11.1 – Rock Deformation

Do Now Describe the different types of stress.

Do Now Describe the different types of stress. Tensional Compressional Shear

Key Words Influence Formation

Vocab Words Anticline Deformation Monocline Normal Fault Reverse Fault Strain Stress Strike-slip Fault Syncline Thrust Fault

Rock Deformation Mountains provide some of the most spectacular scenery on our planet. It is theorized that all continents were once mountainous masses and grow by the addition of mountains to their edges As geologists unravel the secrets of mountain formation, they also gain a deeper understanding of the evolution of Earth’s continents.

Factors Affecting Deformation Factors that influence the strength of a rock and how it will deform include temperature, confining pressure, rock type, and time. Deformation is a general term that refers to all changes in the original shape and/or size of a rock body. Most crustal deformation occurs along plate margins. Stress is the force per unit area acting on a solid. Strain is the change in shape or volume of a body of rock as a result of stress.

Temperature and Pressure Rocks deform permanently in two ways: brittle deformation and ductile deformation. Brittle deformation is the fracturing of an object once its strength is exceeded. Rocks near the surface, where the temperatures and confining pressures are low Ductile deformation is a type of solid state flow that produces a change in the size and shape of an object without fracturing the object. Rocks, where temperatures and confining pressures are high

Rock Type Mineral composition and texture of a rock also greatly affect how it will deform. Rocks like granite and basalt that are composed of minerals with strong internal molecular bonds usually fail by brittle deformation. Rocks that are weakly cemented, like sedimentary rocks or metamorphic rocks, contain zones of weakness and fail by ductile deformation.

Time Forces that are unable to deform rock when first applied may cause rock to flow if the force is maintained over a long period of time. In nature small stresses applied over a long period of time plays an important role in the deformation of rock.

Types of Stress Rocks are exposed to many different forces due to plate motions. The three types of stress are tensional, compressional and shear. Tensional – when rocks are pulled in opposite directions Compressional – when rocks are pushed together or squeezed. Shear – when rocks are distorted from different pulling.

Types of Stress

Folds The three main types of folds are: Anticline– are most commonly formed by the upfolding, or arching, of rock layers. Syncline– are linear downfolds in sedimentary strata. Found along with anticlines Monocline – are large step-like folds in otherwise horizontal sedimentary strata.

Folds

Folds

Faults The major types of faults are: Normal– occur when the hanging wall block moves down relative to the footwall block Reverse– are faults in which the hanging wall block moves up relative to the footwall block Thrust – are reverse faults with dips less than 45 degrees Strike-slip - are faults in which the movement is horizontal and parallel to the trend, or strike, of the fault surface

Faults

Joints Joints are fractures along which no appreciable movement has occurred.

Group Activity Copy and label the following pictures: p. 309 Figure 2