The enigmatic origin and emplacement of the Samosir Island lava domes, Toba Caldera, Sumatra, Indonesia

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Department of Geological and Mining Engineering and Sciences


The 60 × 20-km Samosir resurgent dome within the 74-ka Toba Caldera features several clusters of rhyolitic lava domes. Because the previously known Tuk-Tuk and Samosir Fault lava domes occur near the base of a major resurgent dome fault, they appear associated with resurgent uplift, and dating them could provide constraints on the timing of resurgence. Towards that goal, we mapped and sampled the Tuk-Tuk and Samosir Fault lava domes, discovered two additional lava dome clusters in the uplifted interior of Samosir Island, then determined modal mineralogy, major and trace element geochemistry, and 40Ar/39Ar sanidine ages on several samples from each cluster. These quartz-bearing rhyolite lava domes have SiO2 contents ranging from ~ 70-76 wt%, crystallinities of 28-54%, and 40Ar/39Ar ages (~ 74-76 ka) that overlap with those of the Youngest Toba Tuff (YTT), ~ 74-75 ka. Our field observations and the remarkable similarity in mineralogy, geochemistry, and ages among the dome clusters and YTT support the interpretation that these domes represent remnant YTT magma that erupted shortly after the climactic YTT eruption. Later, during resurgence, these pre-resurgent lava domes were uplifted to their present locations, some near current lake level, others up to 440 m above lake level.

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Bulletin of Volcanology