So, the emergence of Anak Krakatau is part of a repeating pattern. But why at this point in the Sunda Strait? The answer to this question is that Krakatau is just one of a series of volcanoes that run down through Sumatra and Java. They mark the meeting point of two tectonic plates -- great slabs of the Earth's crust -- in a destructive plate margin. Here, the Indo-Australian plate is subducted under the Asian plate, and as it descends below the overriding plate, some of the rock melts, and the molten magma rises through the crust above, forming a line of volcanic mountains. It has long been appreciated that the orientation of the line of volcanoes running along the long axes of Java and Sumatra changes sharply at Krakatau. In addition, a shorter line of volcanoes and volcanic islands can be found within the Sunda straits running from the southern tip of Sumatra to the western tip of Java. The interpretation for these features is that the subduction of crust is faster along the Sumatra trench than along the Javan trench, with the change occurring abruptly at the Sunda Strait. In other words, it marks a major tectonic break, a hinging point by which Sumatra is gradually rotating clockwise out of line with Java. Krakatau thus marks a focal point in a bigger tectonic picture, and one that is liable to continue to be a center of cycles of volcanic activity long into the future.
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