Is it true Volcanoes are fairly evenly distributed all over the world?

Answer 1

The vast majority of earthquakes and volcanoes occur at tectonic plate boundaries. Specifically, transform boundaries (plates slide past each other) have the greatest number and largest magnitude earthquakes (i.e. San Andreas Fault - Pacific plate against North American plate). Convergent boundaries (subduction zones), where one plate dives beneath the other, will produce fairly large numbers of volcanoes because the lower plate melts - the hot magma rises through the crust and erupts, creating a volcano (i.e. Juan de Fuca plate beneath North American plate creates the volcanic Cascades Range). There are also earthquakes at subduction boundaries, but they are less frequent and less powerful. Divergent plate boundaries, where crust moves apart (i.e. the Mid-Atlantic Ridge), have both volcanoes and earthquakes that are very frequent, but also very small - and undetected except by delicate seismometers/seismographs. The exception is in Iceland, where the MAR comes to the surface; the Krafla fissure zone is a very seismically and volcanically active surface expression of a divergent plate boundary.