Volcanoes are frequently classified by their size and shape (as is described in the section Volcanic landforms), but they can also be classified by their eruptive habits. Indeed, the type of volcanic eruption that occurs plays an important role in the evolution of a volcanic landform, thus forming a significant link between eruptive habit and volcanic structure. In general, eruptions can be categorized as either effusive or explosive. Effusive eruptions involve the outpouring of basaltic magma that is relatively low in viscosity and in gas content. Explosive eruptions generally involve magma that is more viscous and has a higher gas content. Such magma is often shattered into pyroclastic fragments by explosive gas expansion during an eruption.

In more-detailed classification schemes based on character of eruption, volcanic activity and volcanic areas are commonly divided into six major types, shown schematically in the diagram. They are listed as follows in order of increasing degree of explosiveness:

1. Icelandic
2. Hawaiian
3. Strombolian
4. Vulcanian
5. Pelean
6. Plinian

The Icelandic type is characterized by effusions of molten basaltic lava that flow from long, parallel fissures. Such outpourings often build lava plateaus.

The Hawaiian type is similar to the Icelandic variety. In this case, however, fluid lava flows from a volcano’s summit and radial fissures to form shield volcanoes, which are quite large and have gentle slopes.

Strombolian eruptions involve moderate bursts of expanding gases that eject clots of incandescent lava in cyclical or nearly continuous small eruptions. Because of such small frequent outbursts, Stromboli volcano, located on Stromboli Island off the northeast coast of Italy, has been called the “lighthouse of the Mediterranean.”

The Vulcanian type, named for Vulcano Island near Stromboli, generally involves moderate explosions of gas laden with volcanic ash. This mixture forms dark, turbulent eruption clouds that rapidly ascend and expand in convoluted shapes.

A Pelean eruption is associated with explosive outbursts that generate pyroclastic flows, dense mixtures of hot volcanic fragments and gas described in the section Lava, gas, and other hazards. Pelean eruptions are named for the destructive eruption of Mount Pelée on the Caribbean island of Martinique in 1902. The fluidized slurries produced by these eruptions are heavier than air but are of low viscosity and pour down valleys and slopes at great velocities. As a result, they are extremely destructive.

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The Plinian type is an intensely violent kind of volcanic eruption exemplified by the outburst of Mount Vesuvius in Italy in 79 ce that killed the famous Roman scholar Pliny the Elder and was described in an eyewitness account by his nephew, the historian Pliny the Younger. In this type of eruption, gases boiling out of gas-rich magma generate enormous and nearly continuous jetting blasts that core out the magma conduit and rip it apart. The uprushing gases and volcanic fragments resemble a gigantic rocket blast directed vertically upward. Plinian eruption clouds can rise into the stratosphere and are sometimes continuously produced for several hours. Lightning strikes caused by a buildup of static electricity are common close to Plinian ash clouds, adding one more element of terror to the eruption.

Why are some volcanic eruptions so explosive while others are so spectacular but relatively harmless? The answer involves at least four factors: the amount of gas dissolved in the magma, the viscosity of the magma, the rate of decompression of the magma as it rises toward the surface, and the number of nucleation sites on which the gases can begin to form bubbles. Volcanoes related to converging plate margins (see the section Volcanism and tectonic activity) generally have a high gas content, and their magma is very viscous. This combination is explosive because the gases cannot easily boil out; rather, they remain pent up until they reach the pressure at which they blow the viscous magma into fragments. The rate at which pressure is reduced also controls the explosiveness. If magma moves slowly toward the surface, its dissolved gases will be released slowly and can escape. During the 1991 Plinian-type eruption of Mount Pinatubo, magma moved quite rapidly toward the surface, resulting in retention of most of the dissolved gases. Finally, the speed at which gases are released from magma is affected by the number of small crystals, which can act as nucleation sites where gas bubbles begin to form. At Pinatubo the magma was more than 40 percent small crystals before the eruption, while at the Hawaiian volcanoes Kilauea and Mauna Loa the percentage of small crystals in the magma is very low (less than 5 percent).

Two 20th-century eruptions

There are many gradations among—and exceptions to—the idealized eruption types listed in the previous section, and it is not unusual for an eruption sequence to involve more than one type of activity. For example, the eruptions of Mount St. Helens from 1980 to 1986 followed a sequence of small Vulcanian-type explosions, large Pelean and Plinian explosions, and finally extrusions of viscous lava into a lava dome that capped the vent. The different types of volcanic activity can best be understood by making comparisons, and in this section two specific eruptions are compared—the 1991 eruption of Mount Pinatubo (a classic example of explosive volcanism) and the 1984 eruption of Mauna Loa (illustrative of effusive volcanism).