Volcanic eruptions range from gentle, flowing rivers of molten rock to violent, cataclysmic explosions. A highly explosive eruption is characterized by the rapid fragmentation of molten material, propelling vast clouds of ash and rock fragments high into the atmosphere. The intensity of this event is determined by the fundamental internal characteristics of the molten rock within its chambers. These physical properties dictate whether an eruption will be a relatively calm outpouring or a destructive blast.
The Physics of Trapped Pressure
The mechanical process driving a volcano’s explosivity begins with dissolved gases, known as volatiles, within the molten rock. Magma naturally contains gases like water vapor and carbon dioxide, which remain dissolved under immense pressure deep beneath the surface. As this gas-rich magma rises, the surrounding pressure decreases, causing the dissolved gases to form bubbles, a process called exsolution.
The behavior of these gas bubbles determines the eruption style. If the magma is fluid, the bubbles escape easily, leading to a gentle, effusive eruption. If the magma is thick, however, it resists the upward movement of the bubbles, trapping them inside. This trapping mechanism causes the pressure within the magma column to build up exponentially.
When the internal pressure of the expanding gases exceeds the strength of the overlying rock, the material violently fragments. This sudden release of trapped pressure causes the explosive ejection of pulverized magma, creating massive ash plumes and pyroclastic flows. Explosive volcanism is directly caused by the combination of high dissolved gas content and high resistance to flow.
How Silica Controls Magma Thickness
The physical property controlling a magma’s resistance to flow is viscosity, which is chemically determined by the rock’s silica content. Magma is molten material composed primarily of silicate minerals, containing silicon and oxygen. Within the melt, these atoms bond together to form complex, tetrahedral structures.
Magma high in silica, known as felsic magma, contains a greater proportion of these silicon-oxygen tetrahedra. These structures readily link together to form long chains and networks, a process called polymerization. This extensive interlocking structure increases internal friction, making the magma extremely thick and sluggish.
Conversely, magma low in silica, known as mafic magma, has simpler chemical structures that do not polymerize as readily. This allows the material to flow much more easily, resulting in low viscosity. The greater the silica content, the higher the viscosity, and the more effective the magma is at trapping explosive gases.
Identifying Explosive Magma and Volcano Structures
The type of magma consistently associated with highly explosive eruptions is the high-silica variety, specifically Andesitic, Dacitic, and Rhyolitic magma. Rhyolitic magma, often exceeding 68 weight percent silica, is the most viscous and prone to violent fragmentation. Andesitic magma, with intermediate silica content (55 to 65 weight percent), is also highly viscous and frequently produces explosive events.
These high-viscosity magmas tend to build a specific volcanic landform known as a stratovolcano, or composite cone. Stratovolcanoes are characterized by their steep, conical shape, resulting from the thick magma solidifying before it can flow very far. The mountain is built up over time by alternating layers of viscous lava flows and fragmented pyroclastic material.
Many dangerous volcanoes, such as Mount St. Helens, Mount Fuji, and Mount Vesuvius, are stratovolcanoes fueled by these magmas. This magma is typically generated above subduction zones, where one tectonic plate slides beneath another. This process causes the melting of water-rich crustal rock that is naturally high in silica. The resulting material is both thick and volatile-rich, providing the necessary conditions for a massive eruption.