[Posted originally on another blog of mine.]
Or by another name "supervolcanoes". There are two active calderas or cauldron-type volcanoes (very large volcanoes with huge oval shaped craters) the lower 48 states, the Yellowstone Caldera and the Long Valley Caldera (eastern California). The oval crater for the Yellowstone Caldera measures approximately 36 miles X 48 miles and has erupted three times over the last 2 million years or so. The last major eruption was approximately 630,000 years ago.
The Cascade Volcanoes (Mount St. Helens, Mt. Rainier, Mt. Baker, etc.) are a smaller type of volcanoes, called composite volcanoes. Both types of volcanoes, calderas and composites are both capable of producing pyroclastic eruptions, which are the explosive eruptions which consist of gray clouds of superheated gases, volcanic ash, rock fragments, crystals, and pumice fragments, that are capable of moving across the countryside at speeds of up to 125 mph. Ancient pyroclastic flows (also called ash flows) have been traced from composite volcanoes to a distance of 60 miles and from calderas a distance of 100 miles. Once the ash flows begin their travel, a trapped layer of air beneath them serves as a cushion, allowing for less friction and greater travel distances.
These types of eruptions occur in composite and caldera eruptions because of the presence of silica (quartz) in the magmas. Quartz tends to increase the viscosity of magmas and as the magmas rise towards the surface, the quartz makes the magmas "want to freeze", while the trapped gases and liquids "want to boil". If the viscous magma plugs the volcano neck, pressure builds until the immovable object is overcome by the irresistable force, which results in an often catastrophic explosive eruption, a la Mount St. Helens, Mt. Pinatubo, El Chichon, Krakatau (1883), Mt. Vesuvius, etc....
As explosive as composite volcanoes are, calderas are much more so, as the quartz content is usually higher, i.e., the underlying magmas are generally similar to granite and the calderas are usually larger. Another caldera, which is approximately 1 million years old is the Valles Caldera, near Los Alamos, NM.
ABC is televising a special tonight [8/30/06] (9 PM EDT) called "The Last Days on Earth", which among other disasters, focuses on the damage that might be done to humanity by a large caldera eruption. The last major caldera eruption occurred about 74,000 years ago. That particular eruption of Mt. Toba in Indonesia reportedly put a major hurt on the DNA of early humans, i.e., there were mass casualties, apparently.
[As with typical Indonesian volcanoes, Mt. Toba was probably a large composite volcano, the explosive eruption of which triggered a caldera collapse. That is somewhat different from the caldera-type "supervolcano" mentioned in the first paragraph.]