Wednesday, October 12, 2016

TAN1611 - Post 6

Dredging

We are now nearing the end of the voyage with just 3 days to go, one of those in transit back to Auckland.  At this point we have hang up the geophysical equipment and, for the most part, turned off the multi-beam system to focus on dredging for seafloor rock samples.  This is final scientific objective for this voyage and is crucial for ‘ground truthing’ the geophysical data.

Pic of rock dredge emptied on the deck

The rock dredge is essentially a long, narrow steel basket with some teeth at the front much like those you might see on the bucket of an excavator.  The teeth dig into and break apart seafloor rocks, and with any luck a decent portion of these freshly broken rocks will be funnelled into the basket and stay there while the dredge is retrieved!  Periodically new teeth must be welded onto the dredges because, as one might expect when towing a large piece of metal behind a ship and across rocks, they have a tendency to snap off.  The ships engineers are a godsend when a bit of crafty welding is required.

The general process for dredging is to use the already collected geophysical and seafloor topography data to identify key areas of interest - something like a large volcano might be of interest if it sticks out from the surrounding terrain.  On this trip our main focus has been the Colville Ridge feature so this is where dredging will be focussed.  Upon reaching a site picked for dredging the ship slows down to 1 knot while the dredge is sent to the seafloor, a process that takes about an hour per thousand metres. While the ship creeps forward at a lazy one knot (one knot/h is equal to 1.85 km/h) the dredge (hopefully) collects the all important rock samples.  We give the dredge maximal 15 minutes at the seafloor and then winch it back up to the ship where we gleefully sort, cut, bag, and tag our geological treasures.

This might sound like a laborious task, and it certainly is - this is the most action the ship will see during this three week voyage, it is however key to a good understanding of what is hiding several thousand metres below us.  As I talked about previously, the geophysical and multi-beam data allow us to say something about the rocks making up this patch of seafloor, but there is no substitute for the hard evidence provided by actual rock samples.  With rock samples we can make accurate and extremely detailed observations of the rocks chemical composition, age, and hydrothermal alteration (think Rotorua geyser areas).  This information is used in piecing together a first approximation of the geological history for the region.

We will be dredging for about 50 hours after which everyone will likely go for a well-deserved kip while the ship begins its 30 odd hour journey back to port in Auckland. 

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