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Volcano No. 19, Tonga

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Volcano 19 is a large stratovolcano located in the southernmost part of the Tonga arc. The volcano has a basal dimension of 14 x 12 km and rises from a water depth of 1400 m to 385 m. It has smooth flanks that rise to a complex summit peaking at 450 mbsl. The summit is dominated by an old caldera, an infilling cone, and a younger western caldera, all elongated in a NW–SE direction.

PISCES identified two large hydrothermal fields on Volcano 19. One is situated near the summit of the central cone between 385 and 540 m water depth. This is the biggest and hottest field which is known along the Tonga-Kermadec arc. The upper part of the central cone (an area of 800 x 800 m) is almost completely covered by Fe-oxyhydroxide crusts, formed from diffusely venting low temperature fluids. Chimneys composed of Feoxyhydroxides have formed where the venting fluids reach temperatures of 70°C. More focused, high temperature venting occurs along the narrow, NE-SW–trending ridge at the top of the cone complex. Here, clusters of large barite and anhydrite chimneys occur along a series of vertical dikes and faults. At slightly greater water depths (420–435 m) at the southern end of the ridge, large high-temperature chimneys show vigorous flow. Focused high-temperature venting also occurs at a water depth of 540 m in one of the shallow pit craters on the central cone. Here, small chimneys and low-relief mounds of barite and anhydrite protrude from the sediment in the pit. Clear two-phase venting occurs from the orifices of several chimneys.

A second, large area of low-temperature venting occurs among a swarm of dikes in the south wall of the western caldera. Here, Fe-oxyhydroxide crusts extend for more than 900 m along the base of the caldera wall and over a depth range of 985–850 m. Diffuse venting of warm fluids and mats of Fe-stained filamentous bacteria occur throughout this field. At the center of the field, large clusters of Fe-oxyhydroxide and silica chimneys cover an area of 200 x 300 m.

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10 entries listed. 10 valid minerals.

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References

Schwarz-Schampera, U., et al. (2007), Cruise Report SONNE 192/2, MANGO. Marine Geoscientific Research on Input and Output in the Tonga-Kermadec Subduction Zone.

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