History

Anak Krakatau erupting in July 2018, six months before the flank collapse. (Reuters)

Prior to 416 CE, a single volcanic island stood at this location. That year a caldera collapse formed the islands of Krakatau, Lang (Panjang), and Sertung (Verlaten) around the perimeter of the new, larger caldera. Krakatau underwent a violent eruption in 1883, launching an estimated 12-20 cubic km of material, destroying two thirds of the island, and producing a massive 40 m tsunami from the pyroclastic density currents that were produced. The remnant of the Krakatau volcano is now known as Rakata. On the rim of the 1883 caldera, at 200-300 m depth, a new volcano formed, Anak Krakatau (“Child of Krakatau”), which first began erupting in 1927 and rose above sea level in 1929.

Anak Krakatau and neighboring islands around the 1883 caldera. (From Omira & Ramalho 2020)

Located between Sumatra and Java at 6.102°S, 105.423°E, by 2018 Anak Krakatau grew rapidly to an elevation of 328 m by 2018, with the flanks extending more than 200 m underwater. With a steep foundation and a composition of unconsolidated volcanic material, it had already been flagged as unstable and a risk for tsunamigenic collapse. In 2012, T. Giachetti and colleagues published a modeling study warning that the lack of buttressing support of the structure could result in a flank collapse to the SW dislodging 0.28 cubic km of material, about half submarine and half subaerial. They estimated a subsequent tsunami of ~ 30 m height at the neighboring islands, and 0.3 to 3 meters high at coastlines 30-100 km away on the islands of Java and Sumatra.

The still submarine volcano erupting in 1928. Photos by Charles E. Stehn (Netherlands Indies Volcanological Survey) from the E.G. Zies Collection, Smithsonian Institution.

In June 2018, Anak Krakatau began another in a series of highly active Strombolian eruption periods characteristic of the volcano since its emergence 90 years before. Such periods typically last several months and frequently include ejection of large volumes of lava, magma bombs, tephra, ash, and volcanic gasses. Several hundred eruptions per day and plumes several hundred meters in altitude are not uncommon, with over 1000 eruptions per day and plumes as high as 2-3 km occurring occasionally. During one 9-month period between November 1992 and July 1993, the peak grew 80-100 meters and a seismometer was destroyed by a large magma bomb.

Signs that a collapse was likely on Anak Krakatau were abundant during the June-December eruption activity, as Walter et al. describe: “The load acting on the summit and especially the southern flanks of the island progressively increased over this time by ~54 million tons,” and the rate of subsidence and deformation more than doubled. This amounted to “one of the most rapidly deforming volcano flanks known on Earth prior to its collapse.”

Contents

• History
• Post-tsunami Eruptions and Cone Regrowth
• 22 December 2018 Eruptions and Tsunami
• Tsunami Impact on Nearby Unpopulated Islands
• Tsunami Impact on Populated Coastlines of Sumatra and Java
• Tsunamagenic Flank Failure Scenarios
• Seismic Signals, Early Warning Wystems and Event Documentation
• Lessons Learned
• Event Information
• Krakatau Islands History
• Media Reports, Photos and Video
• Research Articles