Southern Kermadec submarine caldera arc volcanoes (SW Pacific): caldera formation by effusive and pyroclastic eruption

Three newly identified submarine caldera volcanoes (Brothers, Healy, and Ru mble II West), of the southern Kermadec are (within water depths < 1500 m), provide new insights into modes of submarine are caldera formation. Swath MR1 data, seafloor photography, and rock dredge sampling establish the edif ice and caldera morphology, outcrop volcanology, magma compositions, and pe trography and rock properties of erupted components for each volcano. These data reveal the Brothers and Rumble ZI West volcanoes have: (1) concentric , steep and 50-450 m high escarpments, that are interpreted as caldera wall ring-faults, encircling ca. 3-km wide calderas and associated central cone s, (2) heterogeneous flank-draping lava flows, and reworked pyroclastic dep osits, and (3) the predominance of incipiently to poorly vesiculated (mostl y < 40%), moderately dense (1.6-2.4 g cm(-3)) eruptive flows and clasts. Er upted lavas from Brothers and Rumble II West are predominantly dacitic (64- 67 wt.% SiO2), and basalt-basaltic-andesite (64-67 wt.% SiO2), respectively . Morphology of the Brothers and Rumble LI West volcanoes are interpreted ( although the former less confidently) as evidence of incremental caldera co llapse formed by prolonged, episodic, effusive magma withdrawal from summit and rift flank vents. In contrast, the Healy volcano has: (1) a flat ca. 2 .5-km wide ''caldera'' floor flanked by a 250-400 m high, and a relatively shallowly inclined "caldera'' wall, (2) the presence of a near ubiquitous m antle of relatively well sorted, blocks and lapilli over the edifice flanks and caldera, with rare seafloor outcrop, and no obvious evidence of lava f lows, (3) the predominance of rhyodacitic compositions (66-70 wt.% SiO2) co ntents, and (4) extremely vesiculated(> 80%) and low density (0.3-0.5 g cm( -3)) erupted pumiceous clasts. These observations are interpreted as eviden ce of subaqueously quenched, pyroclastic cm eruption(s), sheathed by ambien t water flashing to steam, discharging some 5 km(3) of pyroclastic material from Healy volcano. Caldera collapse is inferred to be penecontemporeous w ith the pyroclastic eruption(s), although it is possible formation as an ex plosive crater occurred, but we consider the latter unlikely given the volu me of erupted material and water depth. Extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas, and syn-eruptive caldera collapse, is possible within water depths of < 150 0-1000 m. In contrast, incremental, post-eruptive caldera collapse is more likely to occur with repetitive effusive eruptions of more mafic compositio ns, at both summit and rift flank sites, within water depths of > 500-1000 m. (C) 1999 Elsevier Science B.V. All rights reserved.