Volcanoes can be classified in several ways, and there is no single universally complete list because different classification systems emphasise shape, composition, eruptive behaviour, geological setting or activity. The most commonly discussed morphological types include shield volcanoes, stratovolcanoes, cinder cones and lava domes. Other important volcanic structures include fissure vents, volcanic fields, calderas, complex volcanoes, submarine volcanoes and plateau forming volcanic provinces.
A volcano can also evolve from one morphological state to another, and a single volcanic region may contain several types of vents. Classification therefore provides a useful framework rather than an absolute set of mutually exclusive categories. The shape of a volcano is strongly related to the physical properties of its erupted materials, especially viscosity, eruption rate and the relative proportions of lava and pyroclastic deposits.
Shield volcanoes are broad, gently sloping volcanic edifices built predominantly from repeated flows of relatively fluid lava, commonly basaltic. Their low profile resembles a broad shield lying on the ground, hence the name. Because basaltic lava can travel long distances before solidifying, successive flows spread widely around the vent. Shield volcanoes can become enormous in area and volume and may form major oceanic islands.
Hawaiian volcanoes such as Mauna Loa and Mauna Kea illustrate large shield volcanoes, although their individual histories and structures are complex. Shield volcanoes commonly produce effusive eruptions, but they are not incapable of explosive activity, particularly when magma interacts with groundwater or when unusual magma compositions and volatile conditions occur.
Stratovolcanoes, also called composite volcanoes, are generally steep sided and constructed from repeated layers of lava, ash, tephra and other volcanic deposits. They commonly occur above subduction zones and can contain magma ranging from basaltic through andesitic to dacitic and rhyolitic compositions. Their magma is often more viscous than the basaltic magma typical of shield volcanoes, and volatile rich magma can produce explosive eruptions. Stratovolcanoes can generate lava flows, ash columns, pyroclastic density currents, lahars and volcanic debris avalanches. Mount Fuji, Mount St Helens, Mount Etna and many volcanoes of the Andes are examples of stratovolcanic systems, although each has distinctive geological characteristics. The term composite reflects their construction from multiple eruptive products rather than implying that every layer is compositionally identical.
Cinder cones are relatively small volcanic cones composed primarily of loose fragments called scoria or cinders, together with ash and other pyroclastic material. They commonly form around a single vent during relatively short lived eruptions, although their activity can sometimes be more complex. Gas rich basaltic magma can fragment into clasts that are thrown into the air and fall around the vent, gradually creating a steep sided cone. Many cinder cones have a bowl shaped crater at the summit. Because the deposits are loose rather than strongly consolidated lava, their slopes commonly approach the natural angle of repose for granular volcanic material. Cinder cones can occur as independent volcanoes or on the flanks of larger volcanic systems.
Lava domes are formed when highly viscous lava accumulates near a vent rather than flowing far from it. Dacitic and rhyolitic compositions commonly produce domes, although dome formation depends on physical properties rather than composition alone. A dome may grow slowly as new magma is pushed into an existing mass, producing unstable spines, lobes and steep slopes. Collapse of a lava dome can generate pyroclastic density currents, making dome forming volcanoes potentially hazardous even when lava movement itself is slow. Dome growth can also plug a volcanic conduit temporarily, allowing pressure and gas to accumulate beneath or within the dome. Subsequent explosive disruption can therefore occur after periods of apparently quiet dome growth.
Fissure volcanoes or fissure vents are systems in which lava erupts through elongated cracks rather than a single central cone. They are especially important at divergent plate boundaries and in large basaltic volcanic provinces. Fissure eruptions can produce extensive sheets of basalt and may feed lava flows that cover large areas. Iceland provides prominent examples because it lies on the Mid Atlantic Ridge and above a region of active mantle upwelling.
Calderas are another major volcanic landform, generally produced when the ground collapses after substantial magma withdrawal during a large eruption, although caldera formation can involve several mechanisms. Calderas may subsequently host lakes, lava domes, new cones and hydrothermal systems. Their large size distinguishes them from the relatively small craters found at many volcanic vents.
Volcanic fields contain numerous vents and small volcanic centres distributed across a broad region rather than one dominant cone. They may contain scoria cones, lava flows, maars, tuff rings and other structures. Maars are broad, relatively low craters formed by explosive interaction between rising magma and groundwater or wet sediments, while tuff rings and tuff cones can form from similar phreatomagmatic processes.
Submarine volcanoes include seamounts, submarine fissures and hydrothermal volcanic systems, many of which occur along mid ocean ridges and hotspots. Large igneous provinces represent another scale of volcanism, involving enormous volumes of magma erupted over relatively short geological intervals and producing extensive basaltic plateaus. These structures demonstrate that volcanism cannot be represented adequately by cone shaped mountains alone.
Another classification concerns activity rather than physical form. Volcanoes may be described as active, dormant or extinct, although these terms are approximate and depend on geological evidence and the timescale considered. Volcanoes can also be classified according to eruption style, magma composition, tectonic environment or hazard characteristics. These classification systems overlap because a shield volcano can be active or dormant, a stratovolcano can have both explosive and effusive eruptions, and a volcanic field can contain several different landform types.
The most useful approach is therefore to treat volcano type as a description of particular structural, compositional and eruptive characteristics rather than a rigid biological style of classification. Understanding these categories helps scientists interpret volcanic histories, compare volcanic systems and assess the processes and hazards associated with different forms of volcanism.
