Why is silica-rich magma explosive?

Amap of early human migration is shown. a flattened globe shows six continents: north america, south america, europe, africa, asia, and australia. purple lines and arrows show from where people originated and in which directions they migrated, starting in 18,000 bce. the lines start in central africa and from there, arrows point west toward northwestern africa, and east toward the middle east and asia. in the middle east, the line branches west into western europe and northeast into russia. the arrows also continue east into and throughout asia, including south into the southeast asian islands and australia. from north siberia, an arrow crosses into north america via the bering land bridge. from north america, the arrows move south into central and south america. © 2012 the exploration company based on this map, where are the oldest human civilizations? australia and africa africa and europe australia and asia asia and north america