Chapter 2-6.docx

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Department
Earth Sciences
Course
Earth Sciences 2240F/G
Professor
Ron Podesta
Semester
Winter

Description
Unit 2– Introduction and Plate Tectonics • Earth’s internal structure Oceans andAtmosphere Crust- thin shell Mantle- Large volume, silicate and oxide minerals Outer Core- Hot, liquid, metal (metals and silicates) Inner Core- Hot, solid, metal Mantle-upper most part is cold and brittle -reacts with large scale processes -mantle and upper crust are considered as one (Lithosphere) LITHOSPHERE-cold brittle and stuck to the crust, top layer ASTHENOSPHERE-thin layer above with some composition but not as much pressure, even be fluid MESOSPHERE- thick zone, very dense rocks, acts as a solid Differentiation- heavy material moving inwards and lighter material moving outwards (gravity) • Internal heat energy o Accretion and impact (learn the kinetic energy equation) -Earth grew with more and more material adding = kinetic energy increased and was important with the thermal history KE=1/2 mass (velocity )2 -1000km radius = 1000 C increase (impossible, earth would have melted) Initial impacts then the impact of Theia o Core formation heat -Theia –side swept earth to cause the moon to be created -kinetic energy was created -differentiation occurred (H-middle / L-outside) The ‘iron catastrophe’, but really just heavies to core and light toward surface o Dissipation of tidal energy -sun and moon raise large tides in the ocean -similar distortions take place in the moon/earths interiors -tide in asthenosphere/mantle =11cm Height -effect=earth slows rotation/moon recede from earth -tidal force is the same as IO—most volcanic object in the solar system o gravitational potential energy - kinetic energy released only when gravity pulls heavier and lighter elements to more stable positions, added to the molten core -couple million years to do so for the “Iron Catastrophe” occurred (earths formation) 1 Understand the Earth-Moon dynamics o Heat from radioactivity Long-Lived Radioactivity-producing heat energy all through earths history -ex. K, Uranium, Thorium Short-Lived Radioactivity-early years radioactivity decay was high due to SLR -Virtually all gone 26 26 -ex. Aluminum- short livedAl , decays to Mg which has a half life of 720,000 years -means that Al is extinct Very great in the past (why?) -know heat flow from all continents and oceans now and hypothesize past and future heat flows -direct heat flow from magma to atmosphere -crust and oceans acted as insulation between the core and atmosphere -we are very close to a “steady state” -internal heat leads to earthquakes and volcanism -long lived radioactive isotopes decrease in abundance- internal heat will die Essentially the only significant heat supplier anymore. - Long-Lived-Radioactivity Isotopes • Note correlation of plate boundaries with earthquakes and volcanic activity (why?) -plates- associated with volcanoes and earth quakes -tectonics- interactions between plates -heat is released from these boundaries from inside the earth 2 Chapter 3: Plate Tectonics Definitions Plate Tectonics -effect suddenly and dramatically earth processes -create continents, oceans and mountain ranges Theory of Plate Tectonics -where and why significant deformations of Earths Surfaces occurs -helps understand theory of evolution Economic Earth Scientists -type and location of metallic mineral deposits, general distribution of earthquake and volcanic hazards (timing/ magnitude is abit thicker) Development of the theory o Continental drift Role of Wegener; evidence, lack of obvious mechanism -Alfred Wegener – Continental Drift – Published 1912 -geological structures/fossil -Pangaea- “super-continent” Sea floor mapping -1940 looking for sunken ships used a large magnetometer -repetitive magnetic records in ocean floor rocks -mapping the floors showed a volcanic ridge line Harry Hess -princeton university -ridges representing spreading centers (earths crusts move in opposite directions/conveyor belt) -Atlantic Ocean was widening by 2.5cm/year -patters discovered by magnetometers are directly reflected by magnetic field o Discovery of Paleomagnetism -second step of the theory of plate tectonics -earth magnetic field protects us from space radiation and solar storms -Easiest understand -The positions of geographic poles are the poles of planet rotation; positions of the magnetic poles are not the same Dynamo Model – water turns turbines that generate electricity - magnetic field surrounds every power line carrying electricity - earth has circulating hot currents in the liquid metal outer core and elecric currents are surrounded by magnetic fields - one metal rotating inside another - Coriolis Effect (rotation of the earths axis deflects the path of other rotating material such as ocean water) - Metal inner core is rotating higher speed than rest of the planet Understand Earth’s magnetism model - magnetic north shifted 6 degrees per day (16 million years ago) - magnetic field is less predictable than you would think 3 - storms in the outer core subside, dynamos start up again uniform patters (may flip magnetic poles 180 degrees) - reverse polarity vs. normal polarity (today) change occurs every few million years Magnetic Field in Rocks Basalt -ingeous rock , Iron -magma cools, magnetite (iron oxide) forms -magnetically susceptible Hot magma-disoriented Cooled magma- organized -show the same degree of tilt as the magnetic field Preservation of evidence of reversals in rock Sea floor spreading o Paleomagnetic pattern; evidence of current motion -pattern was age related -normal to reverse to normal to reverse ridge center to strip -distance/time=rate of spreading o Compensated by Subduction Zones -trenches is where the lithosphere is consumed -mark the region of great compression Hugo Benioff -development and application of instruments (seismographs) -measured earthquake locations Charles Richter -perfected the instrument Importance of seismic tomography -measured earthquake locations -series of earthquake foci defining a thin zone from the trench surface sloped downward and away from the oceans side toward the continental plate side -(aka descending lithosphere) 4 -seismic waves travel more efficiently through colder material than warmer -seismic tomography, define relative cooler ocean floor slabs as they fall from the earths surface toward the core mantle boundary (color coded blue) Slab-pull strong mechanism Three types of plate motion on a sphere Divergent Boundary =   (tension) Convergent Boundary =  (compression) Transform Boundary = sliding beside (shear) Driving force of plate motion o Internal heat engine Inner Core ball -5000-7000 degrees -70% diameter of the moon -spins 0.2 degrees faster than the earths rotation rate Outer Core -crust and mantle (lithosphere) -mobile part is the asthenosphere beneath o Convection to mantle plumes to dissipate heat -hot interior, cold surface -heat transfers in the asthenosphere drive plate tectonics Mesosphere- lower part of mantle = solid Asthenosphere- upper part of mantle = plastic 1 - convection is quicker through asthenosphere Lithosphere- oceanic crust- thing and brittle 2 - convection model shows asthenosphere dragging lithosphere-causing friction down causing subduction zone rd-cant act alone 3 -subduction zone sucks down the oceanic crust (maybe gravity) -oceanic crust will sink through the asthenosphere for no reason other than its heavier (releases heat from the inner earth) o Evidence from Hawaii Lithosphere reaction to a plume -no plate boundary • all the volcanic islands/seamounts further north than the big island of Hawaii are volcanically dead • on the big island, there is increased volcanic activity toward the southern boundary • a new volcanic mound is rapidly building up from the seafloor off the south coast of Hawaii (when it breaks sea level it will be called Loihi) • there is a tightly defined great “plume” of heat (hot spot) centered beneath the big island (and Loihi). Mantle Plume -originate at the core-mantle boundary from a discontinuous zone -exist every 100-200 million years rising anywhere 5 Ultra Low Velocity Zone - seismic shock waves lose velocity there - characterized by large magma chambers -magma pushes up towards the earths lithosphere from hot liquid to hot solid Lithosphere- significant barrier Africa - concentration of hot spots are high - beneath Africa is a super plume -basalt the magma at the top of the plume and will try to push through the lithosphere o Three-pronged break on dome -equal strength in all directions and will break equally into three spaced segments -if only two are created and the third arm is a fail, it is called Aulacogen ­push/pull crust away and to the middle of the convection current ­gravity also works ­­ Slab Pull ­­ 95% of the force due to gravity Model of Earth interior 6 Chapter 4 : Earthquakes Basics ­Syria and Egypt earth quakes killed 1.1 million people (1201) ­China earth quakes killed 830,000 people (1556) ­India tsunami killed 250,000 people (2004) ­Haiti earthquake killed 316,000 people (2010) Seismic Energy • Stress/Strain diagram -normal faults -reverse/thrust faults -strike slip/transform faults Creep-movement of a rock at a break or fracture  resistance across a break or fracture causes the blocks to JUMP along a fault plane releasing energy that produces earthquakes Stress- energy imposed upon a rock Strain- deformation of the rock • Principle of elastic deformation/rebound Elastic Deformation- low levels of stress applied, release of stress rock goes back to normal shape Plastic Deformation- high levels of stress applied, release of stress rock stays deformed Earthquake- sudden release of the stress, there is a lot of energy released Cold Rocks- break easily under little stress Hot Rocks- survive more stress before rupturing • Definitions: focus -weak part where the rupture will start epicenter -point at the surface exactly vertically above the focus fault -larger the fault the larger the earthquake, energy dissipates as it moves away from the focus • Body waves: P-waves -primary jolt or sound, fast compression waves -compress atoms or molecules as it passes them -brittle material will crack -dependent on density of the material they pass through ( increase density=further, decreased density= shorter) S-waves -shear waves, slower than P-waves -go through all objects except liquids Body waves -P and S together because they can go through the body of earth • Surface waves: slow waves 2-3 km/sec Love waves -waves move forward across the surface like a slithering sidewinder snake Raleigh waves -waves move forward but in an up and down backward rotating motion 7 Location Inertia- resistance to motion • Seismograph -measure horizontal motion + measure vertical motion -measured on a constantly revolving drum graph • Triangulation -locate the epicenter you record an earthquake on a minimum of three seismographs -measure the time interval between the arrival of the first P- and the first S-waves at each seismograph • Problem of ‘average’seismic travel time -distance will not be exact but will not be too wildly off Magnitude • Modified Mercalli Intensity Scale (subjective) -The amount of observed damage, the stories recounted by witnesses, extent of damage to structures whose strengths were known • Richter Magnitude Scale (objective) Local Magnitude Scale- original magnitude scale was designed to work within 100 km of an earthquake Nomograph- consists of three strips of paper such that a straight line joined 100 km (on the distance scale, in log units), 3 (on the arithmetic magnitude scale), and 1 mm (on the log amplitude scale) – and he called that the “standard earthquake” **all you need is distance away from your seismograph and amplitude from your instruments graph/join lines together and you have a magnitude line** Logarithmic Magnitude Scale- magnitude of 6 is 10 times more energetic than one of 5 -magnitude of shaking frequency seems to fall off around 7 or 8 -numbers higher mean it affects bigger areas and last longer • Moment Magnitude Scale (paleoseismology) • total measured area of the fault that has ruptured • amount of offset along the fault (i.e. how far did the blocks move), and • the strength of the rocks involved. -same up until level 7 on the Richter scale Paleoseismology – study of ancient earthquakes by “reading” the rock record 8 Destruction • Acceleration- The rate of change of velocity with respect to time.Amoving body can accelerate by changing speed or by changing direction -1G =32 FEET/SECOND/SECOND -horizontal component and vertical Resonance -explains why certain wavelengths of sound – such as might  emanate from a (truly very accomplished) opera singer ­ can build to  the point of shattering a glass]  • Liquefaction Plate Boundary Correlation ex. Wet sand, standing is fine until you start jumping and youll be in a puddle of water -sand is supported by water pore spaces between he sand grains -when the sand is shaken, grains push apart, grain—grain contact is gone and quick sand is produced -shaking in earthquakes cause a sudden change from solid to liquid (causes bridge/highway/dams to collapse) • Divergent: -tensional,small, close to surface -pull opposite sides apart -oceanic crust because it is more thin, easier to pull apart rather than continental • Convergent (compressional) -originate in subduction zones (largest earthquake recorded was in chili 1960 in SZ) -energy is released by subduction zones o Ocean crust subducted under continental crust: largest quakes known o Continent/continent: large and hugely destructive because of high population in the regions -push on the ends, however, you have to apply a huge amount of energy to break the rock (thus a huge amount of energy is released when the rupture finally occurs!) -on the slab higher earthquakes due to shuffling and breaking of cold rock -oceanic slab completely disappears, means two continents would converge causing almost non stop earthquake activity -once oceanic slabs break off, continental-continental fuse together but until this, earthquakes will occur • Transform -shallow earthquakes and no consumption or generation of crust -breaks in oceanic thin crust, earthquakes arise from motion of the minor break -continental crust breaks, enormous friction across the break by very thick slabs of dry crust, -----catastrophic earth quakes can occur • Critical if cutting continents: locked sections prediction • location of the event, • time expected (within limits of hours rather than weeks or years), and 9 • magnitude that is probable. • Success of Haicheng, China -animals are sensitive sense they can hear rocks rupturing -scientist noticed small earthquakes, strange behaviour in animals, increased distruptions in gas and water wells = big earthquake occurred (4 richters) -helped prevent thousands of deaths • Lack of success at Parkfield, California -Earthquake capital of the world -USGS announced 95% chance of earthquake between 1987-1992 Mag 6-didn’t happen -finally one happened in 2004 • Precursors Foreshocks- microfractures in weaker rock sections will produce small releases of energy, little seismic shocks -weak rocks will show the foreshocks -strong rocks will be quiet and happen over a long period of time (the big one) Seismic Gap- period on a fault line where there has been no activity -As the stress begins to break the weaker rock units, the block of lithosphere is actually increasing in volume – a solid rock is less volume than the same rock with air-filled or water-filled cracks -ground will swell due to increased volume o Monitoring techni
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