ESSA06 Chapter 11.doc

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23 Apr 2012
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EESA06 - Chapter 11
Structural Geology: branch of geology concerned with the shapes, arrangemnet, and
interrelationships of bedrock units and forces that cause them
Stress and Strain in the Earth’s Lithosphere
-Tectonic forces move and deform parts of the lithosphere, particularly along plate mar-
gins
-Stress: a force per unit area (can only observe stress of rock bodies that are exposed,
difficult for buried rocks)
-Strain: change in size (volume) or shape, or both, in response to stress
-Compressive stress common along convergent plate boundaries and result in rocks
being defromed by a shortening strain (changes in perpendicular direction to applied
stress)
-Tensional stress: caused by forces pulling away from one another in opposite direc-
tions, results in stretching or extension of material (changes in parellel direction to ap-
plied stress)
-Tensional stress occurs at divergent plate boundaries (since rocks are very weak when
pulled apart, fractures and faults are common)
-Shear 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
direction)
-Shear stress occurs along actively moving faults
-Elastic: if a deformed rock body recovers its original shape after the stress is reduced
or removed (like elastic band)
-Most rocks behave in elastic way under low stress, but after elastic limit is reached, the
rock is deformed permantently
-Ductile: (or plastic manner) rocks behaving this way will bend under pressure and
don’t return to original shape
-Brittle: some rocks will fracture at stresses higher than its elastic limit or greater than
strength of the rock (happens more at Earth’s surface, which forms faults and joints)
-Sedimentary rock at Earth’s surface is brittle
-Understanding and mapping geological structures is important for pertroleum, mining,
and for evaluating problems related to engineering decisions and environmental plan-
ning (bridges, nuclear reactor, buildings)
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