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The volcanic eruptions are complex natural phenomena that consist of escape of molten rock and gas from the earth’s surface. We usually talk about “magma” if the molten rock is inside the earth’s crust and is mixed with gases; it is called “lava” when this is expelled to the surface, losing the gases inside it. Depending on the chemistry of the magma and the quantity of gases inside it, there will be more or less explosive eruptions.
Usually i shield volcanoes are characterized by not very explosive eruptions and with very fluid (not very viscous) lavas while, on the contrary, the cone volcanoes or stratovolcanoes they give rise to eruptions with a generally higher degree of explosiveness and more viscous lava.
Classification of types of rash
The magmatic eruptions are closely linked to chemical nature of the magma and, in particular, its silica content. To put it simply, the higher the silica content, the more viscous the magma will be – so it will “struggle” to flow during eruptions. Viscosity, in fact, measures the resistance of a fluid to flow. To get a clearer idea, think about honey and oil; the former is more viscous than the latter.
A viscous magma tends to trap gases within it. The more gas there is, the more the pressure will tend to rise, giving rise to more explosive eruptions.
This type of magma is typical of the stratovolcanoes (or cone volcanoes) and associated eruptions are called “explosive“. In contrast, less viscous (therefore more fluid) lavas are typically produced by shield volcanoes (Hawaii style so to speak) and the associated eruptions are defined as “effusive“.
To classify the explosiveness of eruptions we use the so-called Volcanic explosiveness indexin English Volcanic Explosivity Index (or, more simply, VEI). This scale was invented by Alfred Lacroix, a French volcanologist who essentially considered two parameters: volume of material emitted and height of the eruptive column. The VEI is based on a logarithmic scale, so between one step and the next the values become 10 times bigger and range from 0 (“quiet” eruption) to 8 (“devastating” eruption).
There are 7 types of eruptions: Hawaiian, Strombolian, Vulcan, Vesuvian (or sub-Plinian), Plinian, Ultraplinian and Large Calderas.
Please note: the following data comes from the sites of INGV, Civil Protection and the Department of Geology of the University of San Diego.
Hawaiian Eruptions (VEI between 0 and 1)
The Hawaiian eruptions they are typically effusive and “quiet”. The erupted lava is low in silica and the height of the lava fountain created varies between 100 and 1000 meters at most. It often happens that real lava lakes are created in the crater of this type of volcano! The name of these eruptions derives its origins from the place where they are particularly evident: the Hawaii and, in particular, the Kilauea and Mauna Loa volcanoes.
This VEI also includes: Icelandic-type eruptions. These are always very fluid and gas-poor lava flows but, unlike the Hawaiian ones, the eruption does not only occur from a “central” crater but also along fissures that extend for several kilometers in length. Icelandic (or fissure) eruptions give rise to very large lava flows which then give rise to the so-called basaltic plateaux.
Strombolian eruptions (VEI between 1 and 2)
The term “Strombolian” is a clear reference to the Stromboli which has been erupting in this way for thousands of years. Volcanoes with Strombolian eruptions are characterized by lava fountains which, at this time, are more intense. This is due to a greater viscosity of the magma, capable of trapping more gas and, therefore, creating small explosions at the moment of the eruption. This type of eruption is also common in volcanoes such as Mount Erebus in Antarctica or Paricutin in Mexico The latter is famous for being the most recent volcano in history: its formation began in 1943 and continued until 1952, developing for 424 years. meters high.
Vulcan Eruptions (VEI between 2 and 4)
Eruptions of this type are characterized by magmas with a content of silica greater than 50%therefore more viscous, which creates a “plug” in the chimney. As we will see, this mechanism is also at the basis of volcanoes with greater explosiveness. The eruptions are characterized by a relatively small volume of pyroclasts emitted (less than one cubic kilometer) and an eruptive column between 10 and 15 km. The name is linked to the island of Vulcano in the Aeolian Islands.
Vesuvian (or sub-Plinian) eruptions (VEI between 3 and 5)
These eruptions are versions weaker than the Plinians (which we will see later). It’s about explosive eruptions which, however, empty a large part of their magma chamber during the first eruption. The eruptive column can reach 20-25 km in height… the same height as over 80 Eiffel Towers one above the other!
Plinian Eruptions (VEI between 4 to 7)
Described for the first time by Pliny the Younger in 79 AD, they are eruptions characterized by silica-rich magmas and, therefore, very viscous. The duration of the eruptions is variable, from several hours to several days and are characterized by powerful explosions, with eruptive columns that can reach even 45 km highor how much the distance between Venice and Padua! These eruptive columns are often transported by the wind and, for this reason, the areas onto which ash and debris will fall are not always known a priori.
Another dangerous phenomenon linked to eruptive columns is that of burning clouds. These occur when the eruptive column collapses, starting to flow down the sides of the volcano like an incandescent river of dust, lapilli and ash at very high speed. We are talking about a flow that can easily exceed 100 km/h and with a temperature between 200 and 700 degrees! To give you an idea, consider that aluminum melts at around 650°C. Whatever lies along the path of the burning clouds has no escape, as happened in 79 AD to the cities of Herculaneum and Pompeii.
Ultraplinian eruptions (VEI between 7 and 8)
Having such a high VEI, we start talking about eruptions highly destructive. Usually these events are so powerful that they manage to destroy the entire volcanic edifice, as happened at Krakatoa in 1883. On the occasion of this ultraplinian eruption the noise of the explosion was so loud that it could be heard from Australia to Maurituis, becoming one of the loudest sounds ever recorded! The eruption column (according to the American Environmental Center) was about 80 km high and obscured an area of approximately 800 thousand square kilometers, keeping it in the shadow for two and a half days!
Fortunately for the human race, this type of eruption is quite rare.
Supervolcanoes or Large Calderas (VEI equal to 8)
At the top of the explosiveness ranking we find supervolcanoes or, better to say, large calderas. These are volcanoes with very high explosive eruptions. According to the USGS we are talking about 1000 cubic kilometers of material emitted, resulting in the creation of a caldera (i.e. a collapse of the “emptied” magma chamber).
In these cases there is often no classic “cone” and, for this reason, on the surface one does not realize that one is in a volcanic area. However, supervolcanoes hide immense potential and, in the event of an explosion, could be more violent than an ultraplinian eruption.
The image shows a comparison between the material emitted in different eruptions over the geological eras. To give you a comparison, consider that the 79 AD eruption of Vesuvius that destroyed Pompeii, according to INGV, had a volume of approximately 4 km3therefore a measurement similar to that of the green dot of Pinatubo, bottom left. On the contrary, all the orange dots correspond to supervolcano eruptions. Currently, no large caldera appears close to erupting and only secondary magmatism phenomena such as geysers and fumaroles are observed. Among the main supervolcanoes in the world we remember Yellowstone hey Phlegraean Fields.
Precursor phenomena of a volcanic eruption
In some cases volcanoes they send out alarm signals which, nowadays, we are able to record and interpret. They are the so-called “precursor phenomena”, i.e. processes that can herald a possible eruption (especially with regard to closed vent volcanoes).
Among the main ones, as reported on the Civil Protection website, we can observe:
- triggering of fractures (i.e. earthquakes) caused by tension in the rocks;
- swelling or change in shape of the volcanic edifice following the rise of magma;
- variations in parameters relating to gravity and magnetic field near the volcano;
- presence of gaseous emanations from the ground;
- Chemical and physical variations of groundwater.
The monitoring of these parameters, at least in Italy, is carried out 24 hours a day by the National Institute of Geophysics and Volcanology (INGV). The type of data collected helps in understanding eruptive phenomena but, let’s remember, long-term predictions are impossible nowadays.
