The 1883 eruption of Krakatoa in the Dutch East Indies (now Indonesia) began in the afternoon of Sunday, 26 August 1883 (with origins as early as May of that year), and peaked in the late morning of Monday, 27 August when over 70% of the island and its surrounding archipelago were destroyed as it collapsed into a caldera. read more
What caused the magma to rise was a migration of comparatively hot basaltic magma into the lower plumbing of Krakatau beginning at least several years and probably decades before 1883. This caused melting of the comparatively cooler dacite magma that had existed in many different batches for millennia. read more