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BLOG 6: How do you get rock samples from an underwater volcano?

Posted by: Ali Green

January 30, 2019

By Emily Fewster

Have you ever been fishing and got your fishing line snagged on something when reeling in your line.…but then managed to get it free? Well that is pretty much what a cracking team of scientists, engineers and mariners are doing to collect rocks from the steep flanks of seamounts in order to unlock the secrets of the sea floor and provide more details about the Australia – Antarctica break up some 34 million years ago.

Essential items needed for dredging seamounts are:

1. A PLAN: A decent map of the seamount and a planned dredge route based on targeting steep cliffs in locations that will allow the RV Investigator to keep its position facing into the wind and swell conditions at the time of the dredge. The Chief Scientist, the Ship’s Master and the Geospatial Mapping team all collaborate to develop the dredge plan. Steep cliff sides are prime positions for good representative rocks.

Colourful 3D image of sea floor indicating terrain levels
Dredge plan and actual dredge data for Nimbus Seamount

2. EQUIPMENT: A long cable, a winch and a very strong dredge chain mesh bag plus a dedicated team of engineers to deploy and retrieve the dredge (this is the equivalent of your fishing rod and fisherman).

large chain mesh bag laying on ship deck
Dredge with chain mesh bag
large spool of thick cable with woman leaning on spool
8000m of 24mm thick cable

 

 

 

 

 

 

 

Dredge time!

To commence the dredge the RV Investigator gets into position behind the planned dredge line and heads towards the dredge start point while engineers pay out the cable with the dredge connected off the back of the ship. Good timing is essential to get the right amount of cable, some 2-3km worth, out in time for hitting the target on the seamount flank. Once the dredge is at the target sight the field operations team, Chief Scientist, Ship’s Master and winch operator closely oversee progress of the dredge on screens that display the changing cable tension. Spikes in cable tension give an indication of if and when the dredge has bitten into the cliff side and has hopefully started collecting some rocks. Ideally, the science team want to see a number of tension spikes up to around 10 tonnes and then a release back to normal tension.

graph - green waves on black background
Monitors in the ship graph the tension in the dredge cable.

Everything going well, the dredge proceeds with a few nice tension spikes and then the ship continues moving forward and winding the cable in until the dredge is hauled up onto the back deck on the back deck and unloaded, much to the delight of the eagerly waiting scientists.

large mesh bag hanging from crane on back of ship with two men standing nearby in protective clothing
Dredge bag returning to deck full of rocks.
six scientists in hard hats look through a pile of rocks on the ship deck
Scientists look through their haul of rocks

 

 

 

 

 

 

 

 

 

 

 

 

 

If the tension is sustained at 11 tonnes then the dredge is stuck and cable needs to be paid back out and the ship may have to manoeuvre around to get it unstuck and moving again. Over 11 tonnes of tension is the built in breaking point of a shear pin holding the dredge facing upwards. The shear pin protects us from losing the dredge and cable by breaking and releasing the dredge mouth to face downwards, hopefully releasing it from the snag point.