Magma is molten rock found beneath the Earth’s surface; an eruption is the process of this material being released onto the surface as lava, ash, or gas. The behavior of this release, whether it is a gentle, flowing stream or a catastrophic explosion, is governed by the physical properties of the magma itself. The style of a volcanic eruption is fundamentally controlled by two measurable factors that dictate how easily the magma moves and how much energy it can store. These two primary factors are the magma’s viscosity and its content of dissolved gases.
Magma Viscosity and Flow Resistance
Viscosity describes a fluid’s internal resistance to flow, a measure of how “thick” or “sticky” the molten rock is. Magma with low viscosity flows easily, much like warm honey, while magma with high viscosity is stiff and moves sluggishly, similar to peanut butter. This property is largely controlled by the magma’s chemical composition, specifically its concentration of silicon dioxide, or silica (\(\text{SiO}_2\)).
Magmas with high silica content, often referred to as felsic magmas like rhyolite, have a complex molecular structure. The silica tetrahedra link together to form long, tangled chains in a process called polymerization, which increases the internal friction and thus the viscosity. These viscous magmas are typically cooler, erupting at temperatures as low as \(650^\circ\text{C}\) to \(800^\circ\text{C}\), which further contributes to their resistance to flow.
Conversely, magmas with low silica content, known as mafic magmas like basalt, have less polymerization, allowing them to remain fluid. These magmas can be considerably hotter, often erupting at temperatures between \(1000^\circ\text{C}\) and \(1200^\circ\text{C}\), which dramatically lowers their viscosity.
Dissolved Gas Content and Pressure Buildup
The second factor determining eruption style is the concentration of volatiles, or dissolved gases, within the magma. These gases are primarily water vapor (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)), and they are held in solution under immense pressure deep beneath the surface. As the magma begins its ascent toward the surface, the pressure exerted by the surrounding rock decreases.
This drop in pressure causes the dissolved gases to exsolve and form tiny bubbles. This bubble formation is the driving force of an eruption, as the gas phase occupies significantly more volume than the dissolved liquid. The potential for an explosive event is directly related to the amount of gas present and its ability to expand.
If the magma has a low viscosity, the gas bubbles can rise quickly and escape relatively smoothly, preventing excessive pressure accumulation. However, if the magma is highly viscous, the stiff, sticky melt traps the gas bubbles, preventing them from migrating and escaping. This imprisonment causes the internal pressure within the magma conduit to build rapidly.
How Viscosity and Gas Combine to Determine Eruption Style
The interaction between magma viscosity and dissolved gas content ultimately dictates the visible style of an eruption, ranging from gentle flows to devastating explosions. Eruptions are classified based on this combined behavior. Low-viscosity (mafic) magma combined with a low to moderate gas content results in an effusive eruption.
In these effusive events, characteristic of shield volcanoes like those in Hawaii, the gas escapes continuously and gently, allowing the lava to flow out non-violently over the landscape. Even with low viscosity, a high gas content can still produce energetic features like lava fountains, but the overall eruption remains non-explosive because the gas is not trapped long enough to generate catastrophic pressure.
In stark contrast, when high viscosity (felsic) magma combines with a high gas content, the result is an explosive eruption, such as the Plinian or Vulcanian styles common to stratovolcanoes. The high silica content traps the abundant gas, allowing pressure to build to a point where the magma fragments violently into ash and pumice, creating massive eruption columns. This fragmentation occurs because the pressure exceeds the strength of the overlying rock and the magma itself.
An intermediate category, such as a Strombolian eruption, involves moderate viscosity and moderate gas content, leading to short, rhythmic bursts of activity. Thus, the physical resistance to flow (viscosity) controls the seal on the system, while the amount of gas provides the driving energy, and the interplay between the two dictates the eventual release.