The pumice clasts are typically aphyric, and the bulk composition of a pumice-rich sample is trachytic. The volcaniclastic sequence also includes pumice beds, reworked volcaniclastic sediments, and highly altered ash deposits that contain accretionary lapilli. Several boundaries are oxidized, suggesting subaerial weathering between eruptions. The relatively thin flows at Site 1138 resemble Hawaiian lavas, and contrast with the generally thicker flows drilled at Sites 1136 and 1137. Most flows have unique flow top breccias that are not easily classified.
All basalts show normal magnetic inclinations.
The massive parts of flow Units 3 to 22 are slightly to locally highly altered, whereas alteration ranges from high to complete in the brecciated zones. Rubbly flow tops are partly to completely altered to clay minerals and abundant euhedral zeolites form the matrix, fill veins, and partly fill vesicles and large voids. Lava clasts are commonly completely altered to brown clay minerals. Sediment filling breccia void space is variably indurated, perhaps due to silicification. Calcium carbonate is absent except from the uppermost basalts directly underlying the volcaniclastic sequence.