Tectonic Plate Movement: Volcanoes

Because the lava is so fluid, trapped gases can bubble out and escape easily, leading to calm, frequent, and effusive eruptions. The runny lava flows rapidly over vast distances before finally cooling and solidifying. Over time, these frequent eruptions build up flat, broad structures made entirely from layers of lava, resulting in gentle slopes of less than $10^\circ$. A classic example is Skjaldbreiður in Iceland.

Destructive Boundaries and Composite Volcanoes

Think about shaking a warm bottle of fizzy drink before taking off the lid—the trapped gas builds up immense pressure until it violently bursts. This is the exact mechanism that powers a composite volcano.

These landforms are created at a destructive plate boundary, where a denser oceanic plate is forced to move towards a lighter continental plate. Driven by the immense weight of the sinking rock, the oceanic plate is thrust downwards into the mantle in a process known as subduction.

As the plate plunges deeper, intense friction and the release of trapped water and impurities lower the melting point of the surrounding rock, causing partial melting. This process generates andesitic magma (or rhyolitic magma), which rises slowly through faults in the continental crust above.

Andesitic magma contains a much higher proportion of silica ($55\%$ to $75\%$). This high silica concentration creates strong molecular bonds, making the magma highly viscous—meaning it is exceptionally thick, sticky, and pasty.

This thick magma completely traps expanding volcanic gases. As the magma rises, gas pressure builds to extreme levels until it triggers a massive, explosive eruption. The sheer violence of the blast fragments the magma, firing out shattered rock and ash. The thick lava cannot flow very far before solidifying, so it piles up close to the vent. These stratovolcanoes are built from alternating layers of this hardened lava and volcanic ash, giving them a narrow base and steep sides angled at $30^\circ$ to $35^\circ$.

Explosive eruptions also generate severe, life-threatening hazards. The most dangerous is a pyroclastic flow, which is a superheated, fast-moving avalanche of gas and ash. Furthermore, when the loose ash mixes with heavy rainfall or melting snow, it can trigger a lahar, a devastating volcanic mudflow.

Comparing Volcano Types: The Role of Magma

Why do some volcanoes look like flat shields and others like towering cones? The fundamental reason is the chemical composition of the magma.

The amount of silica in the magma directly controls its viscosity, which dictates the trapped gas pressure and ultimately determines the eruption style and the resulting landform shape. We can summarise this causal chain:

• $\uparrow$ Silica $\rightarrow$ $\uparrow$ Viscosity $\rightarrow$ $\uparrow$ Gas Pressure $\rightarrow$ Explosive Eruption $\rightarrow$ Steep Slopes

The table below contrasts the key characteristics of both volcano types:

Feature	Shield Volcano	Composite Volcano
Boundary Type	Constructive (Divergent)	Destructive (Convergent)
Magma Type	Basaltic	Andesitic / Rhyolitic
Silica Content	Low ($45\% - 55\%$)	High ($55\% - 75\%$)
Viscosity	Low (thin and runny)	High (thick and sticky)
Eruption Style	Effusive (calm, frequent)	Explosive (violent, infrequent)
Structure	Built from layers of lava only