Answers

2016-06-29T14:50:49+08:00
 Most are located along the edges of crustal plates, especially it seems the Pacific plate.
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2016-07-01T14:33:17+08:00
 Most are located along the edges of crustal plates, especially it seems the Pacific plate. A few are located over 'hot spots' (example: Hawaii's Kilauea volcano). 
Now many on the edges are either near where the seafloor is spreading or where one plate is going under another (subducting). 
A full 1/4 of the active volcanoes on the planet are located in Indonesia (an area where 3 plates are involved). However the entire Pacific ocean has volcanoes scattered around it
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