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. The previous emptying of this magma chamber is often accomplished during a series of effusive eruptions in the volcanic system, even kilometers away from the magma chamber itself.