Busy today trying to finish up the field trip guide for our Department trip to the Owens Valley in California in a little over a week. We'll be stopping a pile of great volcanic locations (and other stops), including the Coso Geothermal Field, Ubehebe Crater, the Mono-Inyo Crater Chain and a number of localities within the Long Valley caldera to see the Bishop Tuff. It should be a blast!
It very elegantly shows that as magma rises, the dissolved gases (mostly water and CO2 with some sulfur compounds) in the magma begin to come out of solution due to depressurization. Bubbles begin to form and if they can't escape, the get bigger and bigger until the magma is more bubble than magma - and that is when you get fragmentation. The bubbles burst and an explosive eruption happens. As you might guess, the stickier the magma (more viscous), the harder it is for bubbles to escape, thus higher the likelihood of an explosive eruption. That is why andesite, dacite and rhyolite - the more silicic magmas that are more viscous - are much more likely to erupt explosively than the less silicic lavas like basalt or basaltic andesite that are less viscous.
The figure is simple, elegant and gets the idea that bubbles lead to fragmentation then lead to explosive activity across wonderfully.