Study Guide Test 2 GLY1000 RHEOLOGY AND DEFORMATION Stress strain relationship and elastic moduli Bulk and Shear Stress a force producing or tending to produce deformation in a body stress is measured as F A the force divided by the area over which the force is applied and defined in such units as dynes per square centimeter or pounds per square feet etc Strain refers to the deformation resulting from the stress i e to the change in the dimensions or shape of the body relative to the original dimensions or shape Elastic Modulus is the mathematical description of an object or substance s tendency to be deformed elastically i e non permanently when a force is applied to it Bulk Modulus is the measure of the incompressibility of a material The more incompressible a material is the smaller will be the value V v and hence the greater the value of the bulk modulus Shear Modulus is a measure of the rigidity of a material The more rigid a material is the smaller will be the value tan and thus the larger the value of the shear modulus Factor effecting type of strain i e brittle versus ductile strain Brittle when a material breaks under stress Ductile instead of breaking the material absorbs the stress by flowing Whether solids are brittle or ductile depends on their temperature and the strain rate Material can be brittle at low temperatures and ductile at higher temperatures Material can be brittle under high strain rates and ductile under low strain rates Consider the following observations A glass window will shatter when struck with a hammer Images seen through original glass windows in old buildings say 100 years old seem distorted When bent by a stress a plastic spoon will break once its elastic limit is reached If heated sufficiently the same spoon can be bent into a permanent loop The views through old glass windows are often distorted because the glass has deformed it has flowed under the constant stress of gravity Yet the glass shatters when struck Is the glass brittle or ductile The answer is it depends The cold plastic spoon snaps under stress Our hot spoon seems to flow Normal reverse thrust and strike slip faults and orientation of fault planes and stresses Faults are fractures in the earth along which there has been displacement of one side relative to the other side Reverse or thrust fault arrow give direction of primary compressional stress This fault motion is caused by compressional forces and results in shortening Normal fault arrows give direction of primary tensional stress This fault motion is caused by tensional forces and results in extension Strike slip fault arrows give orientation of shear couple The fault motion of a strike slip fault is caused by shearing forces The heavy line in diagrams represents the plane fault plane along movement has taken place The half arrows along the fault planes indicate the direction of relative movement Isn t on study guide but could be important Monocline is a step like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently dipping sequence Anticline is a fold that is convex up and has its oldest beds at its core Syncline is a fold with younger layers closer to the center of the structure Synclines are typically a downward fold Fold axis is the closest approximation to a straight line that when moved parallel to itself it generates the form of the fold Axial plane is a plane that intersects the crest or trough in such a manner that the limbs or sides of the fold are more or less symmetrically arranged with reference to it and it is also known as axial surface Plunge is the angle of inclination of the axis as measured from the horizontal Fold tightness is defined by the angle between the fold s limbs Open fold range from 120 to 70 Tight fold range from 30 to 0 Isoclinal fold is a fold in sedimentary rocks where the axial surface and limbs slope in the same direction and at approximately the same angle and has an interlimb angle of between 10 and zero with essentially parallel limbs Recumbent fold is when the center of the fold moves from being once vertical to a horizontal position EARTHQUAKES Understanding of what causes earthquakes in terms of stresses elastic strain slip along faults Regions of the Earth s crust and lithosphere that are subjected to sufficient stresses can undergo elastic strain This strain can continue to build up over long periods of time If the stresses are suddenly released as they might be when rocks slip along a fault the stored elastic strain energy is released some of it in the form of seismic waves that are travels outward away from the point of origin or focus of the earthquake What are the major hazards of earthquakes The main earthquake hazard is the effect of ground shaking Buildings start to lean and sometimes even fall over destroying everything Ground shaking may also cause landslides mudslides and avalanches Ground Displacement can rip buildings apart if the building was built on a fault Earthquakes can rupture dams or levees along a river causing Flooding Fires can be started by broken gas or power lines Some great faults like the San Andreas periodically generate earthquakes along some segments of their lengths but not along other segments why Faults like the San Andreas are so huge that it can generate earthquakes at different segments of the fault because they the fault is so massive The San Andreas Fault is separated into three different segments What can account for an almost periodical instead of random occurrence of earthquakes along parts of the San Andreas Fault Seismologists have discovered that a section of the fault produces 6 0 magnitude earthquakes about every 22 years because there have been earthquakes in 1857 1881 1901 1922 1934 and 1966 Distribution and depth where do most earthquakes occur and how deep are the foci The depth of the foci can be categorized as shallow up to 70 km below the surface intermediate 70km to 300km or deep greater than 300km About 75 of all earthquakes is from shallow focus ones Most earthquakes occur at oceanic and continental plates Why do earthquakes originate in the lithosphere but not in the asthenosphere Underneath the lithosphere is a hotter softer layer of solid rock called the asthenosphere and the rock of the asthenosphere is viscous rather than rigid and deforms slowly under stress like putty Therefore the lithosphere can move across or through the asthenosphere under the forces of plate tectonics
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