of volcanic vents, but this is just the depressurized surface manifestation of a far more significant flow. The rocks of the Earths mantle flow, but they are not molten. The pressure in the mantle is far too great for the rocks to melt. The flow, which results from the intense heat with in the mantle creating convection currents, happens because at those temperatures and pressures, any imperfections in crystalline structures (known as dislocations) allow the material to deform in a plastic manner rather than a brittle manner (as they would under applied pressure at the surface at room temperature). The slow, microscopic, high pressure creep is what keeps the dynamic Earth in operation.