Classical model of a volcano with a permanent shallow magma chamber.
Current model of a volcano in which melted material is stored between the crystals of an extensive mush. The melt separates out from the mush very slowly. Because it is lighter than the surrounding rock, it is unstable, and rises to the surface, with only transient storage in a shallow magma chamber.
Lab experiments show how an instability forms between liquids of different densities and viscosities. In this experiment, the denser, less viscous material (analogous to the melt) is placed above the denser more viscous material (analogous to the crustal rocks). This is upside down compared to the real world, in which the melt is produced below the rocks, and finds its way to the surface when an instability forms a blob to rise.
All images courtesy of Steve Sparks