World Panorama of Volcanoes: A Tour of the Giants of Fire

FAQ: Everything you need to know about active volcanoes

What is an active volcano?

A volcano is said to be active when it has experienced at least one eruption during the Holocene, that is, over the last approximately 11,700 years. This definition includes volcanoes in permanent eruption, those with regular episodes and those that are temporarily dormant but likely to wake up. About 1,500 volcanoes are considered active on Earth, but only a few dozen are continuously monitored because of their proximity to populations.

Why is Merapi considered the most dangerous volcano in the world?

Merapi, located on the island of Java in Indonesia, combines several risk factors. Its activity is near-permanent, with eruptions every two to three years on average. It frequently produces pyroclastic flows — avalanches of burning gas and ash sweeping down the slopes at more than 100 km/h — and deadly lahars during the rainy season. Above all, more than two million people live in its immediate zone of influence, particularly in the city of Yogyakarta. The 2010 eruption caused 353 deaths and required the evacuation of 350,000 inhabitants.

What is the difference between a stratovolcano and a shield volcano?

A stratovolcano is a conical edifice with steep slopes, built up by the accumulation of viscous lava flows and explosive projections. It produces violent eruptions, often accompanied by pyroclastic flows and ash plumes reaching the stratosphere. Vesuvius, Merapi and Popocatépetl are examples. A shield volcano, by contrast, is formed by fluid lava flows that spread over great distances, giving a broad, low shape. Mauna Loa in Hawaii and Piton de la Fournaise on Réunion are typical representatives, characterised by effusive eruptions that are less dangerous for populations.

What is a pyroclastic flow and why is it so deadly?

A pyroclastic flow is a mixture of very high-temperature gases (which can exceed 800 °C), ash and rock blocks that sweeps down the slopes of a volcano at speeds of 100 to 700 km/h. This phenomenon is particularly deadly because it combines several lethal effects: the intense heat burns everything in its path, the speed prevents any escape, and the inhalation of burning ash causes instant asphyxiation. The pyroclastic flow of Mount Pelée in 1902 destroyed the town of Saint-Pierre in Martinique within minutes, killing nearly 30,000 people.

Why is Vesuvius so closely monitored despite its apparent calm?

Vesuvius has not experienced an eruption since 1944, which may seem reassuring, but it is precisely this calm that worries volcanologists. The more time passes without an eruption, the more pressure builds up in the magma chamber, increasing the risk of a violent future eruption. Moreover, the population density around Vesuvius is exceptional: more than three million people live in the red zone, and a complete evacuation would take at least 72 hours. Scientists continuously monitor the precursor signs — earthquakes, ground deformation, gas emissions — to anticipate a sudden awakening.

How do scientists monitor at-risk volcanoes?

Volcanic monitoring relies on a network of sensors deployed around active edifices. Seismographs detect micro-earthquakes linked to magma migration. GPS stations and radars measure ground deformation, which swells when magma accumulates. Spectrometers analyse volcanic gases — an increase in sulphur dioxide is often a precursor sign of eruption. Finally, observation satellites monitor thermal anomalies and ash plumes. All this data is transmitted in real time to volcanological observatories, which issue alerts and coordinate evacuations with the authorities.