With the cruise coming to an end, the scientists can reflect on the ocean floor rocks they’ve gotten! The excitement is palpable. No one has ever seen, touched or examined a rock from this part of the seafloor before and these samples will help to unravel the mysteries of how it came to be. You can read the daily science update to learn about the types of rocks they found. I’m going to talk about how they got them onboard.
These rocks were scraped off the ocean floor with a rock dredge. A rock dredge is a large bag made of metal chain links attached to a heavy steel frame that keeps the bag open. The scientists use the maps they’ve been making to find a good location to collect rocks. The dredge is lowered over the back of the ship on a long cable using a winch. Once the dredge reaches the seafloor, the ship moves forward at the same speed the cable is let out for several hundred meters. The ship stops and the cable is wound in to pull the dredge across the seafloor toward the ship. This is repeated for about a kilometer, then the dredge is lifted back onto the ship. The whole process takes about five hours.
Recovering a dredge
Sometimes we get a bucket-full of rocks and sometimes no rocks at all. It’s even possible that the dredge could get snagged. There’s a weak link built into the dredge system that won’t allow the dredge to pull too hard on the ship and act like an anchor. If the dredge gets snagged, the cable will break at the weak link to free the ship. There are extra dredges onboard in case one is lost this way.
Removing rocks from the dredge
When the rocks reach the deck, they’re sampled for biology before anyone touches them with bare hands. Next, they’re brought to the main lab to be labeled, photographed and measured. Larger rocks are cut open with a tile saw before they’re described and categorized. Once all rocks are described in detail, they are packaged for shipment home. Ellen is our curator and she keeps track of all the rocks. Some rocks will go home directly to different scientists’ labs, but others will be stored at WHOI and any scientist can request them for research.
There you have it – dredging dynamics!
Sally
