There are two kinds of volcanoes: one kind with violent eruptions producing steep slopes and the other kind with voluminous lava flows producing gentle slopes

The character of volcanic eruptions are largely controlled by the viscosity of the magma. Indeed, low -viscosity magma:

1. flows more easily than high viscosity magma
2. allows gases to escape easily whereas gas pressures can build up in high viscosity magmas.

Those two reasons explain why stratovolcanoes result in violent eruptions.