Evidence of Plate Tectonics

1. Paleomagnetism

the permanent magnetic field of a rock that shows the direction of the magnetic field at the time of the rock’s formation

◦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

On the ocean floor

◦A link between deep-focus earthquakes and ocean trenches

◦Shallow earthquakes occur near or at the trench

◦Deep earthquakes occur toward the mainland

2. Earthquake Patterns

◦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

3. Ocean Drilling

◦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=AhSaE0omw9o

 http://www.youtube.com/watch?v=bYv6V5EJAKc

4. Hot Spots

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

Causes of Plate motion

The pulling of old crust down in the mantle because by the downward flow of more dense magma in the convection current

Slab-pull

The down-ward pull, from gravity, on the oceanic lithosphere at the ridge

Ridge –push

http://www.youtube.com/watch?v=ryrXAGY1dmE