1. Paleomagnetism
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◦ when hot, minerals in newly formed rocks
align themselves with the earth’s magnetic
poles
◦ As the Earth’s magnetic poles change so
do the direction of the rock’s magnetic
poles
Therefore: rocks formed millions of years
ago show the location of the Earth’s
magnetic poles at the time of their
formation
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strips of alternating polarity - mirrored on
each side of ocean ridges
one strip of rock has normal polarity (north
pole of the rock faces the north pole of the
Earth)
another strip of rock has reversed polarity
(north pole of the rock face the south pole of
the Earth)
Therefore: Each strip that has a different
polarity had to be formed at different time –
supports sea floor spreading
◦ A link between deep-focus
earthquakes and ocean trenches
◦ Shallow earthquakes occur near or at
the trench
◦ Deep earthquakes occur toward the
mainland
◦ Drilling of the ocean floor from the mid
ocean ridge toward the coast lines
◦ Oldest rocks were found closest to the
shoreline, youngest nearest the ridge
◦ Therefore: The ridge is producing new
oceanic crust while older crust is being
pushed toward the shoreline
◦ Rising magma from the mantle (in the ocean)
produces volcanoes along the floor of the
ocean
◦ As plates move, new volcanoes are formed
along the floor bottom above the hot spot
◦ Chain of underwater volcanoes and islands
from the Aleutian trench to Hawaii – age of
features increase as you move away from
Hawaii
◦ Therefore: The plates are moving over the hot
spot and forming new volcanic features.
◦ http://www.youtube.com/watch?v=AhSaE0omw
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http://www.youtube.com/watch?v=bYv6V5EJ
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Convection Currents basic driving force
◦ Hot, less dense magma rises toward the
crust while cooler, more dense magma
sinks
◦ Plates move because of the uneven
distribution of the Earth’s heat
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The pulling of old crust down in the mantle
because by the downward flow of more dense
magma in the convection current
The down-ward pull, from gravity, on the
oceanic lithosphere at the ridge
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http://www.youtube.com/watch?v=ryrXAGY1
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