Citation

Hekinian R, Muhe R, Worthington TJ, Stoffers P. J. Volcanol. Geoth. Res. 2008; 176(4): 571-582.

Copyright

(Copyright © 2008, Elsevier Publishing)

DOI

10.1016/j.jvolgeores.2008.05.007

PMID

unavailable

Abstract

A submersible dive conducted on Volcano #1 located near 21 09S-175 45W on the Tonga Arc showed that the volcanic edifice with a caldera floor area of 30km2 located at and 450m deep (b.s.l.=below sea level) was constructed recently during episodic volcanism. The sequential volcanic events are recorded along a faulted terrain formed in response to the collapse of the caldera wall. The post-caldera events are marked by occasional eruptions that have built scoriaceous cones associated with low-temperature hydrothermal venting and localized small-scale collapse features. The stratigraphy of the caldera wall indicates that the volcano was built by explosive volcanism alternating with quieter eruptive events. The repeated, violent explosive events formed &le 20m thick sequences composed of alternating fine-grained ash beds and sand- to boulder-sized pyroclastic layers. During quieter volcanic events, dykes and massive flows intruded and/or accompanied the eruption of the volcaniclastic deposits throughout the sections of the wall explored. Massive columnar-jointed flows consist of viscous, silica-rich lavas forming tabular and giant radial-jointed (GRJ) flows formed in large ( 8m in diameter) conduits and extruded onto the sea floor. In addition, massive lava flows forming sill-like complexes were observed underneath and near the giant radial-jointed columnar flows. Also, an intermittent quiet type of eruption produced vesicular lava flows, which are interbedded within the pyroclastic layered deposits. The massive and vesicular lavas consist of andesites and dacites with Ca-depleted (pigeonite) and Ca-enriched (salite) pyroxene, and intermediate (andesine-labradorite) to calcic (bytownite) plagioclase. They are depleted in total alkalis (Na2O + K2O 3%), K2O ( 1%), Zr/Y ( 1.8), Nb/Zr ( 0.01) and light Rare Earth Elements. We interpret that these andesite-dacite series were erupted after undergoing crystal-liquid fractionation in a magma chamber located underneath the caldera floor. 2008 Elsevier B.V. All rights reserved.