Thats the problem with Caldera or super volcanos is that the are like a huge boil over a large area deep under ground and can't really vent due to their depth & size, so when the blow it's extreme.
http://www.semp.us/publications/biot_reader.php?BiotID=164Consider the last one. 74,000 years ago a supervolcano erupted…in Sumatra. It would have been the loudest noise ever heard by man. It would have blasted vast clouds of ash across the world. The resultant caldera formed Lake Toba, 100 kilometres long, 60 kilometres wide…We're talking about 3,000 cubic kilometres of material coming out of that volcano. That's about 10,000 times the size of the 1980 Mount St. Helens eruption which people think of as a large eruption…It was, in short, colossal. Scientists are only now beginning to understand the effects of so much ash on the planet's climate…”*
Chemical analysis of 35 centimeters of ash located in the floor of the Indian Ocean approximately 2,500 kilometers from the Toba volcano tells us that “this eruption was rich in sulphur, [which] would have released a tremendous amount of sulphur dioxide and other gases into the stratosphere which would have turned into sulphuric acid aerosols and affected the climate of the Earth for years…The fine ash and sulphur dioxide blasted into the stratosphere reflects solar radiation back into space and stops sunlight reaching the planet. This has a cooling effect on the Earth…[T]he temperature change after Toba in degrees Celsius would have been about a 5 degree global temperature drop, very significant, very severe global cooling…causing Europe's summers to freeze and triggering a volcanic winter. Five degrees globally would translate into 15 degrees or so of summer cooling in the temperate to high latitudes. The effects on agriculture, on the growth of plants, on life in the oceans would be catastrophic.”
In fact, human geneticists have learned through the study of accumulation of mutations in mitochondrial DNA that the human species was almost wiped out approximately 70,000-80,000 years ago. Probably only five or ten thousand people worldwide managed to survive.