Rock Deformation Chapter 11, Section 1

Factors Affecting Deformation Deformation – all changes in the original shape and/or size of a rock body Stress – the force per unit area acting on a solid When rocks are under stresses greater than their own strength, they begin to deform, usually by folding, flowing, or fracturing Strain – The change in shape or volume of a body of rock as a result of stress Once the elastic limit or strength of a rock is surpassed, it either flows or fractures The factors that influence the strength of a rock and how it will deform include temperature, confining pressure, rock type, and time

Factors Affecting Deformation Rocks deform permanently in two ways: brittle deformation and ductile deformation Rocks near the surface, where temperatures and confining pressure are low, usually behave like brittle solids and fracture once their strength is exceeded – brittle deformation Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an object without fracturing that object The mineral composition and texture of a rock will also affect how it will deform 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

Deformation

Concept Check What is brittle deformation? The strength of a material is exceeded, and the material breaks or fractures.

Types of Stress The three types of stresses that rocks commonly undergo are tensional stress, compressional stress, and shear stress When rocks are squeezed or shortened the stress is compressional Tensional stress is caused by rocks being pulled in opposite directions Shear stress causes a body of rock to be distorted

Types of Stress

Folds During mountain building, flat-lying sedimentary and volcanic rocks are often bent into a series of wavelike ripples called folds The three main types of folds are anticlines, synclines, and monoclines Anticline – formed by the upfolding, or arching, of rock layers Syncline – downfolds, or troughs, found in association with anticlines The angle that a fold or fault makes with the horizontal is called the dip of the fold or fault Monocline – large, step-like folds in otherwise horizontal sedimentary strata Monoclines seem to occur as sedimentary layers have been folded over a large faulted block of underlying rock

Anticlines and Synclines

Concept Check What is a syncline? A down fold or trough

Faults Faults – fractures in the crust along which movement has taken place The major types of faults are normal faults, reverse faults, thrust faults, and strike-slip faults Normal fault – occurs when the hanging wall block moves down relative to the footwall block (result in the lengthening of the crust) Reverse fault – the hanging wall block moves up relative to the footwall block Thrust faults – reverse faults with dips less than 45º (results in the shortening of the rocks) Strike-slip faults – movement is horizontal and parallel to the trend, or strike of the fault surface (caused by shear stress)

Normal Fault

Reverse Fault

Thrust Fault

Strike-Slip Fault

Concept Check What are the major types of faults? Normal, reverse, thrust, and strike-slip

Assignment Read Chapter 11, Section 1 (pg. 308-313) Do Section 11.1 Assessment #1-8 (pg. 313) Print out “Exploration Lab 11 - Investigating Anticlines and Synclines”