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All about Volcanoes

Objective: Look into what a volcano is, how they form, why they are important and where they are mainly located around Earth.

Every year we hear about volcanoes on the news: flights disrupted, villages evacuated, landscapes destroyed… Throughout history the destructive power of volcanoes has both fascinated and terrified people. But what exactly are these ‘mountains of fire’? Why do they form, and where in the world do we find them?

Read on to find out more!

I) What is a volcano?

Volcanoes come in many shapes and sizes, from enormous mountains 1000s of metres high to vents so small that you could walk past and barely notice. They can erupt at the surface, but also beneath ice or water. To make things even more difficult the term ‘volcano’ refers not only to actively exploding vents but also to dormant or extinct volcanoes that were active in the past.
A possible definition of a volcano would be “a vent, crater or other opening in the ground and the structure around it that is currently active or was active in the past”
We all sort of have an idea in our heads as to what a volcano can look like. The important point here is that volcanoes can look like that, but they can also look very different as well. The following diagrams show different structures that are all counted as ‘volcanoes’ to illustrate this diversity.

What many of us perhaps picture when we think of volcanoes: a steep cone with an actively erupting crater. These are known as stratovolcanoes, and are often the most dangerous type of volcano. The photo is of Mayon volcano in the Philippines.

Some of you may have been picturing the less steep 'shield volcano' type of volcano. Hawaii is probably the best-known example of this type, the photograph here shows Mauna Loa in the background.

While the two types shown above are the most instantly recognizable volcanic types, a large number of other eruption styles can also be found. For example here very low viscosity lavas can form nearly totally flat shields also known as 'lava fields'. This image shows Wapi lava field in the US.

This type of volcano may look more typical, but can actually have a fairly different magma feeder system. Monogenetic volcanoes are often found in fields grouping many small cones, each of which usually only erupts once. Here an image of the new Unesco World Heritage site of the Chaine des Puys in central France.

The most active volcanic system on Earth is actually hidden beneath the oceans, and the majority of magma is erupted deep underwater. While subaqueous eruptions are difficult to observe, they may occasionally breach the surface in dangerous explosions or leave pumice rafts floating around the ocean.

Volcanoes are not only erupted under water, but may also be erupted underneath ice sheets. Check out the article on volcano-ice interactions for more good examples of this! This image is of Herðubreið in Iceland, and the flat top shows the approximate level of the ice at the last glacial maximum.

Finally, even inactive volcanoes are still considered volcanoes. The official definition is rather vague, leaving it up to individual interpretation (i.e. if it still looks like a volcano, it is a volcano). This image is of the inactive Volcan de Agua in Guatemala that very much still looks like a volcano.

II) Why do volcanoes form?

There is a common misconception that a large amount of molten rock is present beneath the surface, and that volcanoes occur where this breaks through. This is not the case. The earth is essentially solid from the surface down to the base of the mantle, with at most a few percent melt. The mantle does ‘flow’, but it flows in a similar way to glaciers through slow, solid-state motion.
Thus volcanoes do not form where a subsurface molten ‘sea’ breaches the surface, but rather where small amounts of melt liquid can accumulate and percolate to the surface. There are two main controls on how much melt liquid can form: Pressure and Temperature. Basically, the hotter a rock is the more likely it is to melt (Temperature). But if this rock is being squashed too hard by the rocks around it, it is not able to melt (Pressure).

Graph of pressure and temperature showing how each affects the amount of melt created. High pressures or low temperatures in the subsurface prevent melt from forming and make the presence of volcanoes in the area unlikely. Volcanoes may form in area where regular temperature mantle is at a lower pressure than usual (decompression melting) or where the mantle is hotter than usual. The presence of fluid (water and carbon dioxide mostly) can also influence magma generation.

This means that volcanoes form in places where temperatures are high and/or pressure is low. For example temperature is high in ‘hotspot’ regions such as Hawaii. Alternatively, pressure is low in places where the crust is being pulled apart, such as in Mid Ocean ridges or the East African rift. All of these environments exhibit large and widespread volcanism.
In reality other factors can also influence whether volcanoes can grow, such as the composition of the mantle when it melts (particularly how much water it has in it) and whether the melt liquid produced (magma) can reach the surface. Where the magma is unable to reach the surface it will cool as magma chambers lower in the crust, forming coarser-grained plutonic rocks such as granites.

III) Volcanic Hazards

It is no secret that volcanoes can be dangerous. Over the last 2 centuries alone volcanoes have directly and indirectly killed more than 100 000 people. It is worth mentioning that while this number is scarily high, it is still very small compared to the number of people that die in wars, famines and other disasters.
Nonetheless, especially given that volcanic regions are some of the most densely populated on Earth (for example Mexico, Indonesia, Guatemala or the Philippines), it is essential for volcanologists and other geologists to study and understand volcanic hazards.
To make things easier to discuss the main volcanic hazards can be separated into 3 categories: ‘Falls’, ‘Flows’ and ‘Global Climatic Effects’.

Some examples of natural hazards associated with volcanoes. Many of the deadliest hazards are only indirectly related to eruptions, such as global cooling or lahars.

a) Falls: During volcanic eruptions pieces of rock and magma of varying sizes are thrown up into the atmosphere. Depending on their size (and local wind conditions) these will all fall back to earth at some point. Volcanic bombs, being relatively large, fall back to earth not far away from the vent. Smaller ash particles however may travel a large distance and blanket an entire region in fine rock. These can cause respiratory problems and collapse rooves when enough builds up.

b) Flows: Probably the most typical volcanic hazard, this category includes lava flow and pyroclastic flows. Pyroclastic flow can flow up to several 100 km/h, so represent a major risk for any nearby populations who are unable to react in time. Another major hazard are Lahars, which are mudflows that form when water mixes with loose volcanic materiel. These are known for burying 25 000 people in the Columbian village of Armero back in 1985. Lava itself is usually slow enough to not be a major hazard, although can be dangerous in some specific areas.

c) Global Climatic effects: This category is discussed more in detail in the article on links between volcanoes and climate. To sum things up all eruptions propel a certain volume of ash and gasses into the atmosphere- the largest propel it high enough up that it can circulate around the entire world. This can affect the climate in various ways on both short and long timescales.

IV) Where are volcanoes located?

As section II discussed, there are certain conditions necessary for the formation of volcanoes: higher than usual temperature and/or lower than usual pressures in the subsurface. This naturally means that volcanoes are preferentially concentrated in certain environments, and absent in other areas. It is worth bearing in mind that volcanoes are still very widespread around the world. The floor of oceans is formed of thousands, if not millions of different volcanoes overlapping each other. Recent discoveries suggest that volcanoes are also abundant under the world’s largest ice sheet in Antarctica. Volcanoes are more common in some places than others, but it is relatively rare to find a region where volcanism is entirely absent. Volcanoes occur in different areas for a large variety of reasons, and thus the types of eruptions are varied. The largest and most dramatic volcanoes are mostly concentrated around hotspots and subduction zones, nevertheless all volcanism can be important, especially when its interactions with other parts of the Earth’s system are taken into consideration.

Map showing the locations of most volcanoes around the world, taken from the Global Volcanism Program database. Yellow dots show the locations of major earthquakes. Note how both are the most common at plate boundaries, but both also exist in other areas of the Earth.

Map showing the locations of proposed volcanoes in Antarctica. Blue volcanoes are located underneath the ice and red volcanoes are exposed at the surface.