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GOPH 375
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Chapter 3  Earth = core, mantle, crust  Lithosphere is broken into plates o Divergence zone: seafloor spreading o Convergence zones: plates collide o Transform faults: plates slide past each other  Tectonic cycle: o melted athenosphere flows upward as magma cools to form lithosphere o new lithosphere moves away from oceanic crust on top of underlying athenosphere (seafloor spreading) o when lithosphere edges collide, the denser ground turns downwards and is pulled by gravity into athenosphere where it’s reabsorbed (subduction) less dense buoyant earth on top  Two distinct types of crusts of earth: continental, oceanic o Both float on top of mantle because they are less dense than the mantle o Compositional o Lithosphere/athenosphere have nothing do with composition; it’s how they fail? Lithosphere rocks fall as they are?  Mantle convects; plate tectonics  Wegener said that all the continents had once been united in a supercontinent called Pangaea; broken apart by continental drift o (not Wegener) when Pangaea existed, the rest of earth’s surface was a giant ocean called Panthalassa  Magma is injected into ocean ridges where it’s imprinted by Earth’s magnetic field as it cools to form new rock; the sea floor is pulled away from the ocean ridges. When lava cools, it mineralizes with iron in it. When it cools below the Curie Point the iron is magnetized towards earth’s magnetic field. North pole is regular polarity; South pole is reverse polarity Every few years, magnetic poles (north and south) switch (fig 3.9) o Ocean basins are young compared to earth because it’s constantly reforming. Oldest seafloor rocks are at the edge of the ocean basins  Convection in the mantle: hot rock rises, pulls plate with it  Subduction zones where you have conversion  Mantle convection is driven by the earth’s “need” to release heat  Bathymetry: oceanic ridges (biggest mountains) are volcanoes forming at spreading centers where pull apart and magma rises to fill the gap  Creep: process by which mantle convects o Lattice structure; ## if you squeeze it, the atoms will go to the side, one by one, and then the solid will actually move eventually o This is how solids creep; very slowly. o Convection velocity in the mantle = 5mm/year (slower than glacier)  Plate tectonics are motivated by mantle convection, the earth is attempting to release heat  Areas of new crust (fig 3.8) magnetic. If you’re only creating new crust, the size of the earth would change, but since it doesn’t, so subduction zones are where the crust is destroyed. These two need to be in balance  Plate tectonics (responsible for earthquakes and volcanoes) o Oceanic crust can go under continental crusts a subduction. Oceanic doesn’t go under  Most action happens at plate boundaries, but within plates you can have hot spots (dwellings that bring relatively hot material to the surface o Hotspot melts from the deeper in the mantle, localized volcanoes; anywhere o Hot spot volcanism isn’t tied to plate boundary, can keep erupting or no o Hot spot island chains. When rocks liquefy, they expand and nearby rocks fracture and move out of the way – sudden breaks/slips of rocks are fault movements that cause earthquakes o Oceanic/continental crusts can travel over hotspots o Produce more heat, causing hotter rock with lower density to rise through the mesosphere, athenosphere and lithosphere as magma. Doming effect + gravity brings sides down, volcano forms above HS; volcanoes become extinct as moving plates carry them away from HS. (age of volcano increases as distance from HS increases) o Hotspots can have earthquakes associated with them;  Fig 19 shows how plates have moved along Atlantic ridge  When two countries collide, mountains exist. Low density of continental plates means neither will subduct  East Africa o Has the most hotspots o Hotspots not associated with plates but happens at spreading centers o Crustal spreading; rifting o Red sea has a spreading center under it  Earthquakes are one of the two biggest killers o Epicenter – projection on earth’s surface of the hypocenter (location of energy/stress release) o Epicenters outline exist where rocks have moved past each other; outline tectonic plates o Hypocenters define subducting plates being pulled back into the athenosphere o Seismic-gap method – to find future earthquakes  Tectonic environment: o Divergent plate boundaries: plates pulled apart under tension. Hot rock rises, buildup causes expansion of lithosphere by gravity aided by convecting cells  Convection in the athenosphere causes plate movement o Convergent plate boundaries: plates deform under compression when lithospheres collide; denser plate underneath the less dense plate. Gravity encourages the pulling away from the spreading center. Subduction occurs with ocean Vs Ocean or ocean vs. continental (younger, warmer plate will go on top, other to mantle). Continental vs. continental will lead to continental upheaval because both plates are buoyant & can’t be pulled into dense athenosphere o Transform plate boundaries: points of sheer stress; places where plates fracture to bend around earth’s curvature. In spreading centers, if plates are going in opposite directions it’s a transform fault; if plates are moving in the same direction it’s a fracture zone o Hot spots  Convection in athenosphere, gravity pulling spreading center mountains, and plates going down at subduction zones keepts the lithosphereic plates moving.  Earthquake hazards: figure 3.24 on exam o Divergent (pull apart) motion at spreading centers causes rock tension, leading to rock to rupture. Causes small earthquakes o Plates moving past each other at transform faults and subjected to shear stress. Large earthquakes from irregularities along faults (rigid edges) o In subduction zones pulling plates into athenosphere, and continent-continent collisions, rock deforms under compression – largest earthquakes  Shallow earthquakes occur in cooler, interior rigid of rock in 3 places: the upper portion of the down-going plate, at the bend in the subducting plate, at in the overriding plate. Are more destructive than deep earthquakes that loose most of their energy travelling up.  India keeps hitting Asia, now the highest standing continental area on Earth  Harmonic tremors: a continuous swarm of small earthquakes that occur when magma is moving at shallow depths o Effects of subsurface magma movement (compression/uplifting when magma is injected, down-dropped with the removal of magma) combine with gravitational pull to cause large movements along faults as well as harmonic tremors  3 types of faults o Normal – Spreading . Spreading centers don’t have large earthquakes o Strike-slip – transform o Thrust (reverse)– subduction/collision Chapter 4  Earthquakes caused by: volcanoes, meteors, undersea landslides, nuclear bombs or mostly by sudden movements along faults. Faults/fractures are cracks in earth where the 2 sides move relative to each other. Near surface rock has built up stress that’s released by a series of movements along the fault – felt as earthquakes. Fault is first ruptured at a weak point (hypocenter location at depth where sliding/rupture begins), then spreads all along fault. o Epicenter is crust spot directly above hypocenter; location projected to the surface o When tectonic plates split, the lithosphere fractures. When stress builds up and there’s movement, it becomes a fault  Fault movements: main shock (biggest “quake” doesn’t have to be first) foreshock (medium) aftershock (small) – all part of the same stress release; can span over days o Fault movements are usually restricted to one fault- rupture stops at bends or steps which ends the earthquake o Amount of fault movement at the ground surface differs from that at depth o Fault patches with little/no movement may be locations for future EQ’s  Figure 4.2 geometry o Rupture area: along fault surface where the rock has slipped. Mostly underground. Rupture point means there’s energy being released very close to people o Need not have rupture area  Measurements: dip (2-D vertical cross-section) strike
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