COMPARATIVE VOLCANOLOGY OF SMALL AXIAL ERUPTIVE CENTERS IN THE MARK AREA

We describe selected examples of small volcanic features identified du ring Ocean Drilling Program (ODP) site surveys in the MARK area for wh ich additional observations and sample data have been obtained by mann ed submersible and/or drilling. Based on both morphologic relations an d similarities in chemistry and petrography, Serocki Volcano, the site of bare rock drilling on ODP legs 106 and 109, appears to be a flat-t opped megatumulus, a large rootless vent developed on a lava flow orig inating from an adjacent cone located above the true eruptive fissure. A linear set of five apparent volcanic centers on the east side of th e median valley (the EastMARK cones) differ from one another in morpho logy and degree of tectonic degradation. They formed at different time s and from different vents, but all were localized by damming of die f lows against the steep walls of the linear valley containing them. The two northernmost features are probably megatumuli at the terminal end s of lava flows descending the valley walls. The other three vents are cones with breached craters and probably were more recently erupted i n 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, relativ ely young fissure eruption. The associated lavas are among the geochem ically most primitive in the MARK area but have their counterparts in several subsets of lavas associated with Serocki Volcano in spite of t he morphologic contrasts and large geographic separation between the t wo features. We conclude that a volcano like form, geographic proximit y, and conformance to a linear geometry do not guarantee compositional or temporal coherence, nor is the alignment of apparent volcanic cent ers a reliable guide to underlying tectonic patterns. The chemistry an d petrography of the Serocki and EastMARK basalts resemble older adjac ent lavas and also conform to subtle along-ridge variations. This and the relatively old appearance of their flow surfaces indicate that the se lavas represent the waning stage of an older magmatic event. The mo re recent eruptions in the Snakepit area seem to reflect a renewed inf lux of the same relatively primitive (and presumably hotter) magma tha t initiated the Serocki eruptions. Residual pockets of this hotter mag ma in the shallow crust may provide the heat that drives the present h ydrothermal system.