7.3: Crustal Deformation

Introduction

Enormous stress is imposed on the crust at the boundaries of the lithospheric plates and where convection currents in the mantle tug and tear at the crust above. When a stress is imposed on rock material it will deform (change shape) and often volume. The change in shape as a result of imposing a stress is called strain.

Figure \(\PageIndex{1}\): Folds in quartzite due to compression. (CC BY-SA 4.0; Courtesy Michael E Ritter via The Physical Environment).

There are three basic kinds of stress. Compression occurs when rock masses are pushed together like that which occurs when plates collide. Rocks tend to shorten laterally and thicken vertically when exposed to compressional stress. Compression often creates great folds in rock. Tension, or tensional stress, pulls the crust apart, like that which occurs along diverging plate boundaries. Tension extends the crust causing it to thin and lengthen. Rifting, like that which created the Great Rift Valley of Africa, is a result of tension. When plates slide past one another in opposite directions along transform plate boundaries, a shearing stress is created. Shearing stress cuts the crust into parallel blocks displacing them horizontally relative to one another. Shearing takes place along the San Andreas Fault where the Pacific Plate is moving past the North American Plate.

Digging Deeper into the Behavior of Rock Material

Rocks behave in a variety of ways when stress is imposed on them. The relationship between stress and deformation is shown in stress-strain diagrams like those below. The reaction of rock material to an imposed stress depends on the temperature and pressure conditions. As stress is imposed on rock it starts to deform up to its yield point. Before it gets to the yield point, the rock will undergo elastic deformation (Figure \(\PageIndex{2}\)). Like a rubber band, if the stress is released before reaching the yield point, the rock material will return to its original shape. However, under low temperature and pressure conditions, once the rock reaches its yield point it will break, called brittle failure (Figure \(\PageIndex{3}\)). Brittle failure may occur if stress is imposed suddenly as well.

Figure \(\PageIndex{2}\): Elastic Deformation. (CC BY-SA 4.0; Courtesy Michael E Ritter via The Physical Environment). (Image enhanced using Gemini April 2026).

Figure \(\PageIndex{2}\) is a simple graph illustrating the relationship between stress and strain in a material. The vertical axis is labeled Stress and points upward. The horizontal axis is labeled Strain and extends to the right. A straight diagonal line begins at the origin, where both stress and strain are zero, and rises upward and to the right. The line indicates that as stress (the force applied to a material) increases, strain (the amount the material deforms) also increases. The straight-line relationship shows a proportional response, meaning the material deforms predictably as stress is applied. Function: The graph represents the elastic behavior of a material. In this region, stress and strain are directly proportional, following Hooke’s Law. When the stress is removed, the material returns to its original shape because the deformation is temporary. The slope of the line represents the material’s stiffness, or resistance to deformation.

Figure \(\PageIndex{3}\): Brittle Failure. (CC BY-SA 4.0; Courtesy Michael E Ritter via The Physical Environment). (Image enhanced using Gemini April 2026).

Figure \(\PageIndex{3}\) is a simplified stress–strain graph illustrating how a material responds to increasing stress until it breaks. The vertical axis is labeled Stress and increases upward. The horizontal axis is labeled Strain and increases to the right. A straight diagonal line begins at the origin and rises upward and to the right. This line is labeled Elastic Deformation. At the end of the line is a red dot labeled Yield Point. A label next to the dot indicates Brittle Failure. Function: The graph shows that as stress is applied, the material undergoes elastic deformation, meaning it changes shape but can return to its original form if the stress is removed. Stress and strain increase together in a proportional relationship. The yield point marks the maximum amount of stress the material can withstand while remaining intact. In this example, the material experiences brittle failure immediately upon reaching the yield point. Rather than bending or permanently deforming, it fractures suddenly. The diagram illustrates the behavior of brittle materials, such as many rocks near Earth’s surface, which accumulate stress over time and then break abruptly when their strength is exceeded. This sudden release of stored energy is the fundamental process that produces earthquakes.

Figure \(\PageIndex{4}\): Plastic Deformation. (CC BY-SA 4.0; Courtesy Michael E Ritter via The Physical Environment). (Image enhanced using Gemini April 2026).

Figure \(\PageIndex{4}\) is a stress–strain graph showing how a material responds to increasing stress when it is capable of plastic deformation before breaking.

• The vertical axis is labeled Stress and increases upward.
• The horizontal axis is labeled Strain and increases to the right.

The graph begins with a straight diagonal line rising from the origin. This portion is labeled Elastic Deformation.

At the end of the straight line is a red dot labeled Yield Point. Up to this point, the material deforms elastically, meaning it will return to its original shape if the stress is removed.

Beyond the yield point, the graph becomes nearly horizontal and is labeled Plastic Deformation. In this region, strain continues to increase while stress changes very little. The material is permanently deformed and will not return to its original shape even if the stress is removed.

At the far right end of the plastic deformation region is a blue dot labeled Rupture Point. This point marks the failure of the material, where it finally breaks after undergoing substantial deformation.

The graph illustrates the behavior of a ductile material. Initially, the material responds elastically to stress. Once the yield point is reached, it enters a plastic deformation stage where it bends, stretches, or flows without immediately breaking. Continued stress eventually causes failure at the rupture point.

In geology, rocks deep within Earth may behave in this ductile manner because high temperatures and pressures allow them to deform plastically rather than fracture abruptly. This contrasts with brittle failure, where rocks break suddenly and generate earthquakes.

Upon reaching the yield point under high pressure and temperature conditions, the rock may undergo plastic deformation (Figure \(\PageIndex{4}\)). In this case, once the rock changes shape and if stress is released, it will not return to its original form. During plastic deformation mineral bonds adjust to the stress by breaking, moving about, and then reforming. Once it reaches its rupture point, the material will break.