It was Charles Darwin who said "Nothing, not even the wind that blows, is so unstable as the crust of this earth." and that brings us to Bathtub Volcanoes.
These volcanoes are the deadliest, stickiest, nasttiest, gassiest of the entire lot.
Their eruptions can be earth shaking, cataclysmic- they can have the power of many nuclear explosions. They may bring famine, pestilence, tsunamis, plaques, great pyroclastic flows, darkness for many days or months, global climatic change, and, possibly, very pretty sunsets.
Caldera volcanoes are ones where the diameter of the circular to oval crater exceeds 1 mile. These form when so much lava is erupted (blown out) so rapidly it partially empties the underlying magma chamber. When this happens the summit of the volcanic structure collapses into the emptied magma chamber. Typically the erupted material occurs as airfall or pyroclastic flows.
There have been no recent eruptions from caldera volcanoes- Valley of Ten Thousand Smokes was the last and before it there was Krakatoa (1886) and Tambora (1815)- which caused the year without a summer. Three most famous examples in North America are Yellowstone,Crater Lake, and Long Valley.
Overall at least 138 caldera volcanos, whose crater exceeds 5 miles in diameter, are known. Unlike Crater Lake , many of these have calderas so large and irregular they remained undetected until high-quality aerial or satellite images became available. One of these, LaGarita, located in the San Jauan mountains of southern Colorado is some 20 miles wide and 60 miles long, Yellowstone, with a caldera of 24 by 40 miles is another example. Both these calderas were formed due to the large scale evacuation of a near surface magma chamber (think of the amount of material that has to be blown out to create a hole many miles in diameter and 1,000's of feet deep),and the collapse of the overlying rocks into it.
The eruptions from these caldera volcanoes are largely explosive leading to extensive air fall and pyroclastic flow deposits.
Where they occur:
Island Arcs- Krakatoa, Tambora, Toba, Taupau
Continental Mountains- Crater Lake,
Precambrian Example- Sturgeon Lake- Ontario- a large caldera volcano some 2.7 billion years old
Two kinds of Caldera Volcanoes: