Rock Deformation notes
Rock Deformation notes GEOL 101 001
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GEOL 101 001
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This 5 page Class Notes was uploaded by Madeline Wilson on Friday February 19, 2016. The Class Notes belongs to GEOL 101 001 at University of South Carolina taught by Dr. Knapp in Spring 2016. Since its upload, it has received 27 views. For similar materials see Introduction to the Earth in Geology at University of South Carolina.
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Date Created: 02/19/16
Rock Deformation Phylite is a Low Grade metamorphic rock. Normal Fault Geological Folds Outcrop – exposure of bedrock without being obscured by soil or loose sediments or boulders Folds – imply that an original planar structure, such as a sedimentary bed, has been bent, most typically found in mountain beds Geological Faults Fault – a fracture in earth’s crust resulting from the displacement of one side with respect to the other Involves relative movement of geological layers How Do Rocks Deform? Stress – force per unit area applied to a rock Strain – change in shape and/or volume of a rock body as a result of applied stress Strength – ability of an object to resist deformation, compressive or tensile Types of Stress Compression – pushed together Tension – pulled apart Shearing – moving sideways Deformation of Rocks Folds and faults are geologic structures caused by deformation Geologic folds and faults can range in size from cm to tens of km or more Structural geology is the study of the deformation of rocks and its effects How do we measure deformation? Many rocks occur naturally in layers 1. Sedimentary rocks 2. Lava flows 3. Metamorphic rocks (foliated) Many features within rock bodies are either planar or linear 1. Minerals 2. Foliation Orientation of Deformed Rocks: Strike & Dip Dip is the angle of steepest descent of the bed from the horizontal Strike is a line at right angles to the dip direction Types of Deformation Elastic: temporary change in shape or size of a body that is recovered when the stress is removed Ductile (plastic): permanent change in shape or size of a body that is not recovered when the stress is removed (smooth, continuous plastic deformation) Brittle (rupture): the body undergoes little change under the stress, until it breaks suddenly (earthquakes) Rock Deformation Deep crustal rocks are more likely to deform ductilely than shallow crustal rocks Hotter rocks are more likely to deform ductilely than cooler rocks Most sedimentary rocks are more deformable than igneous rocks Normal Fault Hanging wall moves down the fault plane with respect to the footwall The “normal” type of movement we would expect as a result of gravity Typical of extensional strain resulting from tensional forces Reverse Fault Hanging wall moves up the fault plane with respect to the footwall The “reverse” type of movement we would expect as a result of gravity Typical of contractional strain resulting from compressional forces Strike-slip Fault No hanging wall or footwall because faults are subvertical Blocks move past one another in the direction the fault is striking Typical of shear strain resulting from shearing forces Can be either right-lateral or left-lateral Styles of Continental Deformation Tensional tectonics – extension of continental crust produces normal faults with high dip angles in the upper crust that flatten with depth, forming curved fault surfaces Compressive tectonics – compression of continental crust occurs on low- angled thrust faults - a thrust fault is a low angle reverse fault Shearing tectonics – shearing of continental crust occurs on a nearly vertical strike-slip fault Geologic Maps Geologic maps represent the rock formations exposed at earth’s surface A common scale for geologic maps is 1:24,000 Geologic Cross Sections Diagrams showing the features that would be visible if vertical slicer were made through part of the crust Ductile Deformation Ductile materials undergo smooth, continuous, plastic deformation and do not spring back to their original shape when the deforming force is released Folding of Rocks Upfolds or arches of layered rocks are called anticlines (a sequence of folded rocks with the oldest rocks on the inside of a fold) Downfolds or throughs are called synclines (a sequence of folded rocks with the youngest rocks on the inside of the fold) Types of Folds Symmetrical folds – anticlines and synclines, limb – the sides of a fold, axial plane – an imaginary plane that divides the fold as symmetrically as possible Asymmetrical folds Overturned folds Plunging folds Unraveling Geologic History Geologic history is a succession of episodes of deformation and other geologic processes Can be described in time steps Factors that Affect Deformation Temperature Pressure Rock type Strength Some rocks are stronger than others Competent rocks – rocks that deform only under great stresses Incompetent rocks – rocks that deform under moderate to low stresses Joints Fractures in rocks along which there has been no appreciated displacement Rocks on either side of a fault have moved, whereas rocks on either side of a joint have remained stationary Styles of Continental Deformation Rift Valley (African and Arabian plates) – tension forces create a rift valley, the result of downfaulted blocks Keystone Thrust Fault – compressive forces create a fault in rock layers thrusting them over a section of the same rock, older layers now overlay younger layers San Andreas Fault (Strike-Slip) Armenia Earthquake, 1988, Inverse Fault Appalachian Mountains: Remnants of a Folded Mountain Belt Dating the Order of Deformation Use geometry: 1. Inclusions 2. Cross-cutting relationships Combine with fossils and radiometric dating