Earth Scientists study many different types of volcanoes and can make the observation that there are a wide variety of igneous rocks.
First observations are about composition.
Note names based on SiO2 content and not mineralogy. This is because as the temperature of solids become higher the bonds become weaker. In a melt, silicate tetrahedra become polymerized, that is they become joined together by sharing oxygens. The more polymerization, or the more shared oxygens, the more viscous the magma. Inreased silica ioons increase the degree of polymerization and thus the viscosity of the melt.