The largest known supervolcano (not actually a technical term by the way) is Yellowstone.
Actually they are measured by VEI- Volcanic Explosivity Index
, which measures the
amount of material expelled, as well as the height of the ash cloud. Each number is 10 times stronger than the next number down.
A VEI of 8 is considered "mega colossal" and does the nasty things probably detailed in the documentary you saw. Big ash cloud that would affect
climate and crops as well as ground airtraffic, the huge ejection of searing dust and lava that would destroy the immediate vacinity, the noxious
gasses, etc etc.
To our knowledge, there hasn't been an erruption of VEI 8 in tens of thousands of years.
Mount St Helens was a VEI 5- which means a VEI 8 would be 1,000 times as bad. Krakatoa was a 6.
The worst one in remotely recent memory was a VEI 7 at Tambora in 1815. It happened in Indonesia. The sun appeared to dim and it snowed in America the
following summer- there were significant crop failures. There are only known to have been 4 such eruptions in the last 10,000 years.
VEI-8 volcanic events have included eruptions at the following locations (with estimates of the volume of erupted material at the event):
Aira Caldera, Kyūshū, Japan
Aso, Kyūshū, Japan
Campi Flegrei, Campania, Italy
Kikai Caldera, Ryūkyū Islands, Japan
Long Valley Caldera, California, United States
Lake Taupo, North Island, New Zealand
Lake Toba, Sumatra, Indonesia (2,800 km³)
Valle Grande, New Mexico, United States
Yellowstone Caldera, Wyoming, United States
La Garita Caldera, Colorado, United States (over 5,000 km³)
The only reason I'm not freaked out is because there is only so much that can be done to prepare. If you're seriously afraid of it happening, have
canned food and water on hand (a wise move in any case) and maybe buy a box of those cheap paper masks that landscapers wear- they use them at rock
crushing operations to prevent silicosis, so they'd do the trick. Oh, and don't move to Wyoming- not that anybody ever would.
Second edit to add: Keep in mind that in the list of VEI 8 sites above, we're talking about places that have had them A LONG time ago. Some could
still have potential (I haven't checked) but they could be innactive. A spot on the Earth's surface only remains active while it is over a "hot
spot" in the mantle. As the continents drift, an area moves away from the hotspot and the old caldera's become incapable of going again unless they
arrive over another hotspot. Hence volcanic island chains like Hawaii, etc.
[edit on 14-1-2006 by The Vagabond]