Structural geology

In broad terms, an entire field of geology relating to how rocks bend (fold) and break (fault) is called structural geology.
Some rocks are more resistant to deformation (bending, breaking), and some are less resistant;  it often is linked to their composition and the arrangement of mineral grains within the rock.
In addition to the rock itself, deformation of rocks is dependent upon external influences– notably temperature and stress-regime (meaning directed pressures or pushes!), but also fluid content.
Another important aspect of deformation is the rate at which directed pressure is applied.

Perhaps you’re familiar with the chemistry (or bar-tending) tool of the glass stirring rod?

Of course, if you take a glass rod and quickly bend it, the result is that the rod breaks.
However, if you could apply a relatively weak stress (force) to the rod over a long period of time, perhaps by taping the rod to a bench, and then hanging a small weight on one end, THEN the rod would eventually bend!

This is rather like the old windows that we see in some ancient houses, where the window might be clean, but when looking through it objects appear kind of wavy.  This is because the glass has sunk, and has deformed without breaking!  

Rocks work the same way– if forces are applied “quickly” then the rocks break, and this is what we refer to as faulting.  If instead the forces are not too strong, and occur over long periods of time, and temperatures are elevated, then the rocks can bend, or what we call folding.  Below, in the image associated with the section “Crustal Deformation” we see rocks that have bent, or folded.

Crustal Deformation

Figure 1. Chevron folds with flat-lying axial planes, Millook Haven, North Cornwall, UK

Crustal deformation refers to how rocks in the crust have been deformed– either broken (faulted) or bent (folded).

What sort of forces are necessary to fold rocks and, ultimately,  create mountains?
This section takes a look at these processes.
We will investigate various types of folds as well as different types of faults. If you recall from the plate tectonics sections, earthquakes occur along active fault lines– and in fact, earthquakes are all about the release of energy when rocks break!

Learning Outcomes

• Compare and contrast stress versus strain in the Earth’s crust.
• Identify geologic structures created by deformation.