Some basic ideas.

Styles of deformation

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Types of deformationWhen strata are caught in the vice of converging tectonic plates they respond by deforming. Rocks can deform in many different ways. They can break with the formation and movement along breaks - called faults. Or they can distort in various ways.


AssyntActive faulting is the main way earthquakes are generated but for every active fault there are hundreds (or thousands) of faults which have moved in the geological past and now lie dormant (the thrust faults of the Moine Thrust Belt are examples of dormant faults). The chief thing about faulting is that all the deformation is taken up in a narrow zone with the surrounding rocks only disturbed by being moved.


AssyntAnother common way that layered rocks deform is by folding. You can do this yourself with a thin plank of wood. If you push the ends of the plank together it will warp - or fold (of course if you push hard enough then the plank can break - congratulations - you've made a fault!). To make folds in this way (called buckling) the role of layering is critical - no layering and something else happens.

Distortion (strain)

AssyntIf a rock has no layering and is simply homogeneous (rather like a pat of butter), if it's squeezed it will just thicken up. If a pat of butter is warm enough you can do this. You can quantify the amount of strain by tracking how the butter pat becomes thick or less wide. If you stamp a circle on the surface of the butter and then distort it, notice that the circle becomes an ellipse. In strata originally circular markers will become ellipses as the rock strains too. This is also a way in which the amount of strain can be charted.

The various types of rock response to deformation can be found in NW Scotland. but why the differences?

One of the most important things governing how a rock will respond is its composition - not only what it is made of (types of grains) but also how they are arranged (layers or random).

Folded turbidites.Then there are the conditions under which it is deformed. The most important of these are pressure (i.e. how deep they were when they were being deformed) and how hot. Depth is important because breaking rocks creates new volume (voids). The further down you go the harder it is to do this - so most faulting in geology is limited to the top 10 km or so in the Earth. There's still another 25 km of crust beneath this in most continents, and over 6000km to the centre of the Earth. So what influences how these deeper rocks deform?

Temperature. The hotter a material in general, the weaker it becomes (take common house-hold materials like chocolate or butter). In geology the a critical point for becoming weaker is passed when a rock reaches 70% of its melting point (measured from absolute zero). As this varies from material to material, the temperature dependence on rock strength is strongly linked to rock composition. Minerals like halite (rock salt) can distort easily under geological conditions at temperatures of less than 100C. In contrast granite or quartzite will need to be at temperatures in excess of 400C to be anything like as weak.

Faulting.And the final controls on how a rock will deform depend on how the deformation is applied (or how the sample is loaded in material science terms). Rocks respond differently if they are extended or compressed. Further, the amount of load can control the response. High stress (lots of force per unit area) tends to break a rock while a low stress may result in a slower distortion . The other chief control is how quickly the rock is distorted (strain rate). Do it quickly and the rock is more likely to break, do it slow and the rock will distort. Again there are plenty of household materials that respond in the same way (e.g. a plastic ruler).

The study of all these responses is called rheology - a relatively easy thing to quantify for household objects but very difficult for rocks under geological conditions - when mountain building events can take place over millions or years rather than minutes and seconds.

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