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EARTHSC 1G03 (139)

Chapter 11 - Geologic Structures

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Earth Sciences
Maureen Padden

CHAPTER 11 - GEOLOGIC STRUCTURES CAUSES OF ROCK BREAKAGE AND BENDING Stress and Strain in Lithosphere: Tectonic forces cause rock deformation – causing change in orientation, location, and shape.  Stress is measurable force per unit area at a point  Strain is change in size (volume) and/or shape Forces: Compressive Tensional Shear Pushed in f/ opp. Sides Pulled f/ opp. Sides - Causes shortening - Causes stretching or breaking Convergent zones Divergent zones Tranform zones Rocks Response:  Elastic: deformed material recovers original shape after stress is reduced/removed o Elastic limit: the limit to which rock can bend before damage is permanent  Ductile (plastic): bends under stress and remains as such o Permanent damage due to folding or bending of rock layers o Folds: permanent wavelike deformations in layered rocks  High temperatures and pressures  Brittle: rock breaks under stresses o Result in faults or joints; fault: fracture in bedrock along which rocks on one side move/displace relative to the other side  Joint (or joint sets): fracture/crack in bed rock w/o displacement o Compressive forces cause: reverse faulting o Tensional forces cause: faulting o Shear forces cause: strike-slip faulting Rocks response influenced by: stress type, rock type, temperature, pressure, fluids, length and magnitude of stress applied. MEASURING AND RECORDING GEOLOGIC STRUCTURES GEOLOGIC MAPS  Uses standardized symbols and patterns to represent rock types and geologic structures  Geologic maps show the type and distribution of rock types, structural features, ore deposits, etc.  Data obtained either from ground or from remotely sensed data f/ a satellite or airplane STRIKE AND DIP  Sedimentary beds can be deposited as horizontal beds or strata  Tilted due to deposition and lithification  Strike: compass direction of a line formed by intersection of inclined plan and horizontal plane (customarily north)  Angle of dip: measured downward from horizontal plane to the inclined plane  Direction of dip: direction in which the angle of dip is measured GEOLOGIC CROSS-SECTION  Represents vertical slice through portion of Earth GEOLOGIC STRUCTURES FOLDS Fold: bends or wavelike features in layered rock. Fold geometry:  Anticline: upward-arching fold o Oldest layers in middle  Syncline: trough-like fold; downward arching o Youngest layers in middle  Limb: fold contains two limbs – l
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