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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
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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
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
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
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)
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”
