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GEOL 240 lecture summary Part 1 Fall 2011

GEOL 240 lecture summary Part 1 Fall 2011 - Tuesday 8...

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Unformatted text preview: Tuesday 8 ­23 ­11: Introduction to course Course logistics Textbooks Grading Policy Field trip Labs (no switching!) Introductory comments about goals of course; slides Thursday 8 ­25 ­11: Plate tectonics (begin) Paradigm shifts (Mendelian genetics & plate tectonics) Hypothesis vs. theory; differences between scientific usage and general usage of these terms Age of the earth; how do we know this? Radiometric dating of rocks Ages of continental vs. oceanic rocks What features of the Earth does plate tectonics explain? Seafloor bathymetry and locations of earthquake belts; relationship to edges of tectonics plates Three types of Plate boundary Divergent plate boundaries Basaltic lavas (basic characteristics and chemistry); “black smokers”; mineral deposits Rifting of continents and development of new ocean basins Mid ­ocean ridges Continental rifts Successful vs. failed rift arms (aulogogene; examples) Tuesday 8 ­30 ­11: Plate tectonics (cont.) Convergent plate boundaries Subduction ­related volcanism; origin of magmas, behavior of magmas Wadati ­Benioff zones Subduction megathrust earthquakes; percentage of global seismic energy release Pacific Ring of Fire Transform plate boundaries Mechanics of plate ­boundary motion; relationship to spreading ridges California example Thursday 9 ­1 ­11: Continent ­continent collisions and hot spots; begin “Continental Drift” Reasons for continent ­continent collisions; Mt. Everest example; older, inactive examples; implications for mountain building Hot Spots What is a hot spot? Hawaiian chain example Yellowstone and Iceland examples “Continental Drift” Wegener’s theory Major lines of evidence in support of continental drift Reasons why it wasn’t accepted by other earth scientists Tuesday 9 ­6 ­11: Theory of plate tectonics The driving mechanism: Arthur Holmes suggests a workable mechanism to drive relative plate motions Lithosphere vs. aesthenosphere Geomagnetism; the Earth’s magnetic field; magnetic North vs. Geogaphic North; inclination vs. declination Paleomagnetism: Curie Point; geomagnetic time scale Thursday 9 ­8 ­11: Theory of plate tectonics (cont.) Paleomagnetism (cont.): Seeafloor spreading evidence; magnetic seafloor stripes Response of the Earth Sciences community to these new data Measuring plate motions Inputs to plate ­motion reconstructions Apparent Polar Wander curves Wilson Cycle of repeated ocean formation and contraction Tuesday 9 ­13 ­11: No Lecture Thursday 9 ­15 ­11: Stress and faults Force; definition Mass vs. weight Stress (definition, vectors, tractions, examples) Isotropic vs. tectonic stresses Overall stress vs. normal stress and shear stress components Relationship to fault slip; earthquake triggering The three principal stresses Definitions; reasons for different sign convention from Engineering Principal planes; example of earth’s surface Tuesday 9 ­20 ­11: Faults and stress (cont.) Dip ­Slip faults (a) Normal faults  ­accommodate extension  ­divergent boundaries  ­orientation of three principal stresses (examples of Sierra Nevada normal fault in eastern California and mid ­Atlantic Ridge) (b) reverse faults thrust faults (example of Puente Hills blind thrust fault beneath LA) accommodate shortening (contraction) convergent boundaries orientation of three principal stresses (examples of Puente Hills Thrust beneath USC and Mt. Everest) Strike ­slip faults discussion of transform plate boundaries and transform faults and fracture zones right ­lateral vs. left ­lateral orientation of three principal stresses (example of San Andreas fault) Thursday 9 ­22 ­11: Pacific Northwest seismic and volcanic hazards Cascade volcanic arc; major hazards to Seattle (Mt. Rainer) and Portland, Oregon (Mt. Hood); eruption of Mt. St Helen’s; lahars (origin and hazards); depth of slab beneath volcanic arc North ­South shortening of upper plate in western Washington; folds; LIDAR; Seattle fault, age of most recent earthquake Evidence for mega ­thrust earthquake hazard GPS; what is GPS?; Why was it deployed?; How precise are measurements of relative motion from site to site? History of seismic hazard views in Pacific NW; absence of historical earthquakes along the Cascadia megathrust; GPS measurements of east ­west shortening and elastic strain accumulation in upper plate; relationship to locked portion of megathrust Age of most recent large earthquake on Cascadia megathrust; tree ­ ring analysis, tsunami deposits, tsunami modeling, stratigraphic evidence for uplift and subsidence Size comparison of Cascadia megathrust and the fault that slipped in 2004 Mw 9.3 Sumatra earthquake Basin and Range province East ­west extension; horsts and grabens; western and eastern limits of province; rates of extension from GPS; location of most seismic activity in Basin and Range; origin of the Sierra Nevada Wasatch normal fault and seismic hazards to Salt Lake City; proximity to Wasatch fault; amplified seismic shaking risk due to Lake Bonneville lake muds Tuesday 9 ­27 ­11: The transform plate boundary in California Overall rate of relative Pacific ­North America plate motion GPS satellite evidence for plate motion What is GPS? What does it measure? What does it tell us about rates of major fault systems in the western USA? The San Andreas fault Length; rate (percentage of total plate rate on the SAF and its various splays) Historical earthquakes 1906 San Francisco length; magnitude; location; average slip (think of the offset manure pile/barn photo I showed in class); epicenter Devastation of San Francisco Reasons for fire burning out of control for so long Earthquake damage vs. fire damage Apparent city ­wide conspiracy to downplay earthquake’s role 1857 Fort Tejon earthquake length; magnitude; location; average slip along fault Creeping section of SAF Relationship to 1906 and 1857 ruptures Why it creeps Eastern California Shear Zone Where it is (also, Who is buried in Grant’s tomb?) What it is (types of faults, orientation) Relative importance in terms of Pacific ­North America relative plate motion; rate Major Historical Earthquakes in ECSZ 1872 Owens Valley; 1992 Landers; 1999 Hector Mine magnitudes; type of faulting Thursday 9 ­29 ­11: Faults of the Los Angeles metropolitan region Different strands of the SAF system in northern (SAF proper, Hayward fault, Calaveras fault) and southern (SAF proper; San Jacinto fault, Elsinore fault) California More on creeping section Origin of North ­South shortening in southern California; Big bend of the SAF Transverse Ranges (Sierra Madre ­Cucamonga fault system) Faults of LA region Location of most North ­South shortening in region; how we know this Blind thrust faults; what are they? Puente Hills example; how we study blind thrust faults; major LA ­region historical blind thrust earthquakes Evidence for seismic energy release through time in the LA region ...
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