How does the orientation of a rock's stress affect its deformation?

1 Stress and strain

Stress is the force applied per unit area on a rock, and it can be expressed as a tensor, which is a mathematical object that describes the magnitude and direction of the force in three dimensions. Strain is the change in shape, size, or orientation of a rock due to stress, and it can also be expressed as a tensor, which is related to the stress tensor by the elastic properties of the rock. The orientation of the stress tensor determines how the rock deforms under different types of stress.

2 Compressional stress

Compressional stress is the force that squeezes a rock from opposite directions, reducing its volume and increasing its density. Compressional stress can cause rocks to fold, which is a bending or warping of rock layers. The orientation of the compressional stress determines the shape and orientation of the folds. For example, if the compressional stress is horizontal, the folds will be upright and parallel to the stress direction. If the compressional stress is inclined, the folds will be asymmetric and tilted.

3 Tensional stress

Tensional stress is the force that pulls a rock apart from opposite directions, increasing its volume and decreasing its density. Tensional stress can cause rocks to fracture or fault, which is a breaking or sliding of rock layers. The orientation of the tensional stress determines the mode and orientation of the fractures or faults. For example, if the tensional stress is horizontal, the fractures or faults will be vertical and perpendicular to the stress direction. If the tensional stress is inclined, the fractures or faults will be oblique and parallel to the stress direction.

4 Shear stress

Shear stress is the force that slides a rock along a plane, causing distortion and rotation. Shear stress can cause rocks to deform by ductile or brittle mechanisms, depending on the temperature, pressure, and strain rate. Ductile deformation is when rocks flow or deform plastically without breaking, while brittle deformation is when rocks crack or snap under stress. The orientation of the shear stress determines the direction and angle of the shear plane and the sense of movement. For example, if the shear stress is horizontal, the shear plane will be horizontal and the movement will be left-lateral or right-lateral. If the shear stress is inclined, the shear plane will be inclined and the movement will be reverse or normal.

5 Stress and strain measurements

Geologists use a variety of tools and methods to measure and analyze the stress and strain in rocks, such as strain gauges, strain markers, strain ellipses, stress indicators, and stress tensors. Strain gauges measure the change in length or angle of a rock due to stress, while strain markers are natural or artificial features that indicate the amount and direction of strain in a rock. Strain ellipses are graphical representations of the strain tensor that show the principal axes and ratios of strain in a rock; likewise, stress indicators are features that reveal the direction and magnitude of stress in a rock. Stress tensors are mathematical models that describe the stress state in a rock using the stress indicators and the elastic properties of the rock. By examining these factors, geologists can infer the tectonic history, deformation mechanisms, and structural evolution of the earth's crust and mantle.