How do we Know Tectonic Plates Move?

Evidence #1:  Matching Coastlines of S. America and Africa

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As we mentioned earlier, the idea that the continents moved is as old as the  most accurate maps.   As soon as these became available, people began to notice how similar the coasts of South America and Africa are to each other.  This gave rise to the theory of continental drift.

In addition to the similarities in shapes of SA and Africa there are several other pieces of evidence to suggest that Plate Tectonics is occurring.

Evidence #2:  Age, Heat, and Magnetic Orientation of Rocks at Mid-Ocean Ridges

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Heat Flow

HEAT FLOW MEASURES THE AMOUNT OF HEAT LEAVING ROCKS IN THE CRUST.

HEAT FLOW MEASUREMENTS ARE ALWAYS HIGHEST ON THE ROCKS ON MID OCEAN RIDGES.

THIS MEANS THAT ROCKS AT THESE SPREADING CENTERS ARE RECENTLY FORMED FROM MAGMA RISING TO EARTH'S SURFACE. 

 

Age

Using radiometric dating we can determine the age of rock in the Earth's crust.  We will discuss exactly how this is done in our next unit.  

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RADIOMETRIC DATING SHOWS THAT THE ROCKS AT MID-OCEAN RIDGES ARE THE YOUNGEST.

AS YOU MOVE FARTHER FROM THE RIDGE THE ROCKS BECOME OLDER.

THIS PATTERN IS SYMETRIC ON BOTH SIDES OF THE RIDGE.

THIS TELLS US THAT NEW ROCKS ARE CONSTANTLY FORMING AT THE SPREADING CENTER, THEN PUSHING OUTWARDS AS THEY AGE. 

 

Magnetic Orientation

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MAGNETIC MINERALS WITHIN THE IGNEOUS ROKCS THAT FORM FROM COOLING MAGMA ARE ORIENTED POINTING STRAIGHT AT MAGNETIC NORTH.

ABOUT EVERY 14,000 YEARS THE MAGNETIC POLES ON EARTH REVERSE, MOVING THE MAGNETIC NORTH POLE UNDERNEATH ANTARCTICA.

BECAUSE OF THIS REVERSAL, THE BANDS OF IGNEOUS ROCKS CREATED AT SPREADING CENTERS CREATE DISTINCT BANDS OF ROCKS WITH ALTERNATING MAGNETIC ORIENTATIONS.

For 14,000 years all of the rocks formed at the spreading center will be pointing North.  In the next band, the minerals will be orienting towards the south because that is where magnetic north is.

LIKE THE AGE, THE MAGNETIC ORIENTATION OF ROCKS AT A SPREADING CENTER IS SYMETRICAL ON BOTH SIDES OF THE BOUNDARY.  

So as we travel farther and farther out from the spreading center the rocks get older and older, and the magnetic orientation of the rocks changes with the magnetic pole reversals.

 

Evidence #3:  Earthquake and Volcanoes Happen More Frequently at Plate Boundaries

Earthquakes don’t occur randomly across the world, but happen across limited belts which also contain most of the world’s volcanoes.  These belts mark the location of plate boundaries.  These areas are so active because the stress of 2 plates against each other and eventually gives in the form of a volcano or earthquake.  The largest belt of activity is the “Ring of Fire” surrounding the Pacific Ocean which is home to 90% of all quakes.

Evidence #4:  Matching Fossils found on Different Continents

Paleontologists have found bones of the exact same species of organisms on continents that are separated by vast oceans.  It is extremely unlikely that the same species evolved twice in two different places.  It is equally unlikely that land based animals could migrate across an entire ocean and survive.

 

The only rational explanation left is that the two continents were once together, allowing the organisms to roam over both territories. 

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