Magmatic Volatiles and Rheology

There are many questions about magmas that pertain to the origin of pyroclastic rocks. Why are some magmas quietly effusive, others violently explosive, and still others alternate between both characteristics? Why do some magmas build high-standing pyroclastic cones, some build small cinder cones, and still others enormous shield volcanoes or lava plateaus? Some of the most scenic volcanoes are towering cones composed primarily of pyroclastic debris knit together by a skeletal framework of lavas and dikes that help maintain the high-standing edifice. Is the commonly observed change from initial explosive to later more quiet eruptive activity within the same volcano caused by a decrease in the amount of dissolved volatiles, or is it caused by differences in the way rising magma interacts with its environment during its ascent? Which are the most important processes that cause magma to break up into particles?