How are volcanoes distributed?

Volcanology - The study of the volcanoes

The doctrine of the volcanoes is a natural science that combines many disciplines. Knowledge of geology, mineralogy, geochemistry and geophysics are necessary to understand the processes inside the earth that lead to the formation of volcanoes and the manifestations of volcanism. The living world of our planet is strongly influenced by active volcanism. Large outbreaks have a direct impact on the economic, social and cultural structures of our civilization.
The birthplace of modern volcanology can be found in Italy, where the first volcanological observatory was built on Vesuvius in 1841. But even earlier scholars from all over the world dealt with volcanism.
We Central Europeans are rarely affected by active volcanism, but around 2000 volcanoes are active worldwide, of which around 50 erupt every year. It is assumed that more than half a billion people live in the sphere of activity of active volcanoes. These numbers alone show that exploring volcanism cannot only be of academic interest! In addition, volcanoes are important suppliers of raw materials. The volcanic soils are extremely fertile, which is one of the reasons for the high population density in volcanic areas.
The processes inside the earth are extremely complex and have not yet been fully explored. People have already spared on the moon, but the exploration of the earth's interior is only progressing with difficulty. The deepest hole ever drilled just reached a depth of 12 km. If you consider that the earth has a diameter of 12800 km, this is a tiny pinprick in the earth's crust. At this point I would like to try to address some aspects of volcanism and to clarify the most important connections, without discussing the topic in any way exhaustively.


Our earth is a very dynamic planet, which in many ways resembles a living organism. Continents migrate, oceans arise and disappear again, mountains pile up and are eroded. Many processes that shape landscapes depend on climatic factors. The power of the water can wash out gorges and caves, frost and wind grind mountains and ice blasts stone. The products of these erosive processes are sedimentary rocks that are often deposited in layers several thousand meters thick and fill sea basins. But where does the primary rock from which the continents were formed come from, where does the atmosphere with its wind and rain come from, where do the oceans and the ice of the glaciers come from? All primary rock on earth was created from magma, i.e. a rock melt that we can only find in the interior of the earth today. A good 4.5 billion years ago, our earth was like a fireball of molten rock, which gradually cooled and formed a thin crust. The young earth's crust broke in many places and the magma emerged, giving rise to the first volcanoes. However, many fluids were also dissolved in the magma and escaped as gas. Most scientists today agree with the theory that even the primordial atmosphere and the water of the oceans were exuded by volcanoes. Only in the course of the eons did the atmosphere change to the mixture of nitrogen, oxygen and carbon dioxide that we breathe today. Algae and bacteria were involved in this metamorphosis. The origin of all life lies in the latter. The archaeobacteria developed near hydrothermal springs and fed on sulfur. Indirectly, we owe the emergence of life on our planet to volcanism.

Volcanic forms

According to the definition, volcanoes are openings in the earth's crust from which glowing rock escapes from the interior of the earth. As long as this molten rock is inside the earth, it is called magma. If it emerges from the surface of the earth, it is called lava. The rocks that emerge from the solidified lava are the volcanites. If the lava is exploded, it is called tephra, or pyroclastics. Depending on the grain size of these pyroclastics, one speaks of volcanic ash (> 0.2 cm), lapilli and bombs (< 6,2="">
The volcanoes themselves can erupt in many different ways and take on very different shapes. These forms are closely associated with the place and type of formation of the volcanoes. The type of eruption of a volcano and the strength of its eruptions are also directly related to the place of its formation and the associated lava type. A basic distinction is made between explosive and effusive volcanism. The latter is characterized by red-hot lava fountains and lava flows, in which the lava flows out calmly. Volcanoes of this type have a slight slope, or even consist of only one production crevice. Explosive volcanoes often produce gray eruption clouds that rise high into the sky when they erupt. The lava of these volcanoes is richer in gas, more viscous (highly viscous) and cooler than that of "red volcanism" and has a high, destructive potential. The classic type of volcano of this genus is the stratovolcano, or stratovolcano, the beautiful, symmetrical cone forms that can be several 1000 meters high. A systematic distinction is made between 10 volcanic forms and structures to which we will later assign the place of their genesis and the associated lava type.

Stratovolcano: This type, also known as stratovolcano, has relatively steep slopes and is made up of alternating layers of loose rock (tephra) and solid lava flows.

Complex volcano: A type of volcano with multiple cones and craters.

Shield volcano: Shield-shaped volcanoes with a flat slope which are mainly built up from layers of lava flows.

Crevasse volcano: These are actually just cracks in the ground, on whose shoulders slight bulges from lava flows formed.

Caldera: A caldera is a large hollow shape. Most of the volcano fell into the emptied magma chamber after an eruption. A new volcanic cone can grow in the caldera.

Summer volcano: These volcanoes are characterized by a double peak like Vesuvius, with one peak representing the edge of a caldera.

Table Mountain Volcano: Table mountains are flattened at the top and are formed when eruptions occur under the ice.

Maar volcano: Maars are explosive funnels and have a negative shape.

Lava dome: A lava dome is an extremely viscous lava flow that forms dome structures. Domes can arise in this way or in the summit areas of explosive stratovolcanoes.

Cinder cone: Are mostly of monogenic origin. Many parasitic craters are cinder cones.

Plate tectonics and the origin of the magma

The forces of plate tectonics, which cause the continents to move between 2 and 20 centimeters per year, are considered to be the engine of volcanism. The solid earth's crust is subdivided into numerous plates that float on the plastic rock of the asthenosphere, a boundary layer between the earth's crust and mantle. Convection currents in the earth's mantle move the plates in different directions. Everyone can observe how convection currents work with the so-called lava lamps. There, wax is heated by a light source, which then rises in a liquid, cools on the surface and then sinks again. The plastic rocks in the earth's mantle behave similarly to wax, only that they rotate in counter-rotating cells and transport the continental plates like conveyor belts. The panels can collide at the seams, rub against one another, or drift away from one another. Plates can break and melt together. In the first case a new ocean forms, in the second a mountain range. Both processes are accompanied by earthquakes and volcanic eruptions. The plates are divided into continental plates and oceanic plates. The earth's crust is on average 30 km thick under the continents and only 7 km thick under the oceans. Extreme values ​​arise during mountain formation, where the continental plates can be up to 70 km thick. Subduction zones also form at colliding (convergent) plate boundaries. These can arise at continental margins as well as at plate boundaries where two ocean plates collide with each other. When two plates collide, one slides under the other and dips into the layers of the upper mantle. Temperatures there are around 800 degrees Celsius. Usually this temperature is not enough to melt rocks. But the oceanic crust contains a lot of water and fluids, which lowers the melting temperature of the rock. Partial melting is the result. Part of the resulting magma rises due to the difference in density and can escape behind the subduction zone in volcanoes. Subducting two oceanic plates creates a volcanic island arc behind a deep sea trench (Japan, Indonesia, Aleutian Islands), one of the plates is of continental origin and a coastal mountain range with volcanoes (Cascades Volcanoes, Alaska, Andean volcanoes). Due to the gas-rich, viscous magma that forms in subduction zones, both forms are indicative of highly explosive, gray volcanism such as can be found in the circumpacific fire belt.

The plunging plates off the continental margins of an ocean pull the rest of the plate behind as they sink. This then tears apart in the middle of the ocean. This seam at the divergent, oceanic plate boundaries is called mid-ocean ridges because an underwater mountain range of lava is formed there. The incoming magma practically fills the cracks created by the subduction. The magma exiting here differs from the magma that is created during partial melting at the subduction zones. It is less differentiated and resembles the original jacket material. This results in basalts, the lavas of which are less gas-rich and less fluid and therefore emerge effusively at the mid-ocean ridges. The volcanoes here are like long crevices. If all the mid-ocean ridges are lined up, you get a 75,000 km long, undersea volcanic mountain range. Only in Iceland and in the Ethiopian Danakil desert do parts of a mid-ocean ridge lie above sea level. A system of fissures runs through Iceland, marking the border between Europe and North America. In Iceland, the seabed was also pushed up by a mantle plume. This is a stationary magma bubble that rises from the earth's mantle. Like a welding torch, a large extraction chimney cuts through the earth's crust and creates volcanoes. Since the earth's crustal plates (oceanic as well as continental) move over this mantle plume, a volcanic chain is created in which only the youngest volcanoes are active. This form of volcanism is also known as "hot-spot". Other examples are the shield volcanoes of Hawaii and the caldera volcanoes of Yellowstone. Obviously there are differences in the magma development of oceanic and continental hot spots. If the volcanoes of Hawaii erupt effusively, the Yellowstone volcano is highly explosive.
The continental equivalent of a mid-ocean ridge is a rift valley, or rift. Wherever continents break, oceans are born! The earth's mantle arches beneath a rift. A crack develops on the apex of this bulge, which widens into a tectonic rift with echelon fractures. The shoulders of the fracture wander apart and the resulting basin is filled with lava from the earth's mantle, so that new, oceanic crust is created over millions of years. A typical example is the East African Rift Valley. It stretches over a length of 6500 kilometers through East Africa and flows into the ocean ridge of the Red Sea. Shield volcanoes such as Killimanscharo and Nyiragongo arise on the rift shoulders. In the central area of ​​the future seabed, stratovolcanoes such as the Ol Doinyo Lengai are forming, although its recently extracted, soda-carbonate lava is atypical. But 95% of this exotic volcano consists of alkaline basalts, which are typical for volcanoes of this type.

In summary, one can say that the type of magma - and thus the eruption behavior of a volcano - strongly depends on the place where it originated. Starting from a trunk magma with the chemical composition of a mantle rock, different magmas can arise. The decisive factors for this are the pressure and temperature conditions, as well as the time that a magma has to "mature" in a magma chamber. Added to this is magma that is created by partial melting under the influence of fluids.

Magma, Lava, and Volcanic Rock

The basic raw material of most magmas is silicon dioxide. Such silicate melts have temperatures between 800 and 1200 degrees Celsius. On the other hand there are magmas, which have calcium carbonate as a basic material and are much cooler (500 - 600 degrees) and thinner. However, these magmas are rare and their formation has not yet been adequately researched.
Silicatic magmas are differentiated according to their SiO2 content. The rule of thumb is that the more SiO2 it contains, the richer the gas and the more viscous (highly viscous) the magma, the more explosive the volcano's eruptions. Magma rich in SiO2 is also referred to as "acidic magma" because it has a lower pH value than "basic magma". A typical rock that arises from a basic magma is basalt. Basalt lava is produced effusively by volcanoes like Hawaii. At the other end of the spectrum is rhyolite, a SiO2-rich lava rock produced by explosive volcanoes like the Yellowstone volcano. A volcanite made of magma with a medium SiO2 concentration is andesite. As the name suggests, this rock is typically found in volcanoes in the Andean region.
Magma that emerges from the surface of the earth is largely degassed and is then called lava. However, rising magma can also get stuck in the earth's crust and cool down there. These rocks are called magmatites, intrusiva, or plutonites in technical jargon. Chemically, these rocks correspond to the volcanites, but differ in their structure. Since they cool down over longer periods of time than volcanically extracted rocks, they have a larger grain size.
A decisive criterion for the type of eruption of a volcano and the strength of an eruption is, in addition to the viscosity (flowability or toughness) of the magma, the gas content of the melt. Volcanic gases such as carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride and water vapor are dissolved in the magma and are mainly released through changes in pressure and temperature conditions. The gases can then collect in large bubbles, which carry lava fragments with them as they rise, or escape in an explosive manner. Particularly violent, explosive eruptions occur when igneous gases are released as a result of pressure relief, as was the case with the great eruption of Mount St. Helens when a flank sheared off in a gigantic landslide. Similar phenomena can be observed when domes collapse. Here, too, gas is released explosively, which results in large eruptions.
If rising magma comes into contact with groundwater or if water penetrates a magma chamber, phreatic explosions follow due to the 2500-fold increase in volume of the water vapor, which can even blow up an entire volcano, as happened in Krakatau in 1883. Such catastrophic eruptions can hurl the finest fragments of rock - so-called ashes - into the upper layers of the atmosphere. The ascent height of such particles is not only determined by the explosive force of the eruption, but also by the thermal dynamics within an eruption cloud. If the hot gas flow breaks off, the eruption cloud can collapse and cause pyroclastic currents that rush to the valley at enormous speeds and have a great destructive potential. Such pyroclastic currents also arise when a lava dome collapses or when large parts of a dome break off.
Eruptions in which the ash particles rise into the stratosphere and spread out in the form of an umbrella are called plianic eruptions. This expression was named after Plinus the Elder, who died in 79 AD in the massive eruption of Mount Vesuvius.

Types of eruptions and the volcanic explosive index (VEI)

Before I go into more detail on the classification of the eruption types, a few words about the "volcanic explosivity index", or VEI for short. The explosiveness of an outbreak is measured with the help of this logarithmic scale. There are 9 levels (0-8), whereby the increase by one level corresponds to a tenfold increase in explosivity. An exception is the transition from 0 to 1, which means a gain by a factor of 100. A VEI of 0 corresponds to the effusive activity in which only lava flows or lava lakes arise.

Hawaiian eruptions: Are typical of the Hawaiian volcanoes. The magma contains little gas, has temperatures of over 1000 degrees Celsius and is thin. Lava fountains are lower than 2 kilometers. These volcanoes also form lava lakes. VEI: 0 - 1

Strombolian activity: Named after the permanently active Stromboli volcano off Sicily. In the case of the rather small but regular explosions, lava fragments can be ejected up to 10 kilometers high. The average height of the eruptions is only a few 100 meters. At the same time, lava flows can flow out. VEI: 1 - 2

Vulcanian activity: Named after the Vulcano volcano, the namesake of all volcanoes. Stronger explosions occur here, often under phreatic influence.Particles are ejected up to 20 km high. VEI: 3 - 4

Plinian eruption: Ascent of the umbrella-shaped eruption cloud up to 60 kilometers high. These are the most powerful eruptions of the dangerous volcanoes with acidic, viscous and gas-rich magmas. A particular danger of Plinian eruptions comes from pyroclastic currents, which arise when the eruption column collapses. This form of eruption takes its name from Pliny the Younger, who documented the catastrophic volcanic eruption of Vesuvius in 79. VEI 5 ​​- 8

Plinian eruptions can be further broken down into Subplian and Phreatoplinian:
Subplinian Outbreaks: Ascent of the particles to a height of 30 km. An umbrella is formed.

Phreatoplinian: The eruptions take place under the action of water. Eruption clouds rise up to 40 km high.

Pelean eruption: This summarizes the activity in which only pyroclastic currents, glowing clouds and glowing avalanches are formed. They are caused either by the collapse of a cathedral, the collapse of an eruption cloud, or by explosions that are directed to the side.
All of these phenomena have in common that a super-hot gas cushion forms on which a mixture of glowing lava rocks, rubble, ash and gas rushes to the valley. They can reach speeds of up to 400 km / h and reach a temperature of 800 degrees Celsius, although the extreme values ​​can be higher. At the other end of the spectrum, there are also pyroclastic currents, which are less powerful and move on the verge of debris avalanches. Typical deposits of Pelean eruptions are ignimbrites, which are characterized by a chaotic structure and flamed structures. Large chunks are enclosed in a fine matrix. Elongated minerals are regulated in the direction of flow. The deposits of surges fill in the terrain and level the landscape. This form of eruption was named after the Pelée volcano on Martinique in the Caribbean. In 1902 the city of Saint-Pierre was destroyed by a cloud of fire. 29,000 people died at that time.

Super volcanoes

Eruptions with a VEI of 8 have not been observed in historical times and are considered a criterion for so-called super volcano eruptions. Huge masses of rock and gas are blown into the atmosphere, which influence the global climate for many years. The tephra emissions are more than 1000 cubic kilometers. The latest example of such an eruption is the Taupo eruption in New Zealand. It erupted about 26,500 years ago. About 75,000 years ago, the Toba eruption in Sumatra destroyed almost all of humanity. Genetic research suggests that around 15,000 people survived the disaster. The last eruption of the Yellowstone volcano was 640,000 years ago. This volcano has been making headlines these days for signs of recurring activity. For comparison there were only 5 eruptions with a VEI of 7. Known, catastrophic eruptions such as Vesuvius (VEI 4), Mount St. Helens (VEI 5) and Krakatau (VEI 6) appear almost against the eruption of a "super volcano" small.

Effects of volcanic eruptions on the climate

But the smaller outbreaks also have a major impact on our climate. Volcanic ashes and gases are distributed globally in the higher atmospheric layers. The fine particles reduce solar radiation and so-called aerosols are even able to reflect UV radiation. As a result, temperatures drop. In the event of an outbreak, so-called greenhouse gases such as carbon dioxide are also promoted, which has the opposite effect. In the balance the temperature-lowering effects predominate and in the years after a major outbreak, quite cool summers and cold winters have been observed. The year 1816 was known as the "year without a summer". It was the coldest year on record. The eruption of Tambora, which erupted a year earlier, is said to be to blame. This eruption had a VEI of 7.
Volcanic eruptions must therefore not be viewed as a singular event, but must be viewed in their entire complexity and in a global context. A good example of this is the above-mentioned "year without a summer" (which in today's times would make things like sun protection films, but also solar systems, superfluous). In addition, researchers have found that dust particles in the air after a volcanic eruption influence the scattering of sunlight, which in turn is said to have a positive effect on photosynthesis in plants. So it is important to put volcano research into a larger context.

Post-volcanic phenomena

Active volcanism is accompanied by numerous phenomena such as geysers, hot springs, mud volcanoes, mud pools and fumaroles. Such phenomena are summarized under the term "post-volcanic phenomena" because they often mark the end of active volcanism. You can just as well observe these phenomena in an inter-volcanic stage, when the volcano is only at rest. You can read more about these phenomena here. There is also a picture gallery with graphics.

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