Using submarine lava pillars to record mid-ocean ridge eruption dynamics

Submarine lava pillars are hollow, glass-lined, basaltic cylinders that occ ur at the axis of the mid-ocean ridge, and within the summit calderas of so me seamounts. Typically, pillars are similar to 1-20 m tall and 0.25-2.0 m in diameter, with subhorizontal to horizontal glassy selvages on their exte rior walls. Lava pillars form gradually during a single eruption, and are c omposed of lava emplaced at the eruption onset as well as the last lava rem aining after the lava pond has drained. On the deep sea floor, the surface of a basaltic lava flow quenches to glass within I s, thereby preserving in formation about eruption dynamics, as well as chemical and physical propert ies of lava within a single eruption. Investigation of different lava pilla rs collected from a single eruption allows us to distinguish surficial lava -pond or lava-lake geochemical processes from those operating in the magma chamber. Morphologic, major-element, petrographic and helium analyses were performed on portions of three lava pillars formed during the April 1991 er uption near 9 degrees 50'N at the axis of the East Pacific Rise. Modeling r esults indicate that the collected portions of pillars formed in similar to 2-5 h, suggesting a total eruption duration of similar to 8-20 h. These va lues are consistent with observed homogeneity in the glass helium concentra tions and helium diffusion rates. Major-element compositions of most pillar glasses are homogeneous and identical to the 1991 flow, but slight chemica l variations measured in the outermost portions of some pillars may reflect post-eruptive processes rather than those occurring in subaxial magma bodi es. Because lava pillars are common at mid-ocean ridges (MORs), the concept s and techniques we present here may have important application to the stud y of MOR eruptions, thereby providing a basis for quantitative comparisons of volcanic eruptions in geographically and tectonically diverse settings. More research is needed to thoroughly test the hypotheses presented here. ( C) 2000 Published by Elsevier Science B,V. All rights reserved.