We describe selected examples of small volcanic features identified during Ocean Drilling Program (ODP) site surveys in the MARK area for which additional observations and sample data have been obtained by manned submersible and/or drilling. Based on both morphologic relations and similarities in chemistry and petrography, Serocki Volcano, the site of bare rock drilling on ODP legs 106 and 109, appears to be a flat-topped megatumulus, a large rootless vent developed on a lava flow originating from an adjacent cone located above the true eruptive fissure. A linear set of five apparent volcanic centers on the east side of the median valley (the EastMARK cones) differ from one another in morphology and degree of tectonic degradation. They formed at different times and from different vents, but all were localized by damming of the flows against the steep walls of the linear valley containing them. The two northernmost features are probably megatumuli at the terminal ends of lava flows descending the valley walls. The other three vents are cones with breached craters and probably were more recently erupted in situ. The Snakepit hydrothermal area is located near the highest and widest part of a 35 km-long ridge that appears to be a major, relatively young fissure eruption. The associated lavas are among the geochemically most primitive in the MARK area but have their counterparts in several subsets of lavas associated with Serocki Volcano in spite of the morphologic contrasts and large geographic separation between the two features.

We conclude that a volcano like form, geographic proximity, and conformance to a linear geometry do not guarantee compositional or temporal coherence, nor is the alignment of apparent volcanic centers a reliable guide to underlying tectonic patterns. The chemistry and petrography of the Serocki and EastMARK basalts resemble older adjacent lavas and also conform to subtle along-ridge variations. This and the relatively old appearance of their flow surfaces indicate that these lavas represent the waning stage of an older magmatic event. The more recent eruptions in the Snakepit area seem to reflect a renewed influx of the same relatively primitive (and presumably hotter) magma that initiated the Serocki eruptions. Residual pockets of this hotter magma in the shallow crust may provide the heat that drives the present hydrothermal system.