A caldera is a cauldron-like volcanic feature on large central volcanoes, a special sort of volcanic crater (from one to several kilometers in diameter), formed when a magma chamber was emptied.The depression then originated either in very big explosive eruptions or in erosion and collapse of the magma chamber roof.s the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions.