What characteristic of magma mainly determines its explosivenes?

The explosiveness of magma is primarily determined by its viscosity, which is influenced by its chemical composition, particularly the silica (SiO2) content. High-silica magmas tend to be more viscous, trapping gases and increasing pressure, leading to explosive eruptions. In contrast, low-silica magmas are less viscous, allowing gases to escape more easily and resulting in less explosive activity.