4.1: Stress and Strain

Forces and Stress

Stress is a force distributed over an area. Rocks are subject to stress—mostly related to plate tectonics but also to the weight of overlying rock.

Stress can fall into two categories: normal stress, which acts at right angles to a surface, and shear stress, which acts parallel to a surface. Normal stress can be subdivided into compression, in which rock is squeezed, and tension, where rocks are being pulled apart. Where plates collide, rocks experience compression, and where plates diverge, rocks experience tension. Shear stress is common at transform plate boundaries where plates move side by side.

We can describe the stress applied to a rock by breaking it down into three dimensions—all at right angles to one-another (Figure \(\PageIndex{1}\)). If the rock is subject only to the pressure of burial (the lithostatic stress), the stresses in all three directions will likely be the same. If it is subject to both burial and tectonic forces, the pressures will be different in different directions.

Figure \(\PageIndex{1}\): Shear stress acts parallel to surfaces, common at transform boundaries where plates slide past each other. Normal stresses act at right angles to surfaces and include compression, which occurs at convergent boundaries, and tension, which occurs in rifts and areas under extension. "Illustrations of Stress" by Karla Panchuk is licensed under CC BY 4.0. Access a detailed description.

Strain and Deformation

Rock’s response to stress is called strain (deformation), but not all stress results in strain. Several factors influence how rocks respond to stress, including composition, physical properties, temperature, the amount and duration of stress, and the rate at which stress is applied.

Rock can respond to stress in three ways: it can deform elastically, deform plastically, or break (fracture) Figure \(\PageIndex{2}\). Elastic strain is reversible; if the stress is removed, the rock will return to its original shape, just like a rubber band that is stretched and released. However, plastic strain is not reversible. As already noted, different rocks at different temperatures will respond in different ways to stress. Higher temperatures generally lead to more plastic behavior. Some rocks or sediments respond more plastically when they are wet. Another factor is the rate at which the stress is applied. If the stress is applied quickly (for example, because of an extraterrestrial impact or an earthquake), rocks are more likely to fracture.

Figure \(\PageIndex{2}\): Responses of geological materials to stress. Dashed lines show elastic strain, curved lines show plastic strain, and X marks the fracture point. A deforms little and fractures at high stress. B is brittle, with no plastic deformation and fractures after minimal elastic strain. C deforms the most, fracturing after significant elastic and plastic strain. Diagrams on the right show A and C before breaking, and B after breaking. "Responses of Geological Materials" by Steven Earle is licensed under CC BY 4.0.

The response from placing rock under stress is highly variable. The rock might fracture or break without significant movement of either side of the rock. Fracturing is particularly common in volcanic rock, which shrinks as it cools. Sometimes there is significant movement and faulting can occur (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): This fence at Point Reyes National Seashore was laterally offset during the 1906 San Francisco earthquake. The original fence has been replaced but in the same location. "1906 Earthquake - Offset Fence" by Dave Schumaker via Flickr is licensed under CC BY-NC-ND.

Rocks are sometimes stretched or squeezed, tilted, or even folded (Figure \(\PageIndex{4}\)). The varieties of deformation that can occur when a rock is under stress can be truly amazing.

Figure \(\PageIndex{4}\): A syncline in California’s Mojave Desert. Synclines are folds where the limbs dip inward toward the center of the fold. Here, the youngest rocks are near the top and center of the fold. “Barstow Syncline” by Allison Jones is licensed under CC BY 4.0.

Measuring Deformation

Much like crime scene investigation, unraveling deformation in rocks helps us understand the geological history of a region. For this reason, it is important for geologists to quantify deformation by measuring the orientations of geological features. One of the key features to measure is the orientation of bedding. Sedimentary beds are deposited in horizontal layers, so if the layers are no longer horizontal, then we can infer that they have been affected by tectonic forces and have become either tilted, or folded. The orientation of a bed (or any other planar feature) can be expressed with two measurements: first, the compass orientation of a horizontal line on the surface—the strike—and second, the angle at which the surface dips below a horizontal plane —the dip (Figure \(\PageIndex{5}\)). Dip is measured in a plane perpendicular or at a right angle to the strike.

Figure \(\PageIndex{5}\): Strike and dip for tilted sedimentary beds. Water provides a horizontal surface. The strike and dip symbol is a T with the long horizontal bar representing the strike direction, and the small tick mark indicating the dip direction. The dip angle is written next to the tick mark. "Strike and Dip Map Symbols" by Karla Panchuk, a derivative of "Strike and Dip", is licensed under CC BY 4.0. Access a detailed description.

To better understand strike and dip, it may help to imagine a vertical surface, such as a wall in your house. The strike is the compass orientation of the wall and the dip is 90˚ from horizontal. If you could push the wall so that it is leaning over, but still attached to the floor, the strike direction would be the same, but the dip angle would be less than 90˚. If you pushed the wall over completely so it was lying on the floor, it would no longer have a strike direction and its dip would be 0˚. When describing the dip it is important to include the direction. In other words. if the strike is 0˚ (i.e., north) and the dip is 30˚, it would be necessary to say “to the west” or “to the east.” Similarly if the strike is 45˚ (i.e., northeast) and the dip is 60˚, it would be necessary to say “to the northwest” or “to the southeast.”

Strike and dip are also used to describe any other planar geological features, including joints (fractures without displacement) and faults (fractures with displacement; see Jointing and Faulting). Strike and dip are just two of the many measurements geologists record on maps to show structural relationships. You can find more information on strike and dip and other features and measurements that appear on geologic maps in the Appendix. Figure \(\PageIndex{6}\) shows an example of how to depict the beds that make up an antiform fold on a map.

Figure \(\PageIndex{6}\): Cross section (top) and map (bottom) showing rock layers folded into an anticline (convex up fold). North is at the top of the map, with strike and dip symbols indicated. An igneous dike dips 80 degrees to the west. See text for detailed descriptions of the cross-section and map. "Cross-section to Map" by Steven Earle is licensed under CC BY 4.0.

The beds on the west (left) side of the map are dipping at various angles to the west. The beds on the east side are dipping to the east. The middle bed (light gray) is horizontal; this is denoted by a cross within a circle. The igneous intrusion is dipping at 80˚ to the west.

Query \(\PageIndex{1}\)