Four small seamounts that rise 800 to 1,200 m above the flanks of the East Pacific Rise at the Pacific-Rivera plate boundary were examined and sampled during four dives of DSV-3 Turtle. They are all volcanic cones, at least two of them with summit craters. Samples from three of the volcanos are tholeiitic basalt that chemically resembles the basalt erupted at the rise crest, although they are less fractionated. One of the cones within 15 km of the spreading axis has very young flows of sheet and pillow lava on its summit, including some en echelon pillow walls. The other seamounts, farther from the plate boundary, appear extinct and generally have more weathered lava with thicker ferromanganese crusts. They also have extensive flows of hyaloclastite that probably formed in deep-water phreatomagmatic eruptions. Typical hyaloclastite landforms are stone streams of rocks that have moved down side slopes of volcanos and been stabilized by precipitation of volcanogenic ferromanganese cements.

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