By about the 15th century BC a massive volcanic explosion known as the Minoan eruption blew the island of Thera apart, casting more than four times the amount of ejecta as the explosion of Krakatoa and generating a tsunami in the enclosed Aegean that threw pumice up to 250 meters above sea level onto the slopes of Anaphi, 27 km to the east. Any fleet along the north shore of Crete was destroyed and John Chadwick suggests that the majority of Cretan fleets had kept the island secure from the Greek-speaking mainlanders. The sites, save Knossos, were destroyed by fires. Mycenaeans from the mainland took over Knossos, rebuilding some parts to suit them. They were in turn subsumed by a subsequent Dorian migration.