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Lecture

GOPH 375 Lecture Notes - Seismogram, Wave Equation, Love Wave


Department
Geophysics
Course Code
GOPH 375
Professor
All

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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)

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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.
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