Detailed survey of the hydrothermally active brothers volcano by an autonomous underwater vehicle
|Location||International Geological Congress,oslo 2008|
|Author||De Ronde, Cornel۱; Embley, Robert۲; Baker, Edward۲; Yoerger, Dana۳; Davy, Bryan۱; Resing, Joseph۲; Dziak, Robert۲; Merle, Susan۲; Walker, Sharon۲; Nakamura, Ko-ichi۴|
|Holding Date||23 September 2008|
Brothers volcano forms an elongate edifice 13 km long by 8 km across that strikes NW-SE, as part of the active Kermadec arc, northeast of New Zealand. The volcano has a caldera with a basal diameter of ~3 km and a floor at 1,850 m below sea level, surrounded by 290 to 530 m high walls. A volcanic cone of dacite rises 350 m from the caldera floor and partially coalesces with the southern caldera wall. Up until July 2007, three hydrothermal sites had been located on Brothers; on the NW caldera wall, on the SE caldera wall, and on the dacite cone (itself host to 3 separate vent fields: summit, upper flank, NE satellite cone).
Combined, multiple hydrothermal plumes rise ~750 m through the water column upwards from the caldera floor. The NW caldera vent site is a long-term hydrothermal system that is today dominated by evolved seawater but has had episodic injections of magmatic fluid with radiogenic dating of sulfide chimneys and other mineralization indicating ages up to 1,200 years. The SE caldera site represents the main upflow of a relatively well-established magmatic-hydrothermal system on the seafloor where sulfide-rich chimneys are extant. The cone site is a nascent magmatic-hydrothermal system where crack zones localize upwelling acidic waters. Each represents diverse parts of an evolving hydrothermal system, any one of which may be typical of submarine volcanic arcs.
In July-August, 2007, the autonomous underwater vehicle ABE (Autonomous Benthic Explorer) was deployed 8 times at Brothers volcano, providing 161 km of survey track over a total survey time of 96 hours. One of the main aims of deploying ABE at Brothers was to map in high resolution (~1 m) the bathymetry of the caldera, including the walls and dacite cone, by consistently flying ~50 m above the seafloor. In addition, ABE was fitted with a CTD and an optical backscatter sensor to detect particulate-rich (black smoker) plumes. Oxidation/reduction potential (Eh) and pH sensors also enabled hydrothermal venting to be detected, while a magnetometer measured the magnetic signal of the underlying volcanic rocks. Various navigation systems meant ABE had the ability to follow tracklines with a repeatability of 10 m line spacing, or better. A new bathymetric map (2-m grid) shows the main dacite cone at Brothers to be onlapping with an older, more eroded cone to the NE. The caldera walls are highly incised with steep gullies, particularly in the vicinity of the NW caldera vent site. The various sensors showed a high density of venting over the NW caldera and cone sites and in addition, discovered a new vent field in the western part of the caldera. Magnetic anomaly measurements confirmed the location of the active hydrothermal vents fields and also the inactive SE caldera field. Detailed surveying of Brothers volcano by ABE has provided the missing link between surface ship surveys and those conducted by manned submersibles and/or remotely operated vehicles.