Volcanic eruptions as we know them today have a place in our lore as terrifyingly powerful acts of nature. Gods have been made of them and cultural touchstones built around them. They remain a disaster that we can do nothing to prevent and can only try to avoid. That said, the volcanoes of human recorded history have literally nothing on the volcanoes of earth's past. They are tiny pimples in comparison to the planet-changing, climate-destroying, extinction-event-triggering behemoths we have evidence of in the geologic record. Geologists categorize these monsters as Mega Colossal and the public calls them supervolcanoes. This list captures some of the largest, most powerful explosive eruptions on the planet, not necessarily the largest or the most volume of ejecta. The Deccan traps are not on this list, for example, because they were not explosive in nature despite the fact they erupted for longer and covered more landmass than some of the explosive events.
These eruptions are all classified as VEI 8, the most cataclysmic you can get. Explosive volcanic eruptions are classified using the Volcanic Explosivity Index, or VEI. It is a logarithmic scale, which means that an increase of one in VEI number is equivalent to a tenfold increase in volume of erupted material. The VEI scale does not distinguish between different types of volcanic ejecta ( ash, lava, ignimbrite), only power. On this scale, VEI 8 eruptions are so cataclysmic that they often form deep circular calderas rather than cones because the downward withdrawal of magma causes the overlying rock mass to collapse into the empty magma chamber beneath it. To give a sense of the VEI scale, St. Helens was a 5. This is a list of the largest, most earth-shaking events in the planet's history - best as we can tell. Let's all hope that there are no new VEI 8s to add to this list any time soon.
The boundaries in the imagery with each entry are as approximate as can be determined from the geologic record.
27 Million Years Ago
La Garita Caldera
This is the granddaddy of powerful volcanic eruptions on earth - at least as best as we know from looking at the geologic record. In the San Juan Mountains in southwestern Colorado, around 27 million years ago, a violent explosion formed what is known as the La Garita Caldera and spewed more than 3,107 cubic miles of lava — enough to put down a 40-foot layer on an area the size of California. The explosion was so large that, in a 2004 report in the Bulletin of Volcanology, researchers suggested adding a ninth level to the VEI scale, so that La Garita could be measured as a 9.2. That would make La Garita the only known magnitude 9 eruption. This caldera is one of a number of calderas that formed during a massive ignimbrite (igneous rocks made up of crystal and rock fragments) flare-up in Colorado, Utah and Nevada from 40–18 million years ago, and was the site of massive eruptions about 28 million years ago, during the Oligocene Epoch.
It is the most energetic event to have taken place on Earth since the Chicxulub meteor impact which, at 240 teratons, was approximately one thousand times more powerful than La Garita and which is believed to have caused the K-T extinction event which killed the dinosaurs.
2.1 Million Years Ago
Island Park Caldera
Located in what is now Idaho and Wyoming, Island Park is one of the world's largest calderas, with approximate dimensions of 80 by 65 km. Its ashfall can be found from southern California to the Mississippi River near St. Louis. This super-eruption of approximately 600 cu mi produced 2,500 times as much ash as the 1980 eruption of Mount St. Helens. Island Park Caldera also contains the Yellowstone Caldera and the smaller and younger Henry's Fork Caldera, which was formed 1.3 million years ago and which is clearly visible today.
75,000 Years Ago
Lake Toba Caldera
The Toba eruption, the largest-known explosive volcanic eruption within the last 25 million years, occurred at what is now Lake Toba. The eruption was large enough to have deposited an ash layer approximately 6 inches thick over all of South Asia. There is one site in central India where the Toba ash layer today is up to 20 ft thick and parts of Malaysia were covered with 30 ft of ash. An enormous amount of sulfurous acid and sulfur dioxide were ejected into the atmosphere by the event. The exact year of the eruption is unknown, but the eruption appears to have lasted perhaps two weeks, and the ensuing volcanic winter resulted in a decrease in average global temperatures 5 to 6 °F for several years. Very few plants or animals in southeast Asia would have survived, and it is possible that the eruption caused a planet-wide die-off.
26,500 Years Ago
Taupo Volcanic Zone
This particular eruption of the Taupo Volcano was the world's largest known eruption of the past 70,000 years, and it generated approximately 430 km³ of ash fall deposits. Rock fragments and particles from the eruption covered much of the central North Island of New Zealand with ignimbrite (igneous rocks made up of crystal and rock fragments) up to 200 meters deep. Most of New Zealand was affected by ash fall, dropping an 18 cm ash layer on the Chatham Islands, 1,000 km away. Modern Lake Taupo partly fills the caldera generated during this eruption.
Though Taupo has had more violent eruptions than cone volcanoes, they are fortunately less frequent. It began erupting about 300,000 years ago and while the present day caldera was created by this Oruanui eruption, the ‘Taupo Eruption’ happened 1800 years ago and was considered one of the most violent eruptions in the 5,000 years - turning the sky red over Rome and China.