Why do magma erupt only sometimes explosively?

I am doing some experiments to answer this question.


How?
I am doing decompression experiments using shock tube type apparatus.
A rapid decompression might cause an explosive eruption, and I am simulating this process.



What determines the eruption styles?
According to my experiments, pressure difference DP and vesicularity phi are important.
I show you a movie how these two parameters are important.

Movie

This movie shows the response of bubbly fluid to a rapid decompression.
From the top

large phi & large DP
large phi & medium DP
small phi & medium DP
large phi & small DP
negligible phi & medium DP

See?