ESSA06 Chapter 11.doc

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Environmental Science
Nick Eyles

EESA06Chapter 11Structural Geology branch of geology concerned with the shapes arrangemnet and interrelationships of bedrock units and forces that cause themStress and Strain in the Earths LithosphereTectonic forces move and deform parts of the lithosphere particularly along plate marginsStress a force per unit area can only observe stress of rock bodies that are exposed difficult for buried rocksStrain change in size volume or shape or both in response to stressCompressive stress common along convergent plate boundaries and result in rocks being defromed by a shortening strain changes in perpendicular direction to applied stressTensional stress caused by forces pulling away from one another in opposite directions results in stretching or extension of material changes in parellel direction to applied stressTensional stress occurs at divergent plate boundaries since rocks are very weak when pulled apart fractures and faults are commonShear stress produced when stresses act parallel to a plane like putting deck of cards in your hands and then shearing the deck by moving your hands in the opposite directionShear stress occurs along actively moving faultsElastic if a deformed rock body recovers its original shape after the stress is reduced or removed like elastic bandMost rocks behave in elastic way under low stress but after elastic limit is reached the rock is deformed permantentlyDuctile or plastic manner rocks behaving this way will bend under pressure and dont return to original shapeBrittle some rocks will fracture at stresses higher than its elastic limit or greater than strength of the rock happens more at Earths surface which forms faults and jointsSedimentary rock at Earths surface is brittleUnderstanding and mapping geological structures is important for pertroleum mining and for evaluating problems related to engineering decisions and environmental planning bridges nuclear reactor buildings
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