PLATE TECTONICS

 

Plate tectonics is THE unifying theory of the Earth.

 

Before the beginning of the 20th century, people thought that the Earth was shrinking.  It was cooling off, and shrunk like a prune.  The wrinkled surface explained the mountains.  In the early 20th century, after the discovery of radioactivity, it was thought that the Earth was heating, not cooling, and the idea of an expanding Earth began.

 

Evidence for an  expanding Earth:  The continents appear to line up.  From one side of the Ocean to the other.  But expansion does not explain the mountains (compression).

 

Darwin recognized that there is a tremendous amount of erosion in the Andes.  How could the mountains remain so tall?  The answer came to him after an earthquake.  The land was being uplifted.  Why?  The answer would come in the 1960s.  Continetal drift and plate tectonics.

 

Text Box: The first well crafted idea for continental drift (the moving of the continents) was by Alfred Wegener, who recognized that the continents fit together.  A german meteorologist, in 1914 he published a paper pointing out that the continents matched if put together and that there are certain fossils that also match up if the continents are put together.

 

His reconstruction is shown at left at 300 million years ago.  Shaded areas are shallow seas.  He fit the whole world into a single continent which he called Pangea, meaning “all lands”.

 

But how do we fit the continents?  Not to the edge – at the seaside – but at the edge of the continental shelf, the shallow part of the continent that is underwater.  Then things match up earily well!

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Africa and South America fit almost perfectly.  The dark areas indicate areas of overlap.  The fit is much better when the continental shelfs are considered (lighter areas).  Australia and Antarctica match up very well also. 

 

 

 

 

 

 

 

 

 

 

Wegener didn’t stop there.  He realized that there are certain fossils in the ancient rock record that also match up when the continents are placed back together.  But Wegener’s ideas were never accepted.  How could something as massive as the continents possibly move?

 

The truth cannot be hidden forever, and evidence in support of evolution was being discovered from many different fronts.

 

The mountain belts on different continents line up when the continents are put together.

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The ages also match up.  As scientists perfected ways for dating rocks, they find that the ages correlate between the continents.

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Ancient evidence of glaciers also match up when the continents are put back together, as do the fossils.

 

Text Box: Left. Reconstruction of glacial coverage of Pangea. On all of these continents, South America, Africa, India, Australia and Antarctica, there is evidence for glacial activity. The arrows give the direction of movement. Right. Geographic extent of the Carboniferous age fossil Glossopteris, a fern. If the continents are all put together, its geographic extent is well explained.
 


Why stop there?  There is still more.

 

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The ring of fire is the region where earthquakes and volcanoes occur. 

 

 

 

 

 

 

 

 


The Hawaiian Islands and the Emperor Sea mounts.