3-D image of Brothers volcano looking to the NW. The cone at Brothers is prominent in the southern part of the caldera and rises about 350 meters (1150 feet) above the caldera floor to a depth of about 1,100 meters (3600 feet) below sea surface. An extensive hydrothermal field occurs atop this cone and also on the summit of the smaller cone on its flanks. A bigger hydrothermal vent field again is perched on the NW caldera walls. Click image for larger view and image credit.
Here We Are Again Over “Brothers Volcano”
July 31, 2007
Cornel E.J. de Ronde
Geologist
GNS Science
Lower Hutt, New Zealand
Here we are again over Brothers volcano with the R/V SONNE. It feels like I have gone full circle with expeditions to Brothers, as it was aboard the SONNE in late 1998 that I first had anything to do with this particular volcano. It was then that we discovered for the first time, that Brothers was host to some active hydrothermal venting on the seafloor. We imaged (by towed cameras and video) tall chimneys perched on the steep walls of the NW caldera site at Brothers, and several of our instruments attached to the camera array confirmed that indeed, hot, metal-rich fluids were being expelled inside the crater (caldera) at Brothers. That was an exciting time, and was followed by three NZAPLUME cruises in 1999, 2002 and 2004 that re-visited Brothers and mapped and sampled in detail the “black smoke”, or hydrothermal plumes that emanated from numerous active chimneys on the volcano.
In late 2004 we dove four times on the vent sites at Brothers with the Japanese manned submersible Shinkai 6500, followed in 2005 by five dives with the American submersible Pisces V. These dives provided us with a wealth of information about the location of the vent sites, samples of the metal-rich chimneys themselves and the hot water (and gases) that were being expelled by the chimneys, and the discovery of new species of vent-related animals. Combining the information gathered from the dive expeditions together with that from the NZAPLUME and previous SONNE cruises, meant that we are able to build up a picture as to how the hydrothermal systems at Brothers may have formed, thus enabling us to understand how these submarine volcanoes evolve.
This is a illustration of a section through Brothers volcano showing the evolution of the hydrothermal systems there. Initially, the NW caldera site was formed, followed by the emplacement of the volcanic cone in the southern part of the caldera where a second hydrothermal field was formed, which has a lot of gases originating from the magma below eventually finding their way to the seafloor and being expelled. Click image for larger view
A good example of one of the many active hydrothermal chimneys (commonly known as “black smokers”) that occurs at the NW caldera site. It is the detection of smoke emanating from chimneys such as this that enabled us to locate numerous hydrothermal vent fields along the Kermadec arc during the NZAPLUME cruises. Click image for larger view
But for all the work we have done at Brothers volcano, and we have done more at this volcano than any other along the Kermadec arc, there is so much still that we don’t know. For example, while we have bathymetric data that allows us to picture the volcano at a large scale, the resolution of that data is about 25 meters (~80 feet) which means we don’t know any of the details of the seafloor at Brothers except what we saw outside the portholes of the submersibles. So it is a bit like going from looking down at Brothers volcano from a small plane 2 km up in the air, to then looking outside your car window at the rocks nearby. That is why this time around we are deploying the autonomous underwater vehicle known as ABE (Autonomous Benthic Explorer), from the Woods Hole Oceanographic Institute. ABE can literally be programmed to ‘fly’ over the seafloor, usually about 50 m above bottom - without being tethered to the ship - where it can map out features with a resolution of about 1 meter (3.28 feet)! That means ABE will ‘see’ large chimneys (some of which are 7 meters tall) and could map out a small car. Effectively, this greatly enhances the detail of our maps of the caldera at Brothers – a bit like looking at something through the thick glass of a Coke bottle to removing the bottle and seeing things clearly for the first time. ABE will also be bristling with a number of other sensors, such as a magnetometer, which will tell us something about the areas within the caldera where hot water reached the seafloor from below, and where older (now dead) chimneys might occur. Other sensors will measure and locate the numerous hydrothermal plumes inside the caldera so that in the end we will have a wonderful 3-D virtual model of the volcano that will show exactly where all the chimneys and hot springs are, all draped over a very detailed map. Finally, if we are lucky, will we use the Kiel 6000 ROV (remotely operated vehicle) onboard to dive on any new or different sites at Brothers that ABE might discover to video and sample chimneys and hot water. Exciting times ahead!
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