Volcanoes: a dangerous spectacle
The region near the Calbuco volcano, in southern Chile, was placed on red alert after the volcano erupted on April 22, 2015. © Rafael Arenas / Reuters
Text by: Nenad Tomic
5 mins
On the surface of our planet, there are several hundred active volcanoes and each year geologists record between 30 and 50 eruptions of varying size.
During the eruption, the volcanoes offer a breathtaking spectacle, but are very dangerous for the neighboring population.
More than 2000 years ago, the city of Pompeii and its population was decimated by Vesuvius in southern Italy, while in 1883, the Krakatoa volcano in Indonesia claimed tens of thousands of victims.
In order to understand this natural phenomenon of our planet, we are going to ask ourselves about their formation, their different shapes and their distributions on the surface of the globe.
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The mechanism of eruptions
For millions of years, the earth's relief has been shaped by volcanic eruptions. When a volcano wakes up and becomes active, it is under the earth's crust, about ten thousand meters deep that the phenomenon begins its journey to the surface. In extremely hot spots, the rock is in a liquid state. It is the magma which, under the effect of the pressure of the earth's crust, is pushed towards the exit. The magma moves slowly in the cracks that appeared as a result of the movement of the tectonic plates. It is through this conduit, which geologists call "the chimney", that the magma leaves by projecting ashes, water vapor or small solidified rocks. Once the magma reaches the surface, it is called "lava".
Volcanologists distinguish two main types of eruptions: effusive and explosive.
An effusive eruption occurs when lava flows from the volcano out of the crater more or less slowly.
It is impossible to stop the lava which destroys everything in its path.
On the contrary, the explosive eruption results in earthquakes and the projection of rocks around the crater and large amounts of ash in the air.
This type of eruption presents a great danger: the speed of the phenomenon often catches the population by surprise, which does not have time to evacuate the surroundings of the volcano.
Most volcanoes are located at the bottom of the ocean and form in areas where two tectonic plates move away from each other (rift zone).
Several islands were formed in this way, such as the Canary Islands archipelago or that of Cape Verde.
In areas where two tectonic plates are in a subduction zone, that is to say when they move towards each other, the magma, rising to the surface, forms volcanoes.
But there are also so-called “strange” areas where volcanoes appear in the middle of a large tectonic plate.
These "fire points" or "hot spots" are rare and scientists are always looking for explanations for their formation, because they are located in the center of a plate.
This is the case of the volcanoes on the Hawaiian Islands, the Piton de la Fournaise on the island of Reunion or the volcano of Mount Cameroon.
The Ring of Fire
98% of the planet's volcanoes lie on the edge of the Pacific Plate that scientists call “the Pacific Ring of Fire”.
The volcanoes are aligned along this plate surrounding the ocean for 40,000 kilometers.
Thus, there are many volcanoes along the west coast of the South American continent (Peru, Chile), in Central America (Mexico), in Asia (Japan, Philippines, Indonesia), and in Oceania along the Tonga faults. and Kermadec in northern New Zealand.
The Pacific Ring of Fire.
© Anoushka Notaras / RFI Knowledge
On the side of the Atlantic Ocean, there are volcanoes at the level of the ocean ridges (Canary Islands and Cape Verde Islands).
In Europe, two regions stand out for their active volcanoes: Italy and Iceland.
In Italy, the Vesuvius which overhangs the city of Naples is a permanent threat for the population which is often brought to evacuate.
In addition, Stromboli, in the north of Sicily, regularly emits large quantities of lava, the last of which occurred in 2019.
In Iceland, many active volcanoes erupt very frequently.
Icelanders and Europeans still remember the eruption of Eyjafjallajökull in the spring of 2010. The volcano expelled a gigantic quantity of ash causing air traffic to stop in Europe for several weeks.
The "super volcanoes"
Geologists have also discovered volcanoes which, when erupting, can throw up huge amounts of lava and ash. Generally speaking, a volcano releases about 1 km³ of lava and ash. For example, the Pinatubo in the Philippines, during its eruption in the summer of 1991, which is considered the largest eruption of the twentieth century, released 10 km³. For a super volcano, these projections of ash and pumice stone can even range between 1,000 and 3,000 km³, a quantity largely sufficient to cover a territory as large as France!
There are at least two "super volcanoes" listed in the world. In the center of Lake Toba on the island of Sumatra in Indonesia sits a sleeping monster: during its last eruption, 73,000 years ago, it emitted ash that blocked solar radiation into the Earth's atmosphere, plunging thus the entire planet in a cold period known as the "Ice Age" which lasted 1000 years.
In the United States, in Yellowstone National Park, there is a "super volcano" that scientists are watching very closely.
During history, this volcano has already erupted three times considerably modifying the relief of the western American continent.
If this volcano were to erupt, the quantity of projections could cover the entire territory of the United States and the temperature on the earth's surface could drop by 10 ° C for 10 years, threatening the extinction of many animal and plant species.
Our selection on the subject:
To listen :
→ Why do volcanoes roar?
→ What is happening under the Nyiragongo volcano?
→ Why live below volcanoes?
→ Why volcanoes?
→ How can we change our perspective on earthquake and volcanic activity?
To read :
→ Eruption of a volcano in the Canaries: respite or end of the lava flow?
→ “Volcanoes of France”, a book by Frédéric Lécuyer
→ Spain: a hundred houses destroyed in the eruption of a volcano in the Canaries
→ You must always be wary of a dormant volcano!
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Environment
Natural disasters