EESA06 - Chapter Eleven

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Environmental Science
Nick Eyles

Chapter 11 - Page 1 of 7 Chapter Eleven – Geologic Structures  Structural geology: branch of geology concerned with the shapes, arrangement, and interrelationships of bedrock units and the forces that cause them What causes rocks to bend and break? Stress and strain in the Earth’s lithosphere  Tectonic forces move and deform parts of the lithosphere, especially along plate margins  Deformation may cause change in orientation, location, and shape of a rock body  Stress expressed as the force per unit area at a particular point o Hard to measure stress in rocks that are currently buried  We can observe effects of past stress when rock bodies are exposed after uplift and erosion  Strain is the change In size (volume) or shape, or both, in response to stress  Pushed together or squeezed from opposite directions  compressive stress o Common along convergent plate boundaries and typically results in rocks being deformed by a shortening strain o In figure 11.2A, an elongate piece of dough may shorten by bending, or folding, whereas a ball of dough will flatten by shortening in the direction parallel to the compressive stress and elongating or stretching in the direction perpendicular to it. Rocks that have been shortened or flattened are typically found along convergent plate boundaries where rocks have been pushed or shoved together  Tensional stress  caused by forces pulling away from one another in opposite directions o Results in a stretching or extension of material o Ball of dough will elongate or stretch parallel to the applied stress o If applied rapidly, the dough will first stretch and then break apart o At divergent plate boundaries, the lithosphere is undergoing extension as the plates move away from one another o Because rocks are very weak when pulled apart, fractures and faults are common structures  When stresses are parallel to a plane, shear stress is produced o Holding a deck of cards and moving your hands in opposite directions o Results in a shear strain parallel to the direction of the stresses o Occur along actively moving faults How do rocks behave when stressed?  Rocks behave as elastic, ductile, or brittle materials depending on the amount and rate of stress applied, the type of rock, and the temperature and pressure under which the rock is strained  If a deformed body recovers its original shape after the stress is reduced/removed, the behaviour is elastic o Tensional stress on a rubber band Chapter 11 - Page 2 of 7 o Most rocks behave like this at very low stress (a few kilobars) o Once the stress exceeds the elastic limit, the rock will deform in a permanent way  A rock that behaves in a ductile or plastic manner will bend while under stress and doesn’t return to its original shape after relaxation of the stress o Pizza dough behaves like this unless the strain is rapid o Rocks exposed to elevated pressure and temperature during regional metamorphism also behave in a ductile manner and develop a planar texture, or foliation, due to alignment of minerals o Don’t require much of an increase in stress to continue to strain (flat-ish curve) o Results in permanently deformed rocks (folding or bending)  A rock with brittle behaviour will fracture at stresses higher than its elastic limit, or once the stresses are greater than the strength of the rock o Rocks behave like this near the earth’s surface where temps/pressures are low o Rocks here favour breaking rather than bending o Faults and joints form due to the brittle behaviour of the crust  Sedimentary rock at the earth’s surface is brittle and will fracture if hit. If it’s bent then it means that either stress increased very slowly or the rock was deformed under considerable confining pressure (buried under rock) and higher temperatures How do we measure and record geologic structures? Geologic Maps and Field Methods  A geologic map, which uses standardized symbols and patterns to represent rock types and geologic structures, is typically produced from the field map for a given area o Plots type and distribution of rock units, the occurrence of structural features, ore deposits, etc. o Sometimes surficial features, such as deposits by former glaciers, are included, but these may be shown separately on a different type of geologic map Strike and Dip  When originally horizontal rocks are found tilted, it indicates that tilting must have occurred after deposition and lithification  Strike is the compass direction of a line formed by the intersection of an inclined plane with a horizontal plane  Angle of dip – measured downward from the horizontal plane to the bedding plane (an inclined plane); perpendicular to the bedding and horizontal planes  Direction of dip – compass direction in which angle of dip is measured; direction in which a ball would roll down the surface  Dip angle is always measured perpendicular to the strike line  Beds can dip away from the strike line in one of two directions – important to specify the direction  Right hand rule – used to record azimuthal strikes and dips Chapter 11 - Page 3 of 7 o States that strike is recorded in the direction such that the dip is down to the right o Necessary for input into computer mapping programs  Brunton pocked transit contains a compass, a level, and a device for measuring angles of inclination; used for measuring strike and dip Geologic Cross-Sections  Represents a vertical slice through a portion of earth  Constructed from maps by projecting the dip of rock units into the subsurface and is useful in helping visualize geology in 3D Remote Predictive Mapping and 3D Modelling of Geologic Structures on Baffin Island  Investigating art of the Canadian Shield  Hyperspectral satellite images allow discrimination of potentially economically valuable rock types WHAT DO FOLDED ROCKS TELL US?  Folds – bends or wavelike features in layered rock  Often seen in roadcuts or other exposures  When the arches/troughs of folds are concealed (or exist on a grand scale), we can determine the presence of folds by noticing repeated reversal in the direction of dip taken on outcrops in the field or shown on a map  Rock behaved in a ductile manner; yet, rock exposed in outcrops is generally brittle and shatters when hit o Rock isn’t metamorphosed  Folding took place when it was buried at a moderate depth with high temp and pressure  Could also have taken place near the surface under very low strain Geometry of Folds  Folds are usually associated with compressive stresses along convergent boundaries, but are also commonly formed where rock has been sheared along a fault  Folds are wave-like forms; two basic fold geometries o Anti clines  Upward arching fold  Usually the rock layers dip away from the hinge line (axis) of the fold o Synclines  Downward arching fold (trough like)  Layered rock dips toward the syncline’s hinge line  In figure 11.11, two anticlines are separated by a syncline. Each anticline and adjacent syncline share a limb  Each plane is an axial plane containing all of th
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