Why are Cascade volcanoes explosive volcanoes?

The volcanoes of the Cascade Range are explosive because they are formed by the subduction of the Juan de Fuca Plate beneath the North American Plate. This tectonic process generates magma that is characteristically high in silica and viscosity.12

The explosive nature of these volcanoes is driven by several key factors:

• High Gas Content: Subduction-zone magmas are rich in water and carbon dioxide derived from subducted seafloor sediments and assimilated crustal rocks.3
• Viscosity: Because the magma is highly viscous, these gases cannot easily escape as they rise. Instead, the gases remain trapped and build up immense pressure.32
• Explosive Expansion: When the magma reaches the surface, the sudden reduction in pressure causes the trapped gases to expand rapidly, shattering the viscous magma into pyroclastic fragments and resulting in violent, explosive eruptions.32

This combination of high viscosity and high gas content is the primary reason why Cascade composite cones are categorized as explosive volcanoes.312