59.2: Types of Stress

The arrows that appear on the tectonic map above display three types of movement. These are the main types of stress that affect the rock of the lithosphere at plate boundaries. Compressional stress takes place where the arrows point toward each other and plates collide. This type of plate boundary is referred to as convergent because the plates are colliding. Tensional stress occurs where the arrows point away from each other and separation occurs between these plates. This type of plate boundary is referred to as divergent as the plates are moving away from each other. Shear stress results where plates attempt to slide past each other in opposite directions. This type of plate boundary is referred to as transform.

Stress is the force being exerted on the rock at each of these boundaries. Strain is the physical change that results in response to that force. The visible strain that we see in the rock is called deformation.

Let’s explain this in visual terms relative to the before/after GIF from the Ridgecrest earthquake above. The Ridgecrest earthquake is related to movement along the transform plate boundary which slices through southwestern California. Movement along this plate boundary also created the infamous San Andreas fault. This transform plate boundary has created a system of faults as it tries to slide a solid slice of southwestern California, which sits on the Pacific Plate, past the rest of California which resides on the North American Plate. This movement produces shear stress. The force of the shear stress caused a fracture to form in the crust which is a fault. The fault is the strain that occured in response to the stress produced by the shearing force. This type of physical fracturing of Earth’s crust is referred to as brittle deformation.

When rock of the Earth’s crust is subjected to increasing stress it passes through 3 successive stages of deformation:

• Elastic deformation where the strain is reversible.
• Ductile deformation (also referred to as plastic deformation); the strain is irreversible.
• Brittle deformation where the strain results in a permanent fracture of the rock.

Figure \(\PageIndex{1}\): Different materials deform differently when stress is applied. Material “A” has relatively little deformation when undergoing large amounts of stress, before undergoing plastic deformation, and finally brittlely failing. The common metamorphic rock gneiss would fall into this category. Material “B” only elastically deforms before brittlely failing. The common igneous rock gabbro would fit here. Material “C” undergoes significant plastic deformation before finally failing brittlely. Most sedimentary rock types fit into this category. (CC BY 4.0; By Steven Earle from Physical Geology.)

Elastic deformation of the crust would not be noticed. Ductile deformation is where things become interesting in terms of visual effects displayed in solid rock (see photo below). When sufficient stress is applied, the response will be a change in shape. This type of deformation is permanent. Brittle deformation, as described above with respect to the Ridgecrest earthquake, occurs when the physical strength of the rock is surpassed and the rock will break resulting in a permanent fracture, or fault.

Figure \(\PageIndex{2}\): Compressive stress has produced folding in the layers limestone, Mt. Kidd, Alberta. The “upfolds” are geologic structures called anticlines and the “downfolds” are synclines. (With permission for educational purposes from Marli Miller Photography.)

Many factors contribute to how a rock will respond to applied stress including the composition and mineral structure of the rock and the temperature of the surrounding environment. Sedimentary rock often contain water in mineral structures like clay and in pore spaces between grains. This allows sedimentary rock to respond to applied stress by deforming ductilely. Most igneous rock, however, is composed of interlocking mineral crystals and contains very little water. The strong fabric of igneous rock will result in rupture to the same applied stress.