MadSci Network: Earth Sciences
Query:

Re: Is the mid atlantic rift pushing in on the pacific ocean on both sides?

Date: Sun Jun 8 12:10:30 2008
Posted By: David Smith, Director of Professional Development
Area of science: Earth Sciences
ID: 1204085036.Es
Message:

Amy,

What a clever and thoughtful question you asked, one that gets right to the
heart of plate tectonics - what's pushing, what's pulling, and what's going
where?

You are correct to notice that the Eurasian Plate (which includes the crust
under the eastern half of the northern Atlantic Ocean, not just the
continents of Europe and Asia) must also be moving away from the
mid-Atlantic ridge, in the opposite direction to the North American plate.  

Both of those plates meet the Pacific plate around the edges of the Pacific
Ocean.  As the Atlantic Ocean Basin expands, the Pacific Ocean Basin is
indeed getting smaller.  In terms of geography, this is happening
relatively equally on both sides.  The west coast of North America and the
east coast of Asia are both moving toward the International Date Line. 
With the Global Positioning System (GPS - using the same GPS satellites
that people use to find their way in their car these days, but with much
more accurate receivers), we can measure plate motions.  Here is a map
showing the motions of some spots on different plates:
 http://www.geodesy.miami.edu/plate_motion.html

Continental spots are in red, oceanic spots in white.  You can see that the
arrows form curves within a plate.  That's because moving on a sphere is
different from moving on a flat surface and so the plates get around by
rotating around on the surface.  The length of the arrow tells you how fast
the plate is going.  You can see that Eurasia is going a little faster than
North America, but there's not a huge difference.  So, both Eurasia and
North America are rotating away from the center of the Atlantic and toward
the center of the Pacific.  However, the Pacific plate isn't just sitting
around.  It is also moving, to the northwest, away from western North
America and toward Alaska and Japan.  As the Pacific ocean gets smaller,
the Pacific plate underneath it has to go somewhere.  That extra plate is
subducted, pulled back into the mantle and recycled. Most of that
subduction is happening along the northwest side of the Pacific ocean,
causing big earthquakes and volcanoes in western Alaska, the Kamchatka
Peninsula (part of Russia), Japan, and the Phillipines as the Pacific plate
slips down underneath those areas.  

In fact, the fact that the Pacific plate moves rapidly and moves toward the
big subduction zones tells us something very important about plate
tectonics.  It tells us that the most important thing driving plate motions
is the pull of the sinking crust in a subduction zone.  This pull is much
larger that the small push generated at ridges and also larger than the
drag on the plate from the moving mantle underneath.  

Hope that helps answer your question,


Dave Smith, Da Vinci Science Center



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