A plethora of volcanic occurrences, such as explosions, streams of molten lava, clouds of ash, and pyroclastic flows, depict the wide array of processes occurring within the concealed eruptive apparatus beneath volcanoes.
Fundamentally, two primary categories of volcanic eruptions stand apart: effusive eruptions, which mainly yield streams of lava, and explosive eruptions, marked by the expulsion of ash clouds and tephra. However, there are numerous intermediate types as well.
Frequently, the various types of volcanic eruptions play a defining role in shaping the physical characteristics of volcanoes. Nevertheless, it is important to acknowledge that a volcano’s eruptive behavior can transform over its history. Thus, it becomes conceivable to witness diverse eruption patterns within the same volcanic structure. For scientists, this introduces an additional layer of intricacy when attempting to assess the potential volcanic hazards in an area and forecast eruptions.
Numerous categorizations exist to depict the distinct sorts of volcanic eruptions. In 1964, Bernard Géze put forth a triangular diagram rooted in the characteristics of emitted volcanic materials. This diagram features three vertices corresponding to the primary materials: gases, liquids, and solids. Within this triangular arrangement, four sectors are delineated to represent varied scenarios based on a predominant material. These are as follows:
- Vulcanian eruption: Relatively powerful explosive bursts are a hallmark of vulcanian eruptions. These eruptions involve the expulsion of volcanic fragments, ash, and gases into the sky. They are generally of short duration and can give rise to volcanic plumes ascending several kilometers into the atmosphere. These eruptions derive their name from Vulcan, the Roman deity associated with fire.
- Peléan eruption: A Peléan eruption is an exceedingly explosive event linked to the collapse of a volcanic dome or the outburst of a dense lava flow. These eruptions carry significant peril as they can produce pyroclastic flows—rapidly moving mixtures of searing ash, gas, and volcanic rocks—that can swiftly descend the slopes of a volcano, leading to widespread devastation.
- Hawaiian eruption: The effusion of low-viscosity basaltic lava is what distinguishes a Hawaiian eruption from other non-explosive events. These eruptions are typically characterized by the gentle flow of lava from an opening or crevice, creating expansive lava fields. Hawaiian eruptions are commonly linked with shield volcanoes and are recognized for their relatively restrained level of explosiveness.
- Strombolian eruption: A Strombolian eruption is a moderately explosive variant characterized by frequent, moderate-sized explosions. These bursts occur when gas-rich magma is expelled from an opening, resulting in bursts of lava fragments and ash. Strombolian eruptions derive their name from Stromboli, an Italian volcano celebrated for its recurrent and systematic Strombolian activity.
Although this categorization proves effective in describing ongoing eruptions, it presents a challenge when applied to the analysis of historical eruptions. In 1973, George Patrick Leonard Walker introduced an alternative classification based on the scrutiny of volcanic deposits. Three factors are taken into account: the thickness of the deposit, the fragmentation of materials (essentially the fineness of emitted elements during the eruption), and their dispersion. Primarily centered on the fragmentation index (F) and the dispersal index (D), this classification allows for a comparison between contemporary and fossilized eruptions. These two parameters also serve as valuable tools in estimating eruption intensity: F and D escalate in tandem with the eruption’s potency.
The Major Effusive Eruptive Types Are Not Very Dangerous
Hawaiian-type volcanoes exhibit distinct features, presenting expansive, gently sloping cones predominantly constructed from lava material. Meager quantities of ash and scoria are observed within their composition. The caldera, frequently expansive due to collapse, usually encompasses a reservoir of molten lava. Eruptions from these volcanoes can manifest as a cascading fiery veil or a graceful outpouring of exceptionally fluid magma.
Such volcanoes tend to remain in a state of near-constant activity, with eruptions occurring frequently, if not continuously. They do not produce explosions and are therefore considered to be relatively non-hazardous.
Volcanoes of the Strombolian type produce eruptions of fluid molten rock and ejections of coarser materials, such as volcanic bombs. This phenomenon is exemplified by Stromboli volcano, from which the eruption style derives its name, and also by Mount Etna. These volcanic formations boast distinctly symmetrical cones adorned with alternating strata of lava and projections. Nonetheless, certain lava flows possess the ability to dislodge a fragment from the crater, a process termed “gouged out.”
Comparable to Hawaiian-type volcanoes, Strombolian-type volcanic activity tends to be frequent yet relatively unthreatening to human populations.
The Major Explosive Types That Pose Significant Danger
Volcanoes characterized as Vulcanian exhibit a predominant composition of ashes and assorted solid matter, including volcanic bombs and blocks. Moreover, they generate substantial plumes of heated ash that descend across extensive areas, resulting in a significant loss of life. These volcanic formations, featuring dense lava, are acknowledged for their perilous nature due to the potential for forceful eruptions of a magnitude that could entirely obliterate the pre-existing crater. Examples of Vulcanian-type volcanoes encompass Vulcano volcano, alongside Pariou and Vesuvius volcanoes. It is noteworthy to mention that Mount Vesuvius transitions between the Vulcanian and Strombolian types in a cyclic manner.
Certain of the most lethal volcanic eruptions throughout history are linked with Vulcanian-type explosions, like the famous outburst of Mount Tambora back in 1815, which tragically claimed the lives of around 60,000 people. These eruptions possess the capacity to bring about substantial shifts in the Earth’s climate.
Mount Pelée in Martinique is a Peléan-type volcano. The lava, which is too viscous to flow, causes extremely dangerous explosive eruptions, which frequently include pyroclastic flows. Often, what is called an lava spine or lava spire forms (example: Mont Gerbier de Jonc in Velay).
The Surtsey underwater volcano in Iceland serves as a representative of this explosive type. When lava comes into contact with water, it intensely fragments, producing fine ash as a byproduct. This is referred to as a phreatomagmatic eruption.