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 on Kyushu island, Mt. Aso has been active for decades. At approximately 2.2 million years, it is an old volcano that got its start with basaltic eruptions and then formed its major caldera about 300,000 years ago with four huge eruptions, the last being the largest. Since the caldera formed more than 17 new cones have formed inside it. The five main cones are Neko, Taka, Naka-Dake, Eboshi, and Mt. Kishima. Only Naka-Dake is currently active: it has erupted 167 times from seven different craters since 553 CE. Two flank volcanoes, Akai and Omine sit along a fault (Oita-Kumamoto Tectonic Line) extending from the volcano towards Kumamoto City.
The dangers of Mt Aso are many, which put it high on this list. The first is that Kyushu island is densely populated. With what we know about the type of volcano that Aso is and what kinds of eruptions it is capable of, a large explosion could blast outwards in excess of 50km from the vent, leaving little chance of anything in proximity to the volcano surviving. The second is that Aso's magma reservoir is thought to be "charged" according to researchers. Hopefully Mt. Aso would give its human neighbors some warning as well as time to evacuate, otherwise it could be potentially catastrophic.
Democratic Republic of Congo
Nyiragongo is not as well studied, nor monitored as a stratovolcano with its associated hazards should be. Twice in recent history it has had two devastating volcanic eruptions; the first related to its lava lake, and the second from a deadly eruption off its flank. The lava lake itself is somewhat unusual due to the viscosity and high alkalinity of its composition. This allows it to move much more quickly than, say, the lava from a Hawaiian shield volcano... upwards of 60mph. Back in 1977, the crater wall holding the lava lake ruptured and within 30 minutes the entire lake had drained, sending lava flowing to the north, west, and south of Nyiragongo. These lava flows reached up to 62 mph, wiping out several of the surrounding villages and burning almost 300 people alive.
Because of the proximity of Goma, a city of over a million people, the volcano poses a real danger. An eruption in Goma itself from fissures located within a mile of the lake shore that could lead to major explosion, and a lava eruption in nearby Lake Kivu could disturb the carbon dioxide and methane stored in the lake at depth which would lead to the release of an asphyxiating gas cloud that could disperse over a wide area. Either one of these events could lead to catastrophic loss of life.
Unfortunately, the political and humanitarian situation makes it unlikely that the population will heed warnings of an impending eruption and be prepared to evacuate part or whole of Goma as a precautionary measure. Secondarily, the population relies almost entirely on Lake Kivu for drinking water. This reliance is unchanged today and the prevention of enteric disease outbreaks like cholera after a future eruption is a major consideration. These things make Nyiragongo very dangerous indeed.
Iwo Jima is the island remains of a 9km-wide submarine caldera located in the central Volcano Islands of the Izu-Marianas arc. Mt. Suribachi, also called Suribati-yama, is the name of the distinctive cone on the southwest side of the island. Iwo-jima has had at least ten historic eruptions, all in this century, the most recent of which was in 1982. All of the eruptions were non-explosive or small and most were phreatic, caused by the interaction of water and magma. However, the island has undergone dramatic uplift for at least the past 700 years caused by resurgent doming of the caldera. Recently, parts of the caldera have been experiencing remarkably strong uplift of more than 1 meter per year and there is strong hydrothermal activity at present.
It is thought that it is only a matter of time before the caldera erupts, and while few people live on Iwo Jima itself, a large eruption could cause a tsunami that could devastate southern Japan and coastal China including Shanghai and Hong Kong. It has been estimated that the tsunami generated by the eruption could reach 30 meters high. This would cause massive loss of life and property.
Responsible for one of the biggest volcanic blasts in recorded history, Tambora is located on the island of Sumbawa in Indonesia. Back in 1815, after lying dormant for 1,000 years, Tambora rumbled to life first with some heavy activity. The plinian explosion that happened next obliterated the caldera and killed more than 71,000 people on the spot. The plume then went on to cause a volcanic winter responsible for a worldwide famine. Since then Tambora remains active. Minor lava domes have extruded on the caldera floor and a small, non-explosive eruption occurred in 1967 and then again in 2011.
As of 2010, the population has reached 238 million people and climbing, of which 57.5% are concentrated on the island of Java. An event as significant as the 1815 eruption would impact about eight million people. There is constant monitoring of the volcano, however, and the local directorate has created a disaster mitigation map which sets two hazard zones in the areas surrounding Tambora. Scientists say that the mountain has a 30% chance of erupting again in this century.