Magma’s resistance to flow, a property known as viscosity, is a fundamental physical characteristic that governs all aspects of volcanism. Viscosity determines how easily magma can move through the Earth’s crust and dictates the style of a volcanic eruption. Magma with low viscosity flows easily, allowing gases to escape smoothly, which results in non-explosive, or effusive, eruptions, such as those that form the shield volcanoes of Hawaii. Conversely, highly viscous magma traps expanding gases, building immense pressure that is violently released during an explosive eruption, like the kind associated with stratovolcanoes.
The Primary Control: Silica Content
The chemical composition of magma, specifically its content of silicon dioxide (\(SiO_2\)), or silica, is the most influential factor determining its viscosity. Silica forms the basic building block of all silicate minerals in magma. These silica tetrahedra readily link together by sharing oxygen atoms, a process called polymerization, which creates long, complex chains and networks within the molten rock. The degree of polymerization increases exponentially with a higher percentage of silica, leading to a significant increase in the magma’s internal friction. Magmas low in silica, such as basaltic magma (45–52% \(SiO_2\)), flow easily like a thin, watery syrup. In contrast, magmas high in silica, such as rhyolitic magma (65–75% \(SiO_2\)), develop highly polymerized frameworks, making the melt extremely resistant to flow and associated with the most explosive volcanic events.
Thermal State and the Impact of Temperature
Temperature provides the kinetic energy necessary to break the chemical bonds that create the magma’s internal structure. The relationship between temperature and viscosity is inverse: as the temperature of the magma rises, its viscosity decreases because the increased thermal energy disrupts the silicate chains and lowers internal friction. This allows the liquid to flow more readily. Magma types that form at higher temperatures are naturally less viscous, even with similar compositions. For instance, basaltic magmas, which have low silica content, erupt at very high temperatures, often exceeding \(1200^{\circ}C\). This combination of low silica and high heat results in the most fluid lavas. Rhyolitic magmas, due to their high silica content, may erupt at temperatures below \(900^{\circ}C\). Even at these lower temperatures, the strong, pervasive silica polymerization maintains an exceptionally high viscosity.
The Role of Dissolved Volatiles
Magma contains dissolved gases, or volatiles, primarily water vapor (\(H_2O\)) and carbon dioxide (\(CO_2\)), which profoundly affect its viscosity while the magma is still deep underground. When water is dissolved in the liquid silicate melt, it acts as a flux by chemically reacting with the oxygen bonds that link the silica tetrahedra together. This chemical interference effectively chops up the long, complex silicate chains, reducing the degree of polymerization. By breaking these bonds, dissolved water significantly lowers the internal friction and the overall viscosity of the magma.
Mechanical Interference: Crystal Content
The presence of solid mineral crystals suspended within the liquid melt provides a mechanical obstacle to flow, fundamentally increasing the magma’s bulk viscosity by physically impeding the movement of the remaining liquid matrix. As magma cools during its ascent or storage in a magma chamber, various minerals begin to crystallize, forming solid particles known as phenocrysts. Even a modest volume of crystals can substantially raise the effective viscosity of the magma, with this resistance increasing non-linearly as the crystal volume fraction grows. Magmas containing a high percentage of crystals, sometimes referred to as a “crystal mush,” can become so rigid that they exhibit a yield strength, meaning they require a significant force to even begin to flow. The highest viscosities occur in magmas that are both silica-rich and have a high crystal load, as the combined chemical and mechanical resistance creates a magma that is extremely resistant to deformation.