Types of volcanic eruptions

Some of the eruptive structures formed during volcanic activity (counterclockwise): a Plinian eruption column, Hawaiian pahoehoe flows, and a lava arc from a Strombolian eruption.

Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

There are three different types of eruptions. The most well-observed are magmatic eruptions, which involve the decompression of gas within magma that propels it forward. Phreatomagmatic eruptions are another type of volcanic eruption, driven by the compression of gas within magma, the direct opposite of the process powering magmatic activity. The third eruptive type is the phreatic eruption, which is driven by the superheating of steam via contact with magma; these eruptive types often exhibit no magmatic release, instead causing the granulation of existing rock.

Within these wide-defining eruptive types are several subtypes. The weakest are Hawaiian and submarine, then Strombolian, followed by Vulcanian and Surtseyan. The stronger eruptive types are Pelean eruptions, followed by Plinian eruptions; the strongest eruptions are called "Ultra-Plinian." Subglacial and phreatic eruptions are defined by their eruptive mechanism, and vary in strength. An important measure of eruptive strength is Volcanic Explosivity Index (VEI), an order of magnitude scale ranging from 0 to 8 that often correlates to eruptive types.

Eruption mechanisms

Diagram showing the scale of VEI correlation with total ejecta volume.

Volcanic eruptions arise through three main mechanisms:[1]

There are two types of eruptions in terms of activity, explosive eruptions and effusive eruptions. Explosive eruptions are characterized by gas-driven explosions that propels magma and tephra.[1] Effusive eruptions, meanwhile, are characterized by the outpouring of lava without significant explosive eruption.[2]

Volcanic eruptions vary widely in strength. On the one extreme there are effusive Hawaiian eruptions, which are characterized by lava fountains and fluid lava flows, which are typically not very dangerous. On the other extreme, Plinian eruptions are large, violent, and highly dangerous explosive events. Volcanoes are not bound to one eruptive style, and frequently display many different types, both passive and explosive, even in the span of a single eruptive cycle.[3] Volcanoes do not always erupt vertically from a single crater near their peak, either. Some volcanoes exhibit lateral and fissure eruptions. Notably, many Hawaiian eruptions start from rift zones,[4] and some of the strongest Surtseyan eruptions develop along fracture zones.[5] Scientists believed that pulses of magma mixed together in the chamber before climbing upward—a process estimated to take several thousands of years. But Columbia University volcanologists found that the eruption of Costa Rica's Irazú Volcano in 1963 was likely triggered by magma that took a nonstop route from the mantle over just a few months.[6]

Volcanic Explosivity Index

The Volcanic Explosivity Index (commonly shortened to VEI) is a scale, from 0 to 8, for measuring the strength of eruptions. It is used by the Smithsonian Institution's Global Volcanism Program in assessing the impact of historic and prehistoric lava flows. It operates in a way similar to the Richter scale for earthquakes, in that each interval in value represents a tenfold increasing in magnitude (it is logarithmic).[7] The vast majority of volcanic eruptions are of VEIs between 0 and 2.[3]

Volcanic eruptions by VEI index[7]

VEI Plume height Eruptive volume * Eruption type Frequency ** Example
0 <100 m (330 ft) 1,000 m3 (35,300 cu ft) Hawaiian Continuous Kilauea
1 100–1,000 m (300–3,300 ft) 10,000 m3 (353,000 cu ft) Hawaiian/Strombolian Fortnightly Stromboli
2 1–5 km (1–3 mi) 1,000,000 m3 (35,300,000 cu ft) Strombolian/Vulcanian Monthly Galeras (1992)
3 3–15 km (2–9 mi) 10,000,000 m3 (353,000,000 cu ft) Vulcanian 3 months Nevado del Ruiz (1985)
4 10–25 km (6–16 mi) 100,000,000 m3 (0.024 cu mi) Vulcanian/Peléan 18 months Eyjafjallajökull (2010)
5 >25 km (16 mi) 1 km3 (0.24 cu mi) Plinian 10–15 years Mount St. Helens (1980)
6 >25 km (16 mi) 10 km3 (2 cu mi) Plinian/Ultra-Plinian 50–100 years Santa Maria ( 1902)
7 >25 km (16 mi) 100 km3 (20 cu mi) Ultra-Plinian 500–1000 years Tambora (1815)
8 >25 km (16 mi) 1,000 km3 (200 cu mi) Supervolcanic 50,000+ years[8][9] Lake Toba (74 k.y.a.)
* This is the minimum eruptive volume necessary for the eruption to be considered within the category.
** Values are a rough estimate. They indicate the frequencies for volcanoes of that magnitude OR HIGHER
There is a discontinuity between the 1st and 2nd VEI level; instead of increasing by a magnitude of 10, the value increases by a magnitude of 100 (from 10,000 to 1,000,000).