Introduction
Here is one of the most counterintuitive facts in all of Earth science: the volcanoes that look the least dangerous — the ones that barely erupt at all, whose flanks are covered in forests, whose summits are cold and quiet — are often the most catastrophic when they finally go. The reason is viscosity. High-silica magma is so stiff it cannot let its gas escape; the pressure builds for centuries until the mountain explodes rather than oozes. Low-silica basalt, by contrast, flows like a river and vents gas harmlessly. Shape and silence are not safety — chemistry is.
Why does Kilauea in Hawaii erupt gently flowing rivers of lava that people can approach to within metres, while Mount Pinatubo in the Philippines exploded in 1991 with the force of thousands of atomic bombs? Both are volcanoes. Both produce magma. The difference lies almost entirely in the chemistry of their magma — specifically, in the silica (SiO₂) content and the concentration of dissolved gases (volatiles) such as water vapour, carbon dioxide, and sulfur dioxide.
Silica forms the backbone of most minerals in magma. SiO₄ tetrahedra link together in chains, sheets, and three-dimensional networks as magma cools, and in high-silica magmas this polymerisation occurs even in the melt — creating a viscous, sticky substance that flows as reluctantly as cold tar. Low-silica basaltic magma, by contrast, has a much simpler molecular structure and flows at speeds comparable to water on gentle slopes. Viscosity is the key physical property that controls virtually everything about how a volcano erupts: whether lava flows or explosive eruptions dominate, whether gases can escape gradually or build to catastrophic pressure, and how tall and steeply-sided a volcanic edifice can grow.
The relationship between magma composition and eruption style has a simple rule of thumb: low silica = low viscosity = gentle effusive eruption; high silica = high viscosity = explosive eruption. This rule has exceptions and nuances, but it correctly predicts the dominant eruption style of thousands of volcanoes and explains why the Hawaiian volcanoes, the Cascade volcanoes, and the Yellowstone supervolcano represent such profoundly different volcanic systems despite all being powered by mantle heat.
Key Terms
The primary chemical control on magma behaviour. Ranges from ~45% (ultramafic) to ~75% (rhyolitic). Classification: basalt (45–52%), andesite (52–63%), dacite (63–68%), rhyolite (68–75%+). Higher SiO₂ → more polymerised melt → higher viscosity. Also correlates with tectonic setting: basalt at MOR/hotspots; andesite/dacite at subduction zones; rhyolite in continental settings.
A measure of a fluid's resistance to flow. Units: Pascal-seconds (Pa·s). Water: 0.001 Pa·s. Honey: ~10 Pa·s. Basaltic lava: ~10–1,000 Pa·s. Rhyolitic lava: 10⁸–10¹⁴ Pa·s (approaching solid behaviour). Controlled by silica content, temperature (hotter = less viscous), and volatile content (dissolved water decreases viscosity). The most important property of magma for volcanic hazard assessment.
Dissolved gases in magma, primarily H₂O (water), CO₂ (carbon dioxide), SO₂ (sulfur dioxide), H₂S, HF, and HCl. Dissolved at depth under pressure; exsolve (form bubbles) as magma rises and pressure decreases. The volume expansion during exsolution can fragment viscous magma into pyroclasts (volcanic bombs, tephra, ash). Higher volatile content → more explosive potential. Subduction zone magmas typically contain more water than MORB.
The process by which minerals crystallise from a cooling magma and are removed (settle out or are carried away), leaving a residual melt of different composition. Mafic minerals (olivine, pyroxene) crystallise first at higher temperatures from basaltic magma, removing Mg, Fe, and Ca; the remaining melt becomes progressively enriched in SiO₂, K, and Na. Fractional crystallisation can drive a basaltic parent magma toward andesitic, dacitic, or rhyolitic compositions over time.
All fragmental material ejected by a volcanic eruption: volcanic bombs (>64 mm (2.52 in), still molten during flight), lapilli (2–64 mm (2.52 in)), volcanic ash (<2 mm (0.08 in) diameter). Ash is formed when viscous magma is fragmented by the explosive expansion of exsolving volatiles. Fine ash can remain airborne for days to weeks, travelling thousands of kilometres and affecting aviation, human health, and climate (by reflecting solar radiation).