The Brothers volcano is a Pacific Ocean submarine volcano in the Kermadec Arc, 340 kilometres north east of New Zealand’s Whakaari/White Island. Within its oval outline, which measures 13 km by 8 km, it contains a 3 km wide caldera with walls 300-500 m high. A dacite dome rises 350 m from the caldera floor (which lies 1850 m below sea level), with a smaller dome just to its northeast. The caldera walls and the larger dome host numerous hydrothermal vents, which send plumes of hot water 750 m up through the water column. It is the most hydrothermally active volcano known in the Kermadec Arc.
The northwest caldera vent site is a long-lived hydrothermal system that is today dominated by evolved sea-water but has had episodic injections of magmatic fluid. The southeast caldera site represents the main upflow of a relatively well established magmatic-hydrothermal system on the sea floor where sulfide-rich chimneys are extant. The cone site is a nascent magmatic-hydrothermal system where crack zones localize upwelling acidic waters. Each of these different vent sites represents diverse parts of an evolving hydrothermal system, any one of which may be typical of submarine volcanic arcs.
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