1. Paleomagnetism

◦ 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

 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. ◦ 9o 9o AKchttp:// AKc

 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

 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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