which type of volcano erupting has not been witnessed first hand
Calderas form when an enormous magma chamber under, what was probably a pretty ginormous volcano, empties and collapses, causing the land above it to collapse as well. An explosive caldera-forming eruption has never been witnessed first hand, but we know that they should pretty fearsome spectacles. Much smaller calderas atop effusive volcanoes (hawaiian style) have been observed. So, formation: maars form when a vent intersects groundwater and blasts out the overlying material.