Eruption Rate Control On Morphology And Structure Of Submarine Monogenetic Volcanoes - Insights From Sumersible Dives Off Maui And Hawaii Islands

This paper presents the results of submersible dives off Hawaii Islands during four research cruises (R/V Kairei-ROV Kaiko 1998 and 2001, R/V Yokosuka-DSV Shinkai 1999 and 2002) by the Japan Marine Science and Technology Center. Morphologies and structures of submarine volcanic edifices were observed during dives on the Hana Ridge, submarine extension of Haleakala rift zone of East Maui and the south rift zone of Loihi Seamount. Three types of volcanic edifices are recognized: 1) Conical seamount with a small summit and steep slopes [Clague et al., 2000]; 2) Flat-topped cone [Clague et al., 2000] or terrace [Smith et al., 2002] with a wide, flat summit compared to the relative height; 3) Lobate terrace with a hemi-circular lobate outline with a smaller areal ratio of the summit to the base than flat-topped cones. These volcanic edifices are morphologically classified into two groups: Group A (flat-topped cones and lobate terraces) shows a positive correlation of an areal ratio of summit/base with slope. Group B (conical seamounts) has a low areal ratio of summit/base and a positive correlation between a volume and slope. ROV Kaiko and DSV Shinkai dives were carried out on 6 conical seamounts, 5 flat-topped cones and 3 lobate terraces. Conical seamounts with moderate slopes have aprons of elongate pillows and a summit crater filled by lobate sheet flows, which gradually change into elongate pahoehoe flows downslope. Steep cones are surrounded by talus slopes of pillow rubble with elongate pillows sporadically exposed between pillow clasts. Embedded pillow lobes are all elongate downslope, indicating that fragmentation took place while pillows were flowing. Under a very low extrusion rate, flowing lava congealed on the steep slope in a short distance from the source and collapsed into rubble that formed the lower part of the cone. Flat-topped cones have elongate pillows on moderate slopes, and pahoehoe and lobate sheet flows interbedded with pillows on gentle slopes. Flat summits are overlain by lobate sheets but do not have any collapse pits. Successive change in flow morphology from lobate sheets on the summit through pahoehoe sheets into elongate pillows downslope indicates that lava quietly spilt over the crater rim flowed as elongate pillows on steep slopes. Because lobate sheets are present interbedded with pillows in the lower slope, several eruptive episodes with higher extrusion rates of lava were repeated to form the flat-topped cones. Lobate terraces have similar constituents as flat-topped cones but collapse pits on the summit. Lava channels are present both on the bottom of the pits and on the aprons of the terraces, through which lava was rapidly expelled. Lobate terraces are more like a single large inflated sheet flow grown on a gentle ridge that broke out channeled lava at the final stage of growth.