Block-and-ash flow deposit of the Narcondam Volcano: Product of dacite–andesite dome collapse in the Burma–Java subduction complex



Narcondam Island in the Andaman Sea represents a dacite–andesite dome volcano in the volcanic chain of the Burma–Java subduction complex. The pyroclasts of andesitic composition are restricted to the periphery of the dome predominantly in the form of block-and-ash deposits and minor base surge deposits. Besides pyroclastic deposits, andesitic lava occurs dominantly at the basal part of the dome whereas dacitic lava occupies the central part of the dome. The pyroclasts are represented by non-vesiculated to poorly vesiculated blocks of andesite, lapilli, and ash. The hot debris derived from dome collapse was deposited initially as massive to reversely-graded beds with the grain support at the lower part and matrix support at the upper part. This sequence is overlain by repetitive beds of lapilli breccia to tuff breccia. These deposits are recognized as a basal avalanche rather than lahar deposit. This basal avalanche was punctuated by an ash-cloud surge deposit representing a sequence of thinly bedded units of normal graded unit to parallel laminated beds.