Stratovolcano

A stratovolcano, also known as a composite volcano, is the archetype of volcanic mountains: a steep-sided, symmetrical cone towering over the landscape. Examples like Mount Fuji, Mount Rainier, and Mount Vesuvius define this category. Unlike the broad, gentle slopes of shield volcanoes, stratovolcanoes are constructed from viscous magma that piles up near the vent rather than flowing freely.

Internal Structure and Stratification

The term “stratovolcano” is derived from the word strata (layers). These mountains are built over tens to hundreds of thousands of years through repeated eruptions. Their internal structure is a complex layer-cake of:

• Lava Flows: Typically intermediate (andesitic) to felsic (rhyolitic) in composition. These flows are thick and slow-moving, solidifying quickly to steepen the cone.
• Tephra and Pyroclastics: Loose layers of ash, cinders, and blocks ejected during explosive phases.
• Sills and Dikes: Intrusions of magma that solidify underground, acting as “ribs” that help stabilize the growing edifice.

This composite structure makes stratovolcanoes inherently unstable. The loose layers of tephra are prone to erosion and landslides, while the hydrothermal alteration from volcanic gases can weaken the rock into clay, leading to sector collapses.

Eruption Dynamics

Stratovolcanoes are notorious for their explosive power. The magma feeding them is typically high in silica (SiO₂) and dissolved gases. High silica content increases viscosity, trapping gas bubbles within the magma. As the magma rises and pressure decreases, these gases expand violently, blasting the magma into ash and pumice.

Common eruption styles include:

1. Vulcanian: Short, violent explosions that eject bombs and blocks.
2. Plinian: The most destructive type, creating towering eruption columns that can reach the stratosphere, spreading ash over continental scales.
3. Pelean: Characterized by the formation of glowing avalanches of hot gas and rock, known as pyroclastic flows.

The Life Cycle

Stratovolcanoes are often located at subduction zones, where one tectonic plate dives beneath another. This setting provides a steady supply of magma.

1. Growth: The volcano builds upwards. Eruptions are frequent, and the cone shape is maintained.
2. Mature: The volcano may reach a height where the summit becomes unstable. Caldera-forming eruptions may decapitate the cone (e.g., Mount Mazama becoming Crater Lake).
3. Degradation: Once the magmatic source moves or becomes extinct, erosion takes over. The cone is worn down, leaving behind only the hardened volcanic plug (neck) and radiating dikes, such as Shiprock in New Mexico.

Hazards

Due to their explosive nature and tendency to form steep slopes, stratovolcanoes pose multiple hazards. Beyond the eruption itself, they are the primary source of lahars (volcanic mudflows). The height of these mountains often allows them to support glaciers; during an eruption, the melting ice mixes with ash to create devastating torrents of mud that can destroy communities tens of kilometers downstream.