Why do some volcanic eruptions consist mostly of lava flows, whereas others are explosive and do not produce flows?

Understanding the composition of magma is fundamental in foreseeing the behavior of a volcano. Different types of magma—basaltic, andesitic, and rhyolitic—are distinguished by their silica content, with basaltic having the least and rhyolitic the most. Silica contributes to the stickiness, or viscosity, of the magma.

Magma's composition affects its temperature, melting point, and how it interacts with gases. Basaltic magma, with its low silica content, tends to be hotter and more fluid. This fluidity helps facilitate the release of volcanic gases and tends to produce non-explosive eruptions with lava flows. In contrast, rhyolitic magma is cooler, more viscous, and traps gases more easily, which can lead to explosive eruptions when the trapped gases are suddenly released.

Magma viscosity is a critical factor that influences a volcano's eruption style. Viscosity is a measure of a substance's resistance to flow. In the case of magma, it stems from its composition—specifically, the silica content. High silica levels increase the magma's viscosity, making it thicker and less fluid.

Low-viscosity basaltic magma allows volcanic gases to escape gently, forming those classic streaming lava flows. On the other hand, higher-viscosity magmas, such as andesitic and rhyolitic, hinder gas escape, setting the stage for a pressure buildup. This pent-up pressure can lead to a dramatic and explosive release when it finally finds a way out.

The gas content in magma is like a volcanic force multiplier. Volcanoes belch out various gases, primarily water vapor, carbon dioxide, and sulfur dioxide. Magma that's rich in gas can lead to a more explosive eruption because as magma rises towards the surface, the pressure decreases, causing the gases to expand rapidly.

If the magma is low in viscosity, gases can escape continuously, leading to effusive, bubbling lava flows. However, when high-viscosity magma traps these gases, the increasing internal pressure can result in a sudden and violent eruption once the gas finds a means of escape.

The structure of the volcanic conduit—the passage through which magma travels to the surface—is like the volcano's plumbing. An open, unobstructed path allows gases to escape smoothly, thus favoring non-explosive lava flows. If the conduit is twisty, convoluted, or blocked with solidified magma from previous eruptions, it acts like a clogged pipe, trapping gases and generating explosive force.

A volcano's architecture, including whether it has a single central vent or a network of fractures, greatly influences how the magma and its entrapped gases behave. As one can imagine, a volcano with a complex interior structure is more likely to produce a violent eruption than a volcano with a simple, open conduit.