Rocks are subject to stress —forces acting upon them which are related to plate tectonics as well as the weight of overlying rocks. When the stress acting upon a rock causes the rock to become deformed, that deformation is referred to as strain.
Types of Stress
Stresses are sorted into two categories: normal stress acts at right angles to a surface, and shear stress acts parallel to a surface (Figure 13.2). Normal stress is subdivided into compression, when the stresses are squeezing a rock, and tension, when stress is pulling it apart. You would expect to find rocks undergoing compression in regions where plates are colliding, or where they are being buried beneath other rocks. You would find tension where divergence is happening, such as when a continent is beginning the rifting process. Shear stress is characteristic of transform plate boundaries, where plates are moving side by side.
Although Figure 13.2 shows only one set of stress arrows for each scenario, rocks within the Earth are subject to stress from all directions. The relative size of the stresses in different directions will determine how we describe the stress on a particular rock. For example, a rock being stretched as a continent breaks apart might also be buried under hundreds of metres of sediments and other rocks. The weight of the rocks above pushes down on the rock we’re interested in (compression), but the pulling force (tension) is much greater, so we say the rock is under tension.
Rocks experience stress from all directions, but it is possible to break down stresses into three directions, just like a graph with x, y, and z axes. In diagrams showing these three directions, the sizes of arrows representing each direction will indicate the relative size of stresses (Figure 13.3). This makes it much easier to describe the stresses operating on a rock, and to understand what their net effect will be.
How a rock responds to stress depends on many factors. The “how” is not simply a matter of how much strain a rock will undergo, but what type of strain will occur. Is the deformation permanent or temporary? Does the rock break or does it deform without breaking?
Elastic strain is reversible strain. You can think of elastic strain as what happens to the elastic waistband in your favourite sweatpants when you put them on. The elastic stretches to let you in to your pants, and once you’re in them, it shrinks to keep them attached to you. When you take the pants off again, the elastic goes back to its original shape. Similarly, rocks undergoing elastic strain will regain their original shape once the stress on them is removed. As you may recall from the discussion on earthquakes, when rocks snap back to that shape it can have profound consequences and release violent vibrations within the Earth.
Figure 13.4 shows how much of a shape change (strain) results from a given application of stress. In this diagram the segments of lines A, B, and C with dashed lines represent the range of stress for which deformation is elastic, and the materials can go back to their original shape. We will discuss what the different line shapes mean shortly.
If enough stress is applied, the changes that a material undergoes to accommodate the stress will leave it permanently deformed. When the stress is removed, the material does not go back to its original shape. This is referred to as plastic strain. In Figure 13.4, lines A and B have curved segments showing plastic strain. Once stress gets to this point, it isn’t possible to retrace the line to its origin by reducing the stress.
Ductile or Brittle?
Ductile refers to deformation that happens by flowing (e.g., when you roll play-dough into a snake), whereas brittle refers to breaking. In Figure 13.4, the point at which the different materials fracture (break) is marked with an x at the end of each line. The material represented by line B fractures immediately at the end of the region of elastic strain, so we would say it is brittle. In contrast, the materials represented by lines A and C have a period of plastic deformation where they deform for a while without breaking. These materials are ductile.
Weak or Strong?
It can be tempting to think of materials which deform in a ductile fashion as being weaker than brittle materials- contrast play dough with concrete, for example. However, whether a material is brittle or ductile does not tell us anything about how its strength compares to that of other materials. What matters is how much stress is required to cause the material to deform and fail. In Figure 13.4, the materials represented by lines A and C both have regions of plastic strain where they undergo ductile deformation before breaking. Notice that material A fails at higher stress than material B (the brittle material), and C fails at lower stress than B. In this case, the strength of the material B is intermediate between that of A and C even though it is brittle and the others are ductile.
Factors That Determine How A Rock Will Deform
From Figure 13.4 you might conclude that for a given type of rock it is possible to construct a diagram which will tell you how the rock will deform under different amounts of stress. This is partly true- the composition of the rock does affect how it responds to stress. However, there are many other factors that also influence the stress response, including temperature, pressure, whether the rock is wet or dry, and whether the stress is applied rapidly or slowly. What this means is that a diagram like Figure 13.4 is actually constructed for a particular set of conditions. You would have to make a different one if the pressure were different, and a different one yet again if the temperature were different. If the rock were being heated, you would have to account for combinations of pressure and temperature. If you consider the wide range of temperature and pressure conditions that a rock encounters as it buried, heated, and experiences different plate tectonic processes, you can begin to imagine the task that is faced by geologists and engineers who spend time in laboratories attempting to quantify the mechanical behavior of rocks. They have a lot of work to do.
Despite the many factors involved, we can still make some generalizations about how rocks will behave under different conditions. In general, sedimentary rocks are weaker than igneous or metamorphic ones, and will tend to deform in a more ductile fashion than igneous or metamorphic rocks under the same conditions. Higher temperatures and pressures will cause rocks to deform in a more ductile fashion. Water makes rocks weaker and more brittle. Ductile deformation is more likely if stress is applied slowly rather than rapidly.
Stress and Geologic Structures
Many different geologic structures can form when stress is applied to rocks (Figure 13.5). Structures form as a result of fracturing, tilting, folding, stretching, and squeezing. Some structures, like the fractures that make basalt columns (Figure 13.5a), happen when rocks shrink due to cooling, but others are a consequence of plate tectonic forces.
The types of structures that form depend on the plate tectonic setting and other geological conditions, which means they are valuable tools for helping us to understand what actually happened to the rocks. In the following sections we will see how different kinds of structures form, and what information can be gleaned from them.
- You might associate the word plastic with the kind of reusable containers in which you pack your lunch, or the bottles in which pop comes. This can be confusing if you focus on the ability of those containers to be flexible. The word plastic actually refers to the fact that the containers are formed from a material that is molded into a particular shape. ↵