Composite volcanoes, also known as stratovolcanoes, are the steeply-sloped, conical mountains responsible for the most destructive and explosive volcanic events on Earth. Their structure, built up by alternating layers of hardened lava and fragmented rock, signals a history of powerful, intermittent eruptions. The reason for their extreme violence is fundamentally rooted in the underlying chemistry and physical properties of the molten rock that feeds them.
The Role of High-Viscosity Magma
The magma that powers composite volcanoes has a high concentration of silica, making it felsic or intermediate in composition. Silica, a compound of silicon and oxygen, forms complex molecular chains within the melt, which significantly increases its internal friction. This characteristic is known as high viscosity, meaning the magma is thick, sticky, and highly resistant to flow.
This high viscosity is a direct consequence of the magma’s chemical make-up (60 to 70 percent silica by weight). Unlike more fluid types of magma, this sticky material cannot easily release the volatile gases dissolved within it. The magma acts like a thick, impenetrable seal, preventing the escape of gases as the molten rock rises toward the surface.
How Internal Pressure Accumulates
The failure of the magma to degas easily creates the mechanism for the volcano’s explosive power. Magma contains significant amounts of dissolved volatile gases, primarily water vapor, carbon dioxide, and sulfur dioxide, held in solution by the immense pressure deep within the Earth. As the magma column moves upward, the surrounding pressure decreases, causing these gases to come out of solution and form bubbles.
Because the high-viscosity magma resists flow, these gas bubbles are unable to rise and escape to the atmosphere. Instead, the bubbles remain trapped, rapidly expanding and coalescing as the magma nears the surface. This process builds up a tremendous, explosive pressure within the magma chamber and conduit.
An eruption occurs when this internal gas pressure finally exceeds the strength of the overlying rock mass. The failure is catastrophic, resulting in a sudden and violent decompression that shatters the magma into tiny fragments of glass, ash, and pumice. This fragmentation drives the massive explosions, propelling material high into the atmosphere.
Distinctive Eruption Products and Hazards
The explosive fragmentation of high-viscosity magma produces a signature set of dangerous eruption products that define the violence of composite volcanoes.
Eruption Columns and Ashfall
The immediate result is the creation of towering eruption columns that inject fine ash and gases tens of thousands of feet into the stratosphere. This ashfall can travel for hundreds of miles, posing a threat to aviation and infrastructure.
Pyroclastic Flows
The most destructive hazard is the pyroclastic flow, a dense, fast-moving current of superheated gas, ash, and fragmented rock. These flows rush down the volcano’s flanks at speeds up to 700 kilometers per hour and reach temperatures of \(1,000^\circ\text{C}\), incinerating everything in their path. The steep slopes of composite volcanoes contribute to the speed and distance these flows can travel.
Lahars (Volcanic Mudflows)
Another significant danger is the formation of lahars, or volcanic mudflows, which are often the cause of most fatalities associated with these volcanoes. Lahars occur when the ejected ash and debris mix with water, either from heavy rainfall or the sudden melting of the volcano’s snow and ice cap. This slurry can travel rapidly through river valleys, posing a severe threat to communities located far downstream.
Contrasting Composite Volcanoes with Shield Volcanoes
The explosive violence of composite volcanoes is best understood by contrasting them with the gentler, effusive eruptions of shield volcanoes, such as those found in Hawaii. Shield volcanoes are fed by basaltic, low-silica magma, which has a very low viscosity. This fluid, runny magma allows the dissolved gases to bubble out and escape easily without building up significant pressure.
The result is a relatively non-explosive eruption style characterized by lava fountains and extensive, fast-moving lava flows. The low-viscosity lava travels great distances before solidifying, leading to the volcano’s broad, gently sloping shape, which resembles a warrior’s shield. This fundamental difference in magma viscosity separates the catastrophic power of a composite volcano from the flow of a shield volcano.