Volcanology is the scientific study of volcanoes, and classifying these geological features is essential for assessing hazards and interpreting deep Earth processes. Geologists rely on multiple independent criteria, as no single characteristic fully describes a volcano’s behavior or history. Classification systems categorize volcanoes based on their physical shape, the chemistry of their molten material, and their historical eruptive patterns. By applying these distinct classifications, scientists gain a comprehensive view of how and why volcanoes differ across the globe.
Classification by Physical Structure and Morphology
Geologists often begin classifying a volcano based on its visible, external shape, or its morphology. This physical structure is a direct result of the type of lava erupted over time and how that material accumulates. The three primary morphological types are shield volcanoes, stratovolcanoes, and cinder cones, each distinguished by its size and slope profile.
Shield volcanoes are characterized by their broad, gently sloping sides, resembling a warrior’s shield lying on the ground. They are built from successive, highly fluid basaltic lava flows that spread out over great distances before cooling. These volcanoes, such as Mauna Loa in Hawaii, can grow to be enormous, with slopes typically ranging from 2 to 10 degrees.
Stratovolcanoes, also known as composite cones, represent the classic, steep-sided conical shape. They are constructed from alternating layers of hardened lava flows and fragmented rock material, or tephra. Their slopes are much steeper, often between 10 and 30 degrees, because the more viscous lava cannot travel far before solidifying.
Cinder cones are the smallest and simplest type, usually rising no more than a few hundred meters high with steep, straight sides. They form from the accumulation of loose, glassy fragments of scoria ejected from a single vent. Most cinder cones are monogenetic, meaning they result from a single, relatively short-lived eruptive phase.
Classification by Magma Composition and Viscosity
The most fundamental classification centers on the chemistry of the molten material, known as magma, because composition dictates a volcano’s behavior. Silica content (silicon dioxide, \(\text{SiO}_2\)) is the primary chemical factor used, as it directly controls the magma’s viscosity (resistance to flow). The silica content in magma generally ranges from 45% to 75% by weight.
Magmas with low silica content (typically 45–52%) are termed mafic, and they produce the rock basalt upon cooling. This low viscosity allows dissolved gases to escape easily, leading to relatively gentle, effusive eruptions where lava flows freely.
Conversely, magmas with high silica content (above 63%) are termed felsic, and they form the rock rhyolite. The silica molecules combine into complex polymer chains, which significantly increases the magma’s internal friction and makes it highly viscous. Intermediate magmas, such as andesite, fall between the mafic and felsic categories, typically having silica content between 52% and 63%.
High-viscosity magmas trap volcanic gases (such as water vapor and carbon dioxide), preventing them from escaping easily as the magma rises. The pressure from these trapped gases builds substantially beneath the surface. When this pressure is finally released, it leads to violent, explosive eruptions that fragment the magma into ash and tephra.
Classification by Eruptive Style and Activity Status
Geologists also classify volcanoes by their characteristic eruptive style, which describes the manner and intensity of the eruption process itself. These styles are determined by the interplay of magma viscosity and the volume of gas released. The classification uses historical examples to define a spectrum of behavior, ranging from calm outpourings to catastrophic explosions.
Hawaiian eruptions are the calmest style, characterized by the effusive eruption of highly fluid, low-viscosity basaltic lava, often producing spectacular fire fountains. Strombolian eruptions are slightly more energetic, involving frequent, mild explosive bursts of gas that launch incandescent fragments hundreds of meters into the air. These explosions are driven by the continuous formation and bursting of large gas bubbles within the magma.
The most violent styles, such as Plinian eruptions, are associated with high-viscosity, gas-rich magmas. They involve the sustained ejection of gas and ash into the atmosphere, forming massive eruption columns that can reach tens of kilometers in height. This explosive intensity is quantified using the Volcanic Explosivity Index (VEI), which ranges from 0 for the least explosive to 8 for the largest super-eruptions.
For risk assessment, volcanoes are also classified by their activity status: active, dormant, or extinct. An active volcano is one that is currently erupting or showing signs of unrest, or has erupted within the past 10,000 years. A dormant volcano has not erupted recently but is still considered capable of future eruptions based on its geological history. An extinct volcano is considered unlikely to erupt again because it has been inactive for a very long period.