Fizzy pop, sweets and volcanoes
What , I hear you ask is the link between these arbitary things?
Well, I saw an experiment on youtube where they put a mint sweet in a bottle of soda and hey presto, the soda erupts, rather like a volcano. And lava is full of gas and at high pressure(like the soda). If the pressure in the volcano is high enough, it will unblock the solid cap, which will fall into the lava lake, and act a bit like the sweets in the video, thus causing the great eruptive coloumn heights we see in super plinian eruptions. Note how high two liters of coke goes, and how long the eruption lasts, and compare that to the pressures and volumes in a volcano, and you get the scaled up version
However, I'm no Geologist, and this is a guess.