An abyssal-hill site with localized high heat flow on the 3-m.y.-old west flank of the active ridge in the Mariana Trough has sets of oblique fault scarps, mapped by multi-beam transits, and clusters of 50- to 1,000-m-diameter domes examined with a Deep Tow survey. Some of the domes, including small, 20- to 30-m-high rocky knobs encrusted with manganese oxide, crop out through the sediment cover, but, despite heat-flow evidence that they are sites of hydrothermal discharge, these bedrock features are fundamentally different from the hydrothermal mounds found on some mid-ocean ridges. Dredging one dome recovered a rhyodacite pumice that we believe erupted at a depth of 2,600–3,600 m within the basin. The siliceous rock may be an extreme differentiate of a tholeiitic magma, but a more likely origin is partial melting of the basin's sedimentary fill.

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