Pyroclastic flows covered an area of at least 20,000 square km. Rose and Chesner suggested the ash may have reached central Asia and the Middle East. Rose and Chesner, after a study of the shapes of the ash shards, concluded this estimate was too high by a factor of 5 or more. There have been no eruptions at Toba in historical time. Stratovolcanoes in Sumatra are part of the Sunda arc .
Volcanism is the result of the subduction of the Indian Ocean plate under the Eurasian plate.
The subduction zone is marked by the Java Trench. The rate of subduction is 6.7 cm per year.