HR: 1330h
AN: V22C-0592    [PDF]
TI: Pyroclastic Fragmentation of Alkalic Lava in Abyssal Depths at the North Arch Volcanic Field, Hawaii
AU: * Davis, A S
EM: davisa@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AB: Pyroclastic submarine eruptions are generally considered to become less likely with increasing depth due to the increasing hydrostatic pressure of the overlying water column. Volcaniclastic deposits from the North Arch Volcanic Field, north of Oahu, have textural characteristics of explosive fragmentation. The presence of glass spheres and bubble-wall, spindle- and ribbon-shapes, or Pele's hair-like fragments, indicate at least mildly explosive eruptions occurred in water depths greater than 4200 m. Given the tectonic setting and young age of these deposits, there is no doubt that sampling depths closely corresponds with eruption depth. The most abundant volcaniclastic samples from the North Arch are clast-supported breccia of highly vesicular, angular clasts that most likely are near vent, pyroclastic deposits formed from eruption columns of limited height. Interbedded with highly vesicular pillow lava, they form steep-sided cones of low height (50 to 200 m) around the vent sites. Less common stratified samples with graded bedding, include one sample with a layer of imbricated, bubble-wall fragments that probably formed from density currents associated with the collapse of an eruption column. Numerous glass spheres, delicate spindle, ribbon and Pele's hair-like fragments as well as curved bubble-wall glass fragments resembling limu o' Pele occur in the finer size fraction of some samples. Another sample consists of glass fragments dispersed in a marine clay matrix that apparently was reworked and deposited farther from the vents by bottom currents. Glass compositions include low ($\sim$0.4%) and medium ($\sim$0.7%) $K_{2}O$ alkalic basalt, basanite, and nephelinite. Sulfur and chlorine are high reaching a maximum of 1800 and 1300 ppm, respectively. The ubiquitous presence of bubble-wall fragments regardless of glass composition suggests bursts of strombolian-like activity accompanied most eruptions. The coalescing vesicles observed in larger pyroclasts and some pillow lava suggest accumulation of gas, dominated by $CO_{2}$, was driving the eruptions. Initial explosive eruptions apparently were followed by effusive eruptions that formed thin, degassed sheet flows that traveled long distances over nearly flat terrain. If these volatile-rich magmas had erupted in shallow water depth or subaerially, tall fountains would likely have resulted. The great hydrostatic pressure ($>$400 Pa) limited fountain and eruption column heights.
DE: 1020 Composition of the crust
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting