Re: where is the weakest spot in the plates?

The strength of rocks is a fairly complex thing, but there are two basic 
rules that are most relevant.  As pressure increases, strength increases.  
This is because the inward pressure on the mineral grains that make up a 
rock makes it harder to break the bonds between the atoms in those grains 
and form a crack that could grow to become a fault.  Second, as temperature 
increases, rock strength decreases.  This is because higher temperatures 
make the atoms in rocks vibrate faster and that makes it easier to break 
the bonds between atoms and start a crack  growing.  Once the temperature 
reaches a certain critical level, rocks also change their behavior from 
cracking under pressure (like glass or wood) to flowing under pressure 
(like wet clay or silly putty).  Much more detailed information, including 
technical references is available at:
 http://seismo.berkeley.edu/~burgmann/GEO207/GEO207-14.html

In the earth, both pressure and temperature increase as you go downward.  
The pressure increase is pretty much the same wherever you go, but the 
temperature increase varies a lot, depending on tectonics.  As a result, 
temperature gradient (how much the temperature increases over a certain 
downward distance) is the most important determining factor in the strength 
of the lithosphere (which is what the plates are made of).  Geothermal 
gradients, as these gradients are called, can vary from about 10 C degrees 
for every kilometer of depth to about 40 C degrees for every kilometer. 

In areas where there is a lot of heat flowing up from inside the earth, 
such as rift zones  or volcanic arcs, the plates will be very weak and the 
weakest zone of all is probably the mid-ocean ridges.  Another piece of 
evidence for this is the small size of most mid-ocean ridge earthquakes.  
Big earthquakes occur where rocks are strong and hard to break.  In areas 
of low heat flow, such as subduction zones, rocks are stronger and 
earthquakes can be much bigger.  Mid-ocean ridges are not technically "in" 
plates, since they are the boundaries of plates.  The weakest spots within 
a plate are likely to be areas where there is high heat flow in continental 
crust (which is generally weaker than oceanic crust).  Areas such as rift 
zones (East African Rift, Rio Grande rift), and hot-spot tracks 
(Yellowstone, Azores, Hawaii) would be the weakest spots in the interior of 
plates.

Enjoy your studies, plate tectonics is amazing stuff,

David Smith, Geology and Environmental Science
La Salle University, Philadelphia, PA