Explain how magma composition controls eruption style and volcano type

What this dot point is asking

SCSA wants you to explain the link from magma chemistry to eruption style, volcano shape and the hazards produced. The central variable is viscosity, and viscosity is set mainly by silica content, so build your answer around that chain.

Magma composition and viscosity

Magma differs chiefly in its silica content, which controls how runny or sticky it is.

• Low-silica (mafic) magma is hot and runny, with low viscosity. Dissolved gas escapes easily, so pressure does not build.
• High-silica (felsic) magma is cooler and sticky, with high viscosity. Gas cannot escape, so pressure builds until the magma erupts violently.

Gas content matters too: dissolved gases provide the explosive force, and viscous magma traps them.

Eruption style and volcano type

• Effusive eruptions from low-silica magma produce flowing lava that travels far and builds broad, gently sloping shield volcanoes, like those of Hawaii. They are dangerous to property but usually slow enough to escape.
• Explosive eruptions from high-silica magma blast out ash, gas and fragments, building steep-sided composite (strato) volcanoes from alternating lava and ash, like those around the Ring of Fire. They are far more deadly.

Volcanic hazards

Explosive eruptions produce the most dangerous hazards.

• Pyroclastic flows: fast, hot avalanches of gas and ash that destroy everything in their path.
• Ash falls: collapse roofs, damage engines and lungs, and disrupt aviation.
• Lahars: mudflows of ash mixed with water that travel down valleys.
• Volcanic gases: can be toxic and contribute to climate cooling when sulfur reaches the stratosphere.

Measuring eruption size

The Volcanic Explosivity Index, or VEI, ranks eruptions on a scale based mainly on the volume of material erupted and the height of the eruption column. It is logarithmic, so each step up represents a roughly tenfold increase in size. It lets scientists compare eruptions and rank explosive events that pose the greatest hazard.

Why subduction makes explosive magma

It is worth connecting eruption style back to plate tectonics, because the two dot points reinforce each other. The most explosive volcanoes sit above subduction zones, and this is not a coincidence: as the descending oceanic plate heats, it releases water into the overlying mantle wedge. That water lowers the mantle's melting point and drives partial melting, and as the resulting magma rises through and melts the continental crust above, it picks up silica and water, becoming viscous and gas-rich, exactly the recipe for an explosive eruption. By contrast, magma at a divergent boundary or a hotspot rises from the mantle without traversing thick silica-rich crust, so it stays low in silica, runny and effusive. This explains the global pattern: the steep, deadly composite volcanoes of the Ring of Fire are a direct consequence of water-fluxed melting at subduction zones, while the gentle shields of Hawaii reflect mantle melting away from any boundary.