What causes volcanoes to erupt explosively?

Volcanoes erupt explosively when magma is rich in silica, making it thick and viscous. This traps gases within the magma, building immense pressure as it rises to the surface. When the pressure is suddenly released, the magma erupts violently, producing ash clouds, pyroclastic flows, and lava bombs. Explosive eruptions, like those of Mount St. Helens, can cause widespread destruction but also create fertile soils over time.