From Ashes to Solid Rock

The extrusion of lava began only in 1960, and prior to this time, the coarse and fine ashes (pyroclasts) that made up the island were subject to fairly swift erosion. Thus, coastal cliffs and narrow beaches developed around the islands. During much of the period from 1932 through 1950, a sea-level crater lake, which first formed in 1932, was open to the sea. From 1938 until 1941, island building from pyroclast production was more or less balanced by marine erosion of the crater edge and exposed southern flank of the cone. But the extrusion of lava over 1960-61 effectively began a new phase in the evolution of Anak Krakatau. A vitric andesite flow breached the wall of the inner cone to form the first resistant bulwark against wave erosion on the southwest coast. The crater lake was also filled in by lava and other ejecta.

Further lava flows were produced between 1972 and 1975, 1979-80, 1988, and in the mid-1990s, when they not only greatly increased the area of the island, but also converted the majority of the coastline to solid geology. The period since 1960 has seen a continued pattern of episodic activity now featuring both lava and pyroclast output. In the first 50 years of its existence, Anak Krakatau was active for a full year at a stretch on only two occasions (1928-29 and 1960-61), and for some 18 of those years, the volcano was effectively dormant. A similar on/off pattern has continued over the last 20 years.