Behavior of Rocks under Different Types of Stress PDF

## Behavior of Rocks under Different Types of Stress ### Key Concepts - Rocks change their shape and volume when they are subjected to stress. - Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. ### Two types of stress: 1. **Lithostati...

## Behavior of Rocks under Different Types of Stress ### Key Concepts - Rocks change their shape and volume when they are subjected to stress. - Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. ### Two types of stress: 1. **Lithostatic stress**: Rock beneath the Earth's surface experiences equal pressure exerted on it from all directions because of the weight of the overlying rock. It is like the hydrostatic stress (water pressure) that a person feels pressing all around their body when diving down deep in water. 2. **Differential (deviatoric) stress**: In many cases, rock may experience an additional, unequal stress due to tectonic forces. ### Three basic kinds of differential stress - **Compressional stress** - stress that squeezes rocks together. Compression causes rocks to fold or fracture (break). Compression is the most common stress at convergent plate boundaries. - **Tensional stress** - stress that pulls rocks apart. Rocks under tension lengthen or break apart. Tension is the major type of stress at divergent plate boundaries. - **Shear stress** - stress that happens when forces are parallel but moving in opposite directions. It is the most common stress at transform plate boundaries. ### Strain or Deformation - When stress causes a material to change shape, it has undergone strain or deformation. - Deformed rocks are common in geologically active areas. - A rock's response to stress depends on the rock type, the surrounding temperature, and pressure conditions the rock is under, the length of time the rock is under stress, and the type of stress. ### Rock's responses to increasing stress - **Elastic deformation**: the rock returns to its original shape when the stress is removed. - **Plastic deformation**: the rock does not return to its original shape when the stress is removed. - **Fracture**: the rock breaks. - With increasing stress, the rock undergoes: (1) elastic deformation, (2) plastic deformation, and (3) fracture. - At the Earth's surface, rocks usually break quite quickly, but deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically. - Sudden stress, such as a hit with a hammer, is more likely to make a rock break. Stress applied over time often leads to plastic deformation.