CVGHM raised the Alert Level to 3 (on a scale of 1-4) for Krakatau on 26 October due to the presence of multiple gray plumes from Anak Krakatau and an increase in seismicity during 23-26 October. Plumes rose to an altitude of approximately 1 km (3,300 ft) a.s.l. during 23-26 and 30 October. Inclement weather resulted in only intermittent observations. Villagers and tourists were advised not go within a 3 km radius of the summit.

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)

Geological Summary. The renowned Krakatau (frequently misnamed as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.