June 15, 2011 marks the 20th anniversary of Mt. Pinatubo's cataclysmic eruption, which sent huge amounts of ash high into the sky and circling around the globe, lowering global temperatures by 1 degree Fahrenheit (0.5 degree Celsius) over the following year.
On this anniversary, we countdown the largest volcanic eruptions in history as measured by the Volcanic Explosivity Index (VEI) a classification system somewhat akin to the magnitude scale for earthquakes.
The system was developed in the 1980s, and uses factors such as the volume of an eruption, its rate, and other variables to quantify a volcano's power. The scale goes from 1 to 8, and each succeeding VEI is 10 times greater than the last.
There haven't been any VEI-8 volcanoes in the last 10,000 years, but human history has seen some powerful and devastating eruptions. Because it's extremely difficult for scientists to be able to rank the strength of eruptions in the same VEI category, here we present the 10 most powerful volcanoes within the last 4,000 years (within human records) first in order of strength, then within each category, in chronological order.