The Quaternary volcanism is anomalous in relation to both southern Sumatra and adjacent Java/Bali: in southern Sumatra, volcanoes are regularly spaced along and successively less active away from the SFS, but neither rule holds in northern Sumatra. Quaternary volcanics are mainly calc-alkaline andesites, dacites and rhyolites with few basalts they seem less variable, but on the whole more acid, than the Tertiary. The largest concentration of centres, around Lake Toba, includes the >2000 km 3 Pleistocene rhyolitic Toba Tuffs. Quaternary volcanoes (3 active, 14 dormant or extinct) are irregularly distributed both along and across the arc thus they lie fore-arc of the SFS near the Equator but well back-arc farther north. Miocene volcanic rocks, widely distributed (especially W of the SFS), and cropping out extensively along the W coast, include calc-alkaline to high-K calc-alkaline basalts, andesites and dacites. Possible Palaeogene volcanic rocks include an altered basalt pile with associated dyke-swarm in the extreme NW, intruded by an Early Miocene (19 my) dioritic stock and variable pyroxene rich basic lavas and agglomerates ranging from alkali basaltic to absarokitic in the extreme SW. Sumatra Fault System (SFS), which axially bisects Sumatra, include ophiolite-related spilites, andesites and basalts. Late Mesozoic volcanic rocks, widely distributed along and W of the major transcurrent. Late Permian volcanic rocks, of limited extent, are altered porphyritic basic lavas interstratified with limestones and phyllites. The principal volcanic episodes in Sumatra N of the Equator have been in the Late Permian, Late Mesozoic, Palaeogene, Miocene and Quaternary.
Sumatra has been a ‘volcanic arc’, above an NE-dipping subduction zone, since at least the Late Permian.
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