Ductile versus Fragile Rock

• Shear stress – this is when you get slippage or translation on rock. This is most likely to contribute to the twisting of rock in some tangible way.

Rocks deforming means they are under strain. We define strain as any change in the shape, size, or total volume on a rock. Now that you have stress and strain on rock, you can see that it will deform the rock. Rock doesn't deform in a random way. It goes through several stages of deformation as this happens. Let's look at these stages of deformation so you can see how a rock can change as force is applied to it:

• Stage 1 – this is called elastic deformation. Like any deformation, it is completely reversible if the pressure on the rock is removed.

• Stage 2 – this is called ductile deformation. The strain is irreversible by now but the rock is still intact.

• Stage 3 – this is also irreversible and happens as the rock is fractured completely.

DUCTILE VERSUS FRAGILE ROCK

Of course, all rocks do not behave the same. You know intuitively that some rock is fragile and more brittle, while other rocks are stronger and perhaps more stretchy. In geology, we divide these rocks into different types based on how they behave under stress:

• Brittle rock will not be ductile enough to remain intact. It will fracture easily.

• Ductile rock will be more elastic so that it will stretch more or bend before fracturing.

• Temperature affects how ductile a rock will be. Like anything, high temperature means more ductile behavior.

• The presence of confining pressure will diminish the chances of a fracture. Low pressure around a rock will alternatively increase the chances of a fracture.

• Strain rate matters – if the rate of strain is too fast, there is a greater chance for a fracture compared to a slower strain rate.

• Rock composition matter as well – olivine, feldspars, and quartz are all more brittle than clay, calcite, and mica.

• Water presence or absence – if there is water around, the chemical bonds between atoms in a rock are looser so that there is more ductile behavior than you'll see with a drier rock.

Rocks that are closer to the surface of the earth and are higher in quartz content mean that the rock will be more brittle. These rocks, like quartz and feldspar, are strong rocks but not very ductile. Most sedimentary rocks are much more brittle than most igneous rocks and metamorphic rocks.

Deep within the earth, you get to what is called the crustal brittle-ductile transition zone at 15 kilometers deep, below which the rocks are very ductile and total rock strength diminishes. The bottom of the crust has a lot of olivine in it, which is fairly strong, helping the upper mantle layer to be strong here too.

Higher temperatures below the crustal areas and into the mantle of the earth, you then get to a 40 kilometer-deep area, called the mantle's brittle-ductile zone. Rocks in the mantle here are also more ductile deep to that level.

Deformation of rock is such a slow process, you don't see it unless there is an earthquake. Geologists can measure deformation that is somewhat less abrupt in areas where there is change happening in subsidence areas or where there is uplift. It may take years still to succeed in detecting rock deformation.

Deformation is easy to see in areas where there have been sedimentary deposits. Remember the principle of horizontality? If you do not see horizontal layers, you can assume that deformation has happened.