The violent eruption was centred on a small island just north of the existing island of Nea Kameni in the centre of the caldera; the caldera itself was formed several hundred thousand years ago by the collapse of the centre of a circular island, caused by the emptying of the magma chamber during an eruption. It has been filled several times by ignimbrite since then, and the process repeated itself, most recently 21,000 years ago. The northern part of the caldera was refilled by the volcano, then collapsed once more during the Minoan eruption. Before the Minoan eruption, the caldera formed a nearly continuous ring with the only entrance between the tiny island of Aspronisi and Thera; the eruption destroyed the sections of the ring between Aspronisi and Therasia, and between Therasia and Thera, creating two new channels.