∙ Plate tectonics is the underlying paradigm of modern geology
o Unifies much of that data about the earth that was once thought to be unconnected o The theory is relatively new, started in the 1960’s and accepted over 1970’s o The theory of plate tectonics states that the crust and the uppermost mantle are
broken into a series of plates that move around the surface of the earth.
∙ Wegener 1915 noted that continental margins fit together like puzzle pieces, proposed a single supercontinent, called Pangea. South America and Africa fit together particularly
o Fossil Faunas. Across the Southern continents when they were all together (Gondowana), fossils of many different creatures and plants were found across the continents that wouldn’t have been able to swim to get to the other continents, so
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they had to be connected.
o More evidence? Mountain ranges line up along with specific types of rocks. There is also matching glacial evidence on South America, Africa, and Australia. A giant glacier in old Antarctica left marks on these continents that can be seen
∙ Continental drift: where did Wegener go wrong?
o Wegener challenged the existing scientific paradigm, which was that continents
o He had no believable mechanism by which the continents could move ∙ Earth’s Magnetic Field
o Dip needles can measure your latitude on the earth by sensing magnetic fields and
where you are placed on earth
o Apparent Polar Wandering – the poles have wandered in the past, showing where
We also discuss several other topics like What is the difference between a variable and a constant?
the continents moved
∙ Geomagnetic Reversal
o Every few million years, the magnetic poles of the earth reverse. The way it is
now is ‘Normal’ and the opposite way is ‘Reversed’
o Scientists can tell when the poles switch through the colors of certain magmatic
rocks. White means normal, red means reversed
∙ Oceanic Magnetic Stripes
o The rocks of the ocean floor have stripes of the red and the white rock. And there is one distinct line that divides the rock, and the rock is symmetrical in color on
either side. The line is called a ridge axis
o When magma comes through the ridge, it rises and crystalizes, it then gains the
color of the magnetic pole orientation at the time.
o As the magma comes up and solidifies, the ocean floor moves. This is Sea Floor
o The Sea Floor Spreading causes the earth’s plates to move apart. All land masses
are connected to the plates and they move with them.
o As the plates move and grow, some parts of the plates subduct under other plates and are ‘recycled’ back into the mantle. Gravity plays a part in how these plates
move, spread, and subduct.
∙ Geo Trivia
o How many major plates are on the surface of the earth? Answer: 7 main plates o Plates can have only oceanic crust on it, but there is often a mix of oceanic and If you want to learn more check out Why is fear rising when the threat is falling?
∙ Two kinds of crust
o Continental – Thicker, about 2570km thick, less dense, composition: granite, not
o Oceanic – Very thin, only about 710km thick, but more dense, composition:
∙ Three kinds of plate boundaries:
o Divergent – Plates move apart, mantle comes up and makes more oceanic crust. Ex. MidAtlantic Ridge, a ridge in the Atlantic that is broken up into segments We also discuss several other topics like Why is directional solidification important?
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that are connected by transform fault lines.
Juan de Fuca plate makes new crust that is almost immediately consumed
under North America. This plate will disappear soon.
Continental Rifting – Mantle coming up under a continent, creating dense ocean crust that lies lower than the continent, allowing a small linear sea
to form, which can then become a larger ocean. Don't forget about the age old question of What is plane polarized light in organic chemistry?
o Convergent – Plates come together
Oceanic crust coming in contact with continental crust, the oceanic crust is more dense so it will always subduct under the continental crust. When the oceanic crust subducts and starts partial melting in the mantle, the melted material will come up and create volcanoes/mountains
Example – West coast of South America
o Transform – Plates slide past one another
Not common, mostly in the ocean.
Rarely on continents, but there is one in North America in California. It is
called the San Andreas Fault system
∙ Hot Spots
o Mantle plumes under the crust, a collection of superhot magma that begins to rise
in one area
o Magma will rise through the crust and release through it. This creates a volcano on either continental crust or oceanic crust. If enough magma comes through, it
will form an island.
o The hot spot remains in the same spot, and as the plate moves the hot spot will
keep punching through and create a chain of islands (ex. Hawaii)
o Hot spots can also be under continents, an example in North America is in
o Ridge Push – gravity driven force that results from elevated position of the ridge o Slab pull – Reults from sinking of a cold dense slab of oceanic lithosphere under