A schematic for the fluid mechanics of a volcano is represented below.
Graciously borrowed from Figure 1 of the reference cited below
From the caption of that figure:
During explosive eruptions, bubble walls rupture catastrophically at the fragmentation surface. The released gas expands rapidly and magma ascent changes from a viscous melt with suspended bubbles to a gas flow with suspended magma fragments.
My assumption is that this boundary between the magma column and the steam above it is critical to the crystallization of the minerals to form the volcanic rock.
- Does the composition of the volcanic rock change drastically according to the distance between the magma column and the mouth of the volcano at the time the rock is formed?
- (Or) do the rocks melt and recrystallize frequently as the boundary moves, only attaining their final composition before they are ejected into the air?
- How important is the steam/air interface at the top of the volcano to the rock's formation?
I realize this is going to change based on the composition of the magma itself, but I'm looking for the behavior of an "average" rock species, though specific examples are great, too.