What causes some volcanic eruptions to be highly explosive?

Highly explosive volcanic eruptions are primarily caused by magma that is more viscous and has a higher gas content. This type of magma can become fragmented into pyroclastic materials by the explosive expansion of gases. The rapid release of dissolved gases, due to a sudden decrease in pressure, is a key factor. Additionally, the rate at which magma rises to the surface and the presence of small crystals that act as nucleation sites for gas bubbles also influence explosivity. Magmas with a high degree of polymerization, particularly those rich in silicon and aluminum, tend to be more viscous, contributing to explosive eruptions.123