Answer: The amount of silica in the magma
The magma possesses high silica content approx 50-70% and because of that, it has a greater degree of polymerization. Because of that, they have high viscosity.
The more the silica content in the magma the more will be its viscosity. The volcanoes that exhibit mass destruction and massive eruptions possess high viscosity and high content of silica.
Hence, the silica content and the viscosity determine the flow of magma.