What are the largest eruptions in the world?

It is difficult to estimate the relative size of eruptions, because every volcano is different from another, and the eruptions’ characteristics vary a lot. Some volcanoes throw out dense material while others eject less dense lavas and ashes; some volcanoes erupt very quickly whereas others erupt for years.

Explosive eruptions:
A scale created by Chris Newhall and Stephen Self in 1982 helps determine the relative explosiveness of volcanic eruptions: it is called the Volcanic Explosivity Index (VEI). Although the VEI does consider the total volume of material ejected, it does not consider the nature of this material (it neglects its density in particular).
The VEI depends on the overall volume of the eruption, its height, and how long it lasts. It is a logarithmic scale that ranges from 0 to 8: this means that a VEI 8 is ten times more powerful than a VEI 7, and hundred times more powerful than a VEI 6.

Examples of large explosive eruptions in recent history:
Pinatubo, 1991, VEI=6
Krakatoa, 1883, VEI=6
Tambora, 1815, VEI=7

Some of the largest explosive eruptions in Earth’s history:
Taupo, 24,500 years ago, VEI=8
Toba, 73,000 years ago, VEI=8
Whakamaru, 254,000 years ago, VEI=8
Yellowstone, 640,000 years ago, VEI=8

Other eruptions:
Some eruptive events cannot be classified using the VEI, because explosiveness is not their principal characteristic, or because they regroup in fact numerous eruptions on a very long period of time.
Traps, also called flood basalts, are the result of immense eruptions or series of eruptions that can last for several million years. The volcanic deposits of such eruptions are on a complete other scale than the ones of the explosive eruptions mentioned above.

Some of the biggest eruptive events (Traps) in Earth’s history:
Deccan Traps, 60-68 Ma, Volume between 0.5 and 1.5 million km3
Kerguelen plateau, 112 Ma, Volume of 17 million km3
Ontong Java plateau, 120 Ma, Volume between 59 and 77 million km3

Comparison between traps and explosive eruptions:
A comparison between the Ontong Java Traps and the Toba caldera eruption is quite useful to appreciate the different scales of these two kinds of eruptive events.
The Ontong Java plateau is the result of a series of eruptions that lasted about five million years. The lower estimate for the volume of lava ejected during this event is around 50 millions km3. It covered a surface comparable to Alaska, about 1% of the surface of the Earth.

The Toba caldera eruption happened 73000 years ago. 2800 km3 of material were erupted, about 20,000 times less than the Ontong Java plateau. However, this explosive eruption was so instantaneous that it created a great disequilibrium in global climate. The average surface temperature of the globe dropped by at least 3˚C because of the ashes in the atmosphere. Such a climatic change in a short period is thought to be responsible for a bottleneck in human evolution: human population may have been reduced to a few thousand people only at that time.