EESB18H3 Lecture Notes - Seafloor Spreading, Seismic Tomography, Density

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Published on 6 Oct 2012
EESA05 Lecture 3: PY
Lecture 3: Plate Tectonics
How do we get the mountain formations, volcanoes etc; plate tectonics
Development of plate tectonic theory
Consequences of tectonics
Slide : Internal Energy
Whenever you have something moving, have to have NRG to actually move it
Early on, when planets formed, pretty much there were many rock masses that
collided; these collisions had to do with formation of planets
Each mass has lots of kinetic NRG associated w/ it, once collide w/ each other,
the kinetic NRG changes to thermal NRG; very hot
Thru heat differentiation and mass differentiation –> Earth became bigger;
middle = very hot; then surrounding it cooled down; as you get to surface =
oDensity differentiation is what actually facilitates this; there are things that
are less dense than the iron core that’s in the middle
oThe NRG of radioactive decay of core is what’s running the engine (plate
Slide : Cross-Section of Earth
Very dense very hot = inner core
As you move out, get less dense and less hot materials
Radioactive heat thru mantle and the mantle sends that heat to the top layers
Top of mantle = asthenosphere
oAsthenosphere = hot but it’s not a liquid; it’s a very hot solid and because
it’s under so much pressure, even though it’s hot, it acts as plastic
plasticity quality
oHeat rising, goes thru asthenosphere that’s very plastic and on top of the
asthenosphere is the lithosphere which is brittle and hard (not plastic)
ob/c asthenosphere is very hot and under pressure and has plastic quality,
the lithosphere rides on top of the asthenosphere
lithosphere = broken into bits = lithospheric plates and these plates float
around on top of the asthenosphere
on top of lithosphere = crust = what we walk on
ocrust = 2 diff kinds
inner core outer core mantle top of mantle = asthenosphere (plastic)
lithosphere = broken into bits at the very top, what we see is the crust
radioactive NRG drives the bits of lithosphere and pushes them around
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Lithospheric plates:
various lithospheric plates: African plate etc;
lithospheric plates are moving around
osome places where they’re moving away from one another = divergent
osome places where they’re moving towards one another = convergent
oit’s the moving away and towards one another that’s creating everything
that we see
in the oceans, we have the oceanic crust; in the ocean, have mid-ocean
ridges/rifts where in the ocean, hot stuff comes up from the asthenosphere and
comes onto the ocean floor and pushes these oceanic crusts away from one
osomewhere else, the oceanic plates may actually be colliding
oas plates hit, a lot may happen and it changes based on what kinds of
plates we have colliding
owhether 2 oceanic plates, one continental + one oceanic or 2 continental
plates colliding
raw materials of the plates are constantly being recycled = this is part of the
tectonic cycle
mid ocean ridge, for ex: Atlantic mid-ocean ridge and also have big one in
pacific ocean
oalso west coast of south America and North America
omid-ocean ridge = not very high mountains but have volcanoes magma
comes up and out and pushes plates away
ostuff comes out of the ocean floor = basalt
basalt = really dark, lot of iron/magnesium, very dense
the rocks that we see on the continents aren’t as dense and heavy as the
oceanic crust
the differences in density is very impt for what happens
Physics: To see how heat is actually distributed
which movement of NRG is actually impt for tectonic plate mvt?
oAt the molecular level, you have NRG that’s being passed on from one
molecule to the next
oIn this way, get NRG transfer from one location to the next
oVibration of atoms/mocs
oWhen adding heat to the pot, you’re increasing the vibration of the atoms of
the moc in that object
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EESA05 Lecture 3: PY
oBy moving of anything that can flow, whether air/water/something plastic
that’s how you get the heat moving thru convection
Latent heat
oVery impt when considering heat transfer of ocean and atmosphere
oWater has the largest heat capacity of all the liquids/solids on the planet
with the exception of liquid nitrogen/ammonium
oHeat capacity: if you want to heat 1 kg of water (change it from liquid to
vapour), you have to add a LOT of NRG to change the phase of the water
a wonderful quality of water means that we can transfer that NRG
In oceans, within the depths of the ocean, hold a certain amount of NRG
and they distribute that NRG around the globe that’s how climate gets
oSo, the water has all this heat and changes into vapour to change it back
to liquid form, it has to give away that heat that’s the latent heat that’s
being transferred
oWhen it snows, actually feel that it’s not as cold; the water’s actually giving
away that latent heat to freeze up
Which heat movement is impt 4 plate tectonics?
oConvection is what’s driving the transfer
Atlantic ocean is getting wider by 5cm/yr; opening up by 1mm every week
oThe amount of NRG needed and the force = tremendous
oArrows show the relative mvts of these plates and along some boundaries =
moving away from one another, or towards one another
oOne place where you can see it colliding = Indian plate
India and Himalayas; have 2 continental plates colliding and giving rise
to the continental crust where you get the Himalayas
oAnother place with collisions:
West coast of South America
Normasca plate = part of the Pacific Ocean is hitting West coast of South
America to form the Andes (oceanic plate hitting continental plate)
Convergent Margins
o2 plates colliding
Divergent Margins
oTwo plates are actually moving away from one another; can get this in the
middle of the ocean have hot magma from the asthenosphere coming
out thru lithosphere & oceanic crust and pushing apart the 2 plates
oGet destruction of crust
oWhere you have divergence, you’re actually making new crust; deep down
in mid-oceanic ridge, have the basalt, magma coming out new crust
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