|MadSci Network: Earth Sciences|
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
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