Volcanic eruptions are among the most fascinating and destructive natural events that can take place on Earth. However, not all volcanic eruptions are the same, and volcanologists have subdivided them into different types. This classification is based on the mechanisms behind the eruption and the condition in which it happens, and different types are often named after famous volcanoes where a certain behavior has been observed. Different kinds of eruptions can also be divided according to their strength using the Volcanic Explosivity Index (VEI). This measure, which ranges from 0, for non-explosive eruptions, to 8, for the largest eruptions in history, is often related to the kind of eruption that took place.
The three main types of volcanic eruptions are magmatic eruptions, phreatic eruptions, and phreatomagmatic eruptions, and they further subdivided into more types. Magmatic eruptions are the most common type of volcanic eruptions, and involve the expulsion of magma, which is propelled forward by the decompression of gas. This kind of eruption has a wide range of intensity, from small lava fountains to some of the most destructive events in Earth’s history. Magmatic eruptions are subdivided into Hawaiian, Strombolian, Vulcanian, Peléan, and Plinian.
The least destructive type of volcanic eruption is the Hawaiian eruption, named after the volcanoes of the Hawaiian archipelago. Hawaiian eruptions often begin from a fissure vent and result in lava fountains, which can continue for years. The ejected material is usually much smaller than in other types of eruption, but the steady production of small amount of lava leads over time to the creation of a wide shield volcano. Hawaiian eruptions usually have a VEI of 0 or 1.
Eruption of a fissure during the 2022 eruption of Mauna Loa in Hawaii, a Hawaiian eruption with a VEI of 0.
The Strombolian eruptions, named after the Stromboli volcano in Italy, are slightly stronger. These eructions are the result of the bursting of gas bubbles within the magma, which rise and explode after reaching the surface. Strombolian eruptions can last for a long time with loud blasts from the explosion of gas bubbles happening as often as every few minutes. Despite the fact that lava columns from Strombolian eruptions can reach hundreds of meters, these are still mild eruptions, with a VEI or 1 or 2. In rare cases, violent Strombolian eruptions can reach a VEI of 3.
Near Stromboli is the island of Vulcano, which gives its name to Vulcanian eruptions. Similarly to Strombolian eruptions, Vulcanian eruptions happen when the gas bubbles trapped under the magma explode. However, Vulcanian eruptions are much stronger, with columns often rising 5 to 10 km above the volcano. Vulcanian eruptions usually start with a series of small explosions, which can last for a few hours. The lava dome holding down the magma slowly becomes weaker and the eruption becomes more quiet and continuous. These eruptions have a VEI between 2 and 4.
Peléan eruptions are named after Mount Pelée on the island of Martinique in the Caribbean. This volcano erupted in 1902 in what was the most destructive eruption of the 20th century, causing around 29,000 deaths. Peléan eruptions expel large amounts of gas, dust, ash, and lava fragments from the central crater after the collapse of the lava dome. As the material collapses upon itself, it forms a fast-moving pyroclastic flow, which descends the mountain at over 150 km/h. This makes Peléan eruptions extremely dangerous, and they can result in the destruction of entire cities and a large loss of life. The VEI of these eruptions ranges from 3 to 5.
Even stronger than Peléan eruptions are Plinian eruptions. This type of eruption is named after Roman author Pliny the Younger, who observed the eruption of Mount Vesuvius in 79 CE, which buried the cities of Pompeii and Herculaneum. Plinian eruptions begin with the accumulation of gas in a magma chamber located underneath the volcano. After the gas bubbles have reached a certain size, they explode, forcing magma to be expelled from a crater and forming a tall eruptive column. These columns, which are the distinctive feature of Plinian eruptions, can reach up to 45 km into the atmosphere, and powerful winds at the top can spread the expelled material far from the volcano. The area in the immediate vicinity of a volcano after a Plinian eruption is subjected to the rainfall of volcanic rocks, while the ash plume expelled from the volcano slowly falls to the ground, covering everything in a thick layer of ash. Even more dangerous is the pyroclastic flow, which can descend the volcano with speeds of up to 700 km/h, burying cities even several kilometers away from the volcano. The amount of material ejected by a Plinian eruption can be so large that the magma chamber becomes depleted, causing the top of the volcano to collapse. Plinian eruptions can last from less than a day to several weeks, and have a VEI that can range between 3 and 8, but tipically is between 4 and 6. Weaker eruptions, with a VEI of 3 or 4, are sometimes classified as Sub-Plinian, while the strongest ones, with a VEI of 6 or above are referred to as Ultra-Plinian.
Eruption of Mount St. Helens in the United States in 1980, a Plinian eruption with a VEI of 5.
Unlike magmatic eruptions, phreatic eruptions are driven by the expansion of steam, after water comes into contact with magma. The hot temperature of magma causes water to evaporate into steam almost instantaneously, resulting in an explosion of steam, water, ash, rock, and other volcanic material. However, no magma is expelled, and sometimes the phreatic activity isn’t even strong enough to fracture the surrounding rock and cause an eruption. Phreatic eruptions are usually quite weak, but can be the prelude for much stronger magmatic eruptions. This happened for example in 1980, when months of phreatic activity preceded the devastating Mount St. Helens eruption, which was Plinian and had a VEI of 5.
Phreatomagmatic eruptions also result from the interaction between magma and water, but unlike phreatic eruptions, they also expel magma and are much more explosive. There are three different types of phreatomagmatic eruptions: Surtseyan, submarine, and subglacial. Surtseyan eruptions are named after the Surtsey island in Iceland which formed after a volcanic eruption in 1963. These eruptions are similar to Strombolian eruptions, but they take place in water, and are much more explosive. As water is heated by magma, it evaporates into steam, expanding quickly and causing an explosion. Surtseyan eruptions cause the creation of a pyroclastic surge, a mass of gas and rock fragments that develops around the eruption column. These eruptions can also form low and broad volcanic craters.
The Surtsey eruption of 1963, the typical example of Surtseyan eruption, had a VEI of 3.
Submarine eruptions happen underwater, often as a result of tectonic movements which cause volcanoes located along plate boundaries and mid-ocean ridges to erupt. These eruptions are actually much more common than those happening above sea level, but due to difficulties in detecting and studying underwater eruptions, these are still not well known. Submarine eruptions often produce a rounded lava flow with a peculiar shape, called pillow lava. Over long periods of time, submarine eruptions can produce seamounts that can rise above the sea level and form islands.
Subglacial eruptions are also extremely hard to study, as they happen in volcanoes covered by a glacier, often at high latitudes and high altitudes. Like submarine eruptions, subglacial eruptions can form pillow lava, and can cause the ice above the volcano to melt. As the ice melts into water, the eruption can become Surtseyan.