In 2015, an international team of volcano experts (called the Global Volcano Model Network) released a report (linked below) ranking the countries most at risk from volcanic disasters - both from the volcano itself and from the hazards wrought by the volcano. The thought was to increase awareness for the most at-risk areas... to urge monitoring and emergency planning. More than 278,000 people have died in volcanic eruptions since 1600, and only five eruptions caused 58 percent of recorded fatalities. Of the total deaths; 33% were killed by pyroclastic flows, 20% by tsunamis and 14% died in lahars. Only 887 people died from lava. Finally, the remaining 24% of deaths were indirect, of famine, disease, ash, avalanches, lightning and other hazards.
Thus, this is a difficult list to make. Not because there aren't volcanoes that are dangerous, there are. Volcanos are doing their thing around the globe every single day... around 50 at a time in-process of erupting, getting ready to erupt or just releasing gas and steam. Most of the 1,500+ currently active volcanoes are nowhere near human populations so they don't make the news, but others are so intensely monitored that if they hiccup any nearby town, city or port flies into action.
Do you rank a volcano's potential danger by how much activity it's starting to show even if the nearby town has all the plans in place? Or by an inactive volcano's potential for catastrophe if, despite no indicators and minuscule odds, it did go off and wiped out half a continent? Different types of volcanoes also erupt in different ways - some ways more catastrophic than others. A good example is Kilauea which has been active for some time on a populated island. Is it dangerous? Most definitely, but, because it's a shield volcano and because of the composition of its magma, the slow moving lava doesn't pose a deadly threat. So, it's active and it's dangerous, but should it be ranked higher than Mt. Rainier? Mt. Rainier does not seem to be in danger of erupting any time soon, but if this stratovolcano did suddenly spring to life, the resulting pyroclastic flows and lahars would be catastrophic for the many millions of people living within spitting distance of it. Should it rank higher than an actual erupting volcano?
For these reasons, I am not putting ranked numbers on this list, but I will loosely group them - in my amateur opinion - by most dangerous at the top down to dangerous-but-not-as-dangerous at the bottom. I am sure there will be disagreement... which is why no one should use this list as any kind of scientific resource.
Lastly, I am not putting supervolcanoes on this list because the odds of those big boys going off any time soon are miniscule.
* Decade volcanoes were determined by the International Association of Volcanology and Chemistry of the Earth's Interior to be a volcano that exhibits more than one volcanic hazard (people living near the Decade Volcanoes may experience tephra fall, pyroclastic flows, lava flows, lahars, volcanic edifice instability and lava dome collapse); shows recent geological activity; is located in a populated area (eruptions at any of the Decade Volcanoes may threaten tens or hundreds of thousands of people, and therefore mitigating eruption hazards at these volcanoes is crucial); is politically and physically accessible for study; and there is local support for the work.
Located in the Philippines, Mayon is the most volatile of the nation's 22 currently active volcanoes. It is a stratovolcano that sits, like so many, on the collection of subduction zones and plate boundaries known as "The Ring of Fire". It is a very active mountain -- constantly letting everyone know how temperamental it is. This is actually a benefit, as volcanoes that sit quiet and silent are the one you usually have to worry about. In the case of Mayon, the fact that it lets lots of energy out doesn't fully mitigate its danger. In fact, it caused the evacuation of the region in early 2018 when, after weeks of increasing unrest, it erupted lava 700m into the air and turned the sky black with ash.
There are 49 eruptions of Mayon in the historical record, with the (so far) most devastating one being the 1814 tragedy (VEI 4) that claimed an estimated 1,200 lives. The eruption destroyed most villages of the Albay and Camarines province. Mayon's perfectly symmetrical cone sits in the middle province of Albay with a population of over 1.2 million people. The current monitoring and response of the volcano is, so far, adequate to protect the lives of the province... the concern comes from comparing Mayon to similiar volcanos such as Pinatubo or St. Helens. If, instead of a summit eruption, a period of inflation leads to a sudden, explosive eruption at one of the fissures on the volcano’s flanks or beyond then the resulting pyroclastic flows could easily travel well past 20km. The current, evacuated danger zone around Mayon is 7km. The potential for catastrophic loss of life is huge.
Located on the island of Java, Merapi is the most active of the 129 active volcanoes in Indonesia. It is considered one of the most dangerous volcanoes in the world due to several factors. One is that it sits inside one the most dense populations on the globe. Yogyakarta City has a population of 3 million people and sits only 15 miles away. Another 70,000 people live in the immediate vicinity. Merapi is well known for its pyroclastic flows... flows that can reach speeds of 70mph and can travel as far as 8 miles. These flows can also be re-mobilized as lahars.
This stratovolcano has had numerous moderate to large eruptions that appear to have started over 400,000 years ago. These eruptions typically begin with pyroclastic flows and are followed by widespread pyroclastic air fall. Typically, smaller eruptions happen every 2-3 years with larger ones having 10-15 years between them. The most notable eruptions on record occured in 1006, 1786, 1822, 1872 and 1930. It has been postulated that the 1006 eruption covered all of central Java with ash and that the devastation possibly led to the collapse of the Hindu Kingdom of Mataram.
Merapi's type of eruptions are more dangerous than Hawaiian-style, shield volcano eruptions because the magma in stratovolcanoes is charged with gas. As a result, when such magma finds a conduit to the surface, it can build up a sticky, hot, gas-rich plug known as a volcanic dome. The dome can build up higher and higher, and then suddenly collapse under its own weight in an eruption. This creates a very dangerous mixture of hot gas, ash and dome fragments that can flow downhill as much as 5-10 miles. This flow could wipe out anything on ridge tops because of the gas expansion, and could melt glaciers and mix with the meltwater to form very dangerous, hot and mobile lahars.
One of the world's most heavily monitored volcanoes, Vesuvius easily perches high in the ranks of world's most dangerous. This is not just because of its status as a very active and alive stratovolcano, but because it sits in the middle of a city of 3.1 million people. Like many stratovolcanos, this one was formed as a result of plate collision... in this case the African and Eurasian plates. One of several volcanoes that form the Camanian volcanic arc, it squats at the southern end of the chain and is the only one of its companions ( including Epomeo and Campi Flegrei ) that has erupted in the last few hundred years as well as being the only active volcano in Europe.
Vesuvius is also one of the most famous volcanoes in history, having been responsible for the nearly instantaneous destruction of Pompeii in 70 AD. The last time it erupted was 1944, causing some minor damage and killing 26 people. The research being done is showing, however, that the next time it erupts it won't be so gentle and that it is overdue for a large event. It has had 42 eruptions of VEI 3 and up and it is capable of much larger as it erupted at a VEI5 in 1631. It sits on top of a layer of magma that measures 154 square miles and thus, scientists think that the next eruption will be an incredibly forceful plinean explosion, marked by flying rock and ash at speeds of up to almost 100 miles per hour. The summit looms a mere 7.5 miles from downtown Naples and any pyroclastic flow from a volcano like Vesuvius would take only about two and a half minutes to travel that distance.
Were Vesuvius simply to repeat the same size eruption that destroyed Pompeii, there could potentially be 6 million people who would lose power, water and transportation - not to mention how many dead in the pyroclastic flows. Vesuvius is intensely monitored because of the high danger, and there would hopefully be many indicators pre-eruption... but as we know from both experience and record, volcanoes are unpredictable.
Considered one of the most dangerous volcanoes in the US, Mt. Rainier is also the highest peak in the Cascade mountain range. With an active hydrothermal system and thick glacial layer, it has been the source of countless eruptions and volcanic mudflows (lahars) that have surged down valleys on its flanks and buried vast areas of land... land that is now densely populated. The last magmatic eruption was around 1,000 years ago, but small summit explosions were witnessed in 1894. The real danger of Rainier, however, is its lahar potential. This mountain has 26 glaciers containing more than five times as much snow and ice as all the other Cascade volcanoes combined. If even a small part of this ice were melted by volcanic activity, it would trigger truly enormous lahars, lahars that would be incredibly destructive simply by the fact of the mountains great height and steepness.
Mount Rainier has erupted less often and less explosively in recent millennia than its well-known neighbor, Mount St. Helens. However, because of the nearby population centers and the danger posed by catastrophic lahars, its threat level is much higher.