What are volcanoes?

Volcanoes: Fire-breathing dragons, bubbling rocks and lots of steam

When partially or fully molten rock from the deeper reaches of the Earth’s crust and mantle, magma, rises to the Earth’s surface, volcanoes are formed. They can lie deep in the sea, pile up into massive mountains on land, spew ash from afar or simmer quietly. Their manifestations are varied and knowledge about them is limited. It is true that today it is quite easy to guess which volcanic activities have formed the earth – but the current behavior of active volcanoes always holds new surprises.

This chapter provides an introduction to the basics of volcanism. How are volcanoes formed? In what forms are they visible? And what phenomena accompany a volcanic eruption?

How are volcanoes formed?

Volcanoes – a greeting from the heart of the earth

Much of the rock in the Earth’s interior is very hot or even molten, and often both. Only the comparatively very thin crust of the earth forms the solid surface on which we live. In the truest sense of the word the solid basis of our existence! Below the crust are the mantle and the core. Especially in the lower areas of the earth’s crust and in the upper mantle we find large amounts of molten rock, the magma. This magma rises towards the surface in various parts of the earth and, to put it simply, when it breaks through, a volcano is formed. As soon as the magma comes to the surface, it changes its name: from now on it is called lava.

Infographic: structure of the earth's core, image: BR

Heat flow in the earth’s interior

However, the earth’s crust does not consist of a single seamless surface, but of individual plates that move against each other, the so-called continental plates. We call the movement of these continental plates continental drift or plate tectonics. The hot heart of our earth is particularly noticeable at the edges of these plates. If you measure the heat flowing from the Earth’s interior to the surface, you get a special kind of map of the world: it gets red, i.e. very warm, deep down on the mid-ocean ridges. Much less heat arrives at the solid central parts of the continental plates.

heat flux of the earth. Image: JH Davies, DR Davies: Earth's surface heat flux.

Earthquakes and Volcanoes

If you compare this world map with one that shows earthquakes (blue) and active volcanoes (red), the similarity is easy to see. About 1900 active volcanoes are marked here, which are known from the epoch of the Holocene (about the last 12000 years). The cause of the vast majority of volcanoes is plate tectonics: Where continental and ocean plates collide, they slide over one another. In addition to the lava, pyroclastics are released with every volcanic eruption – loose volcanic products of various compositions and sizes.

earthquakes and volcanoes. Image: GFZ Potsdam

Plate tectonics: engine of volcanism

So where do we find active volcanoes on Earth?

  • In tectonically active zones at the plate boundaries: this is the so-called interplate volcanism, which accounts for about 90% of volcanoes
  • However, magma sometimes flows to the surface at so-called “hot spots” far from the plate boundaries. We call this form of volcanism intraplate volcanism. If this happens in places that are covered by oceans, particularly exciting island chains are formed for us divers, which rise deep out of the oceans and can produce exciting ecosystems
Plate tectonics of the earth. Image: BR

Always under pressure: what drives lava to the surface

The rise of magma is largely determined by buoyancy: hot, molten rock is less dense than solidified rock. In particular, when the ambient pressure decreases due to the ascent, and gases dissolved in the melt begin to escape and perhaps even form bubbles. This basic principle is of course very well known to us divers, after all we experience Henry’s law with every dive, which states that at higher ambient pressure more gas can dissolve in a liquid, and Boyle and Mariotte’s law, which states that the volume of a gas is indirectly proportional to the ambient pressure. Especially for the people in the immediate vicinity of a volcano, it is very important how the eruption will proceed: with rather harmless lava spurting out of fissures, or, for example, explosive and therefore unfortunately destructive, because, for example, viscous magma locks the gases in until they can suddenly find a way? And how long will the outbreak last?

Several aspects are decisive for these questions:

1.: How much magma can be erupted?
2. What is the volatile content of magma?
3.: How tough is the magma?
4.: Do mixing processes take place, for example with older magmas or (liquid) water, which can sometimes be very violent?

Volcanic phenomena

It was already the first hour of the day and the day dawned hesitantly, almost drowsily. The surrounding buildings swayed wildly, and although we were in an open, albeit confined, space, we feared the buildings might collapse. Now it seemed advisable to us to leave the city. A frightened crowd follows us, taking what almost amounts to prudence in the panic, to be guided by the advice of others rather than their own.

When we got past the buildings, we stopped. Here, too, all sorts of strange and amazing things happened: the wagons that we had had brought out moved back and forth, although they were standing on completely level ground, and didn’t stay in place even when we put stones under them. The sea retreated and was pushed back by the tremors, at least we saw all kinds of sea creatures stuck on dry sand.

On the other side a black cloud criss-crossed with twitching squiggles that looked like lightning but were larger. (…) Soon the cloud descends to earth and covers the sea, enveloping Capri and making Cape Misenum invisible.

from the second epistle of Pliny the Younger to Tacitus

Jacob Philipp Hackert: Vesuvius eruption in 1774

Every Volcano Spits Differently: Types of Eruptions

But how does a volcanic eruption actually take place, and how do such spectacular natural phenomena as those described by Pliny come about? And how can we better understand volcanoes? These are important questions, because it is only through understanding our environment that we can act safely, both in diving and in volcanology.
As already mentioned at the beginning of the chapter, when a volcano erupts, magma from one or more magma chambers rises to the surface via vents. Sometimes we can observe the emergence of the molten material, which we then call lava, very directly, for example as a lava fountain, as a more or less thin lava flow, or in the case of very viscous and comparatively cool lavas through the bulging of a dome-like structure above the vent, a so-called lava domes. But the eruption of liquid lava accounts for only part of the volcanic phenomena. Some volcanoes also produce large quantities of loose ejecta – lava that ruptured during the eruption and then cooled, called tephra – ranging in size from tiny ash particles to house-sized chunks. If large amounts of ash are deposited near or far from the eruption, subsequent rainfall can create mudflows of volcanic ash and water called lahars. Particularly spectacular, but also destructive, can be clouds of hot ash and larger chunks, the pyroclastic streams that roll down the slopes at speeds of several hundred kilometers per hour in some eruptions.
Sometimes there is also contact of hot rock or even magma with ground or surface water. The evaporation of water, the rapid transfer of heat from the magma to the water, and the more and more effective mixing of the two unequal partners through the formation of cracks can then result in high pressure, which can result in so-called phreatic (when water comes into contact with heated rock) or phreatomagmatic (when water comes into direct contact with the magma) explosions.
During very violent volcanic eruptions, large amounts of ash and volcanic gases can be released into the atmosphere and spread over large parts of the world.
Of course, most volcanoes show only a part of these phenomena, and in very different strengths. This variety and diversity accounts for a large part of the fascination of these “Fire Mountains”.

How can you categorize volcanoes?

Basically, you can sort the strength of volcanic eruptions according to different categories: How much lava comes out, or how much tephra, i.e. loose material, is spat out? This is measured in m³ and km³, and this figure gives the VEI, the Volcanic Explosivity Index. This was developed in order to be able to classify historically more or less well-studied outbreaks. The maximum height of the eruption cloud is sometimes used. However, the VEI can at best be a guide to classifying the consequences of current eruptions.
Because: with the total amount of material that has erupted so far, nothing has been said about the destruction a volcano will cause and what damage it could still cause. A small, rather harmless volcano that has been active for a long time can of course produce a lot of material in total and thus get into a higher index, although the eruption causes comparatively little damage, partly because the residents of the area have enough time to take protective measures seize.
Therefore, there are other classification options. The CPI, the Volcano Population Index, indicates how many people live in the area where an eruption would be dangerous for them. This is particularly relevant for assessing the danger of volcanoes that are active today.
The turbidity index gives an indication of how far fine ash particles and gases can get into the atmosphere and thus influence the weather or even the climate. Sometimes after volcanic eruptions, the ash that blocks the sunlight makes it dark and hazy for a wide area. Sulfur dioxide, a common volcanic gas, can combine with water vapor to form fine droplets of sulfuric acid. During violent volcanic eruptions, such sulfuric acid droplets reach the stratosphere and reflect part of the incoming solar radiation. The largest outbreaks of the 20th century have led to a small but measurable drop in the average global temperature over months and years. A worldwide effect of volcanic eruptions, but its extent is far behind the man-made warming of the earth’s climate.
And in order to describe an eruption vividly, one often resorts to proper names of the eruption types – Hawaiian, Strombolian, Pelean and Plinian. These names come from those who first described such an eruption, or from places and volcanoes where they are commonly seen.

The largest volcanic eruptions, accompanied by miles of ash clouds and violent explosions, are called Plinian . They are comparatively rare, but their occurrence often has far-reaching consequences. One of the last such violent eruptions took place in 1991 at Pinatubo in the Philippines.
Slightly smaller but also impressive are the Pelean eruptions . These are named after the Pelee volcano on Martinique, which in 1902 wiped out the entire town of St. Pierre. Such an eruption is also powerful, but is characterized above all by the fact that the volcano produces a hot ash avalanche. The magma is very viscous and dark and solidifies quickly, causing plugs to form and the pushing magma finding a new path up the mountainside, which can then lead to the sideways and downslope avalanche.
Vulcanian eruptions are also slightly smaller than Plinian eruptions, and are named after the Italian island of Vulcano. They are characterized by the fact that the remains of previous eruptions are explosively blasted out of the crater at the beginning, and only then does the fresh magma flow in.
Strombolian eruptions are slightly explosive volcanic eruptions that promote magma that still quickly forms smaller or sometimes larger plugs. This repeatedly clogs the chimney, which is then blown free with a moderate explosion. Like the eponymous volcano Stromboli, they are often active for long periods of time, but only produce comparatively small amounts, and are sometimes — with due safety margin — downright tourist attractions.
In the Hawaiian eruptions , thin magma flows steadily downhill, often over several months and years.

Six types of volcanic eruptions. Image: Encyclopedia Britannica

What can be seen on the edge of an eruption

A mysterious phenomenon sometimes occurs during a volcanic eruption: lightning suddenly occurs in and around the ash clouds.
These are mostly observed directly around rapidly rising ash: Ash particles are blown far into the atmosphere from the vent at a very high speed. One theory says that the grains of ash rub against each other, creating an electrical voltage that discharges in lightning. The finer the ash, the more likely it is that lightning will occur.

If great pressure builds up inside a volcano, it can suddenly discharge in the form of a veritable explosion. In some cases, the pressure waves from these explosions can not only be heard over long distances, but can also change the density of the air as they pass and can thus be observed directly. An example of such an explosion, in which the atmospheric pressure waves were even detectable worldwide, is the eruption of the submarine Hunga Tonga-Hunga Haʻapai volcano in the Pacific island state of Tonga on January 15, 2022.

Volcanic landscapes

Volcanoes come in many different forms. Basically, the magma can either exit in a fissure and form a fissure volcano , which often happens along mid-ocean ridges. Or the lava comes out of a central volcano at one point. However, this classification based on the type of magma supply does not provide any information about what the volcanoes look like. For that, it makes more sense to look at the different shapes that volcanoes take.

The most well-known image of volcanoes probably originates from the stratovolcanoes, also called stratovolcanoes. One layer of lava lays on top of the other and forms a high cone that can be seen from afar. The lava that forms such forms is very viscous – friable and more solid layers are formed, which are clearly distinguishable from each other.
The high, dark mountains of the stratovolcanoes tend to sometimes violent explosive eruptions because of the viscosity of their magmas. Nevertheless, millions of people around the world live in close proximity to stratovolcanoes.
Mount-Fuji. Image: Captain76 via Wikimedia Commons

The cinder cones that pile up and also look as beautiful as a picture book volcano are called cinder volcanoes. They are formed by the spat out pyroclastics, which pile up and form a steep cone of lava boulders and ash. A round crater is formed at the top, which repeatedly partially collapses and builds up again during an eruption. They usually do not stand alone but, for example, as flank volcanoes on the slopes of stratovolcanoes.

Fimmvorduhals, Iceland. Image: Olikristinn via Wikimedia Commons

Extraterrestrial Volcanoes

We do not only find volcanic landscapes on earth. Venus and Mars, our neighboring planets in the solar system, also have a large number of — probably mostly extinct — volcanoes. For example, Olympus Mons on Mars, the tallest mountain in the entire solar system at an impressive 22km above mean Martian level, is an ancient shield volcano. We also observe very intense volcanic activity on Jupiter’s moon Io, and lava-flooded plains and lava rilles on our own moon also clearly indicate a volcanic history.

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