Abstract

The late Quaternary Toba volcano-tectonic depression, in the Sunda Arc, is the largest resurgent cauldron in the world and lies in one of the largest ignimbrite fields, the Toba Tuffs. Although composed of at least 3000 km3 of acid tuffs, spread over 20000 km2 of mainland Sumatra, the greater part of the Toba Tuffs is believed to be a single ignimbrite cooling unit, formed about 100,000 years ago. The Toba Tuffs also include a 30000-year-old airfall tuff, which was erupted from a centre just N of the Toba depression.

The Toba depression developed through the same stages as other resurgent cauldrons. It formed after lithification of the Toba Tuffs, by collapse along regional faults crossing an eastwards bulge in the Sumatran geanticline. Resurgent uplift raised lake sediments within the depression by about 500 m. The uplift, which was not confined to the depression, was probably caused by continuing influx of magma and it is possible that the Toba depression is still in the stage of post-resurgence volcanism. The eruption of the Toba Tuffs and of all post-ignimbrite volcanism took place on the line of the western marginal fault of the depression. This marginal fault once extended northwards offshore into a zone of Miocene back-arc rifting.

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