One thing that's particularly interesting about the magmatic composition of a stratovolcano is the fact that the lava
is very high in SiO2, but it also has a lot of basalt in it since it's produced at a subduction
zone. We know this because there is a high basaltic content in oceanic crust.
As we learnt in Geology 1020, mafic magmas contain about 50% SiO2, while silicic magmas
contain from 60-75% SiO2.
Incidentally, while the magma from the 1990 eruption was slightly low in its silica content, it's still
considered to be a granitic magma. However, the magma from a stratovolcano still has a relatively high water content
(roughly 5%)(USGS, 2003).
Consequently, the water content makes the magma more fluid because it breaks up its Silica-Oxygen bonds,
and also makes the magma erupt more violently because it expands gas bubbles.