Vertical and lateral splitting of a hydrothermal plume at Steinaholl, Reykjanes Ridge, Iceland

The generation of multiple, neutrally-buoyant intrusions by a single, bubbl e-rich plume [Asaeda and Imberger, J. Fluid Mech. 249 (1993) 35-57] and plu me bifurcation [Ernst et al., Bull. Volcanol. 56 (1994) 159-169; Lavelle, J . Geophys. Res. 102 (1997) 3405-3420], are predicted from theory and experi ments but have yet to be documented for hydrothermal plumes. In contrast, b ifurcation of volcanic plumes (which are dynamically analogous to hydrother mal plumes) is very common [Ernst el al., Bull. Volcanol. 56 (1994) 159-169 ] and thus bifurcation of hydrothermal plumes should be expected [Lavelle, J. Geophys. Res. 102(1997) 3405-3420; Ernst et al., BRIDGE Newslett. 10 (19 96) 76-77]. Recent 38 kHz echo-sounder and water-column chemical tracer stu dies near the Steinaholl vent site (Reykjanes Ridge, Iceland), identified a bubble-rich hydrothermal plume intruding at three levels (two main ones an d a subsidiary one) before reaching the sea surface and spreading there [Er nst et al., BRIDGE Newslett. 10 (1996) 76-77; Olafsson et al., RIDGE Events 2 (1991) 35-38; German et al., Earth Planet. Sci. Lett. 121 (1994) 647-654 ]. The two main intrusions (ca. 100 and 200 m above the vent) show lateral development of two lobes away from the vent and are consistent with the 350 m rise of a bubble plume yielding seafloor and surface gas fluxes of simil ar to 2.5x10(-3) and 0.75x10(-1) m(3)/s, respectively. The bubble-rich core of the hydrothermal plume also penetrates the 150 m deep thermocline and g enerates an intrusion visible at the sea surface. Although bifurcation of t he bent-over plume could have been initiated by crossflow alone [Ernst et a l., Bull. Volcanol. 56 (1994) 159-169], several other processes may also ha ve enhanced plume bifurcation; including interaction with the thermocline a nd entrainment of the plume lobes by horseshoe eddies in the lee of a 150 m high hill on the seafloor. (C) 2000 Elsevier Science B.V. All rights reser ved.