Abstract The submarine Healy volcano (southern Ker- madec arc), with a 2–2.5 km wide caldera, is pervasively mantled with highly vesicular silicic pumice within a wa- ter depth of 1,150–1,800 m. Pumices comprise type 1 white–light grey pumice with ≤30 mm vesicles and weak- moderate foliation, type 2 grey pumice with millimetre- scale laminae, flow banded foliation, including stretched vesicles ≤55 mm in length, and a minor finely vesicular type 3 pumice. All types are sparsely porphyritic, with undevitrified glassy groundmass (68–70% SiO 2 ), which is microlite and lithic free. Coexisting pyroxenes yield magma temperatures of ~950 °C. Pumice density is ≤0.5 g cm –3 and vesicularity is 78–83%. Vesicle size dis- tributions for types 1 and 2 pumice, range from ~20 μm to >20 mm, with a strong power-law relation (with d=–2.5±0.4) for vesicles <1–2 mm. Larger vesicles have variable size modes. The vesicle size distribution and packing indicates rapid magma decompression and as- cent. Consideration of the pressure dependent, solubility of H 2 O at a magma temperature of ≤950 °C and water content of ≤6 wt%, with pumice petrography and vesicle granulometry, strongly suggests a pyroclastic eruption. Reconstructions of the submarine edifice between water depths of 1,000 and 550 m constrain the ambient hydro- static pressure to ~6–9 MPa. Pressures >~9 MPa will lim- it vesicularity to less than the observed 78–83%, whereas pressure <~6 MPa require a more shallower reconstruc- tion of the edifice and larger-volume syn-eruptive col- lapse. Uniformly high vesicularity is interpreted as evi- dence of insulation within an eruption column comprising steam and hot pyroclasts. Most pyroclasts cool, condens- ing and ingesting water into steam-inflated vesicles, and then sink. Progression into pyroclastic mode would ex- pand the eruption column, displace ambient water, reduce the hydrostatic load, and further promote vesiculation and fragmentation. Pyroclasts within the column would quench at these reduced pressures. We argue that Healy eruptions deeper than ~1,000 m cannot be pyroclastic. Volumes for the lower and upper bounds of edifice size are 2.36 and 3.58 km 3 , respectively, but do not account for intra-caldera pumice fill. These volumes are consid- ered to be predominantly primary eruption output, as shown by a dearth of accessory lithics in all pumice, yielding (at an average 81% vesicularity) eruptive pumice volumes of between 10 and 15 km 3 . Some pyroclasts may have risen to the sea surface and be a correlative of the sea-rafted Loisels pumice; the latter occurs in some New Zealand Holocene beach sequences and has a estimated age of 590±80 calendar years. Keywords Healy caldera · Kermadec arc · Pyroclastic eruption · Silicic volcanism · Submarine eruption Introduction Submarine pyroclastic volcanism remains an intractable phenomenon, although it is known that water depth, mag- nitude and rate of magma vesiculation are the predomi- nant limiting constraints (e.g. McBirney 1963, 1971; Sparks 1978; Sheridan and Wohletz 1983; Kokelaar 1986; Wohletz 1986). Initially such pyroclastic eruptions were considered to be mostly restricted to water depths <500 m for a range of magma compositions and volatile contents (e.g. Fisher 1984; Stix 1991), although theoretically deep- er submarine pyroclastic eruptions could occur (e.g. Burn- ham 1983). Although the palaeo-eruptive water depth can be equivocal, extensive pumiceous deposits within older exhumed marine sequences have been interpreted either as Editorial responsibility: J. Gilbert I.C. Wright ( ✉ ) National Institute of Water and Atmospheric Research (NIWA), P.O. Box 14-901, Wellington, New Zealand e-mail: i.wright@niwa.cri.nz Tel.: +64-4-3860300, Fax: +64-4-3862153 J.A. Gamble School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand P.A.R. Shane Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand Bull Volcanol (2003) 65:15–29 DOI 10.1007/s00445-002-0234-1 RESEARCH ARTICLE Ian C. Wright · John A. Gamble · Phil A. R. Shane Submarine silicic volcanism of the Healy caldera, southern Kermadec arc (SW Pacific): I – volcanology and eruption mechanisms Received: 13 September 2001 / Accepted: 30 May 2002 / Published online: 27 July 2002 © Springer-Verlag 2002