Dredging Rock For Christmas
The busy days of OBS deployment and recovery are long passed. We recovered the last station on the 21st of December and all the 48 stations are now standing happily on the deck in the back of the vessel, waiting for their next deployment. The next few days are all in the spirit of rock samples. Dredging and gravity coring are the main activities over Christmas, although the heat flow probe and the Autonomous Underwater Vehicle (AUV Abyss) are also in action.
With every dredge it is a surprise if we are getting any rocks ‒ it is almost like opening Christmas presents
The first dredges have been done. They brought lots and lots of rocks to the surface. With every dredge it is a surprise ‒ are we getting any rocks, and if so, what are they going to be? It is almost like opening Christmas presents. We were expecting mafic rocks for the first dredges, basalts for example, that consist mainly of plagioclase and olivine minerals. Basalt is a typical rock from the oceanic crust. Instead we brought dacitic rocks back to the surface, which contain higher amounts of felsic minerals like plagioclase and quartz. These types of rocks are more characteristic for arc crust. Both types were expected, but to our surprise we also found some muddy sediments with burrow holes of some deep-sea animals together with other clastic sedimentary rocks such as volcanic conglomerates. It was a very interesting recovery!
A dredge brings rock to the surface. © Philipp Brandl
For the gravity coring a different approach is used as it is a method to recover the top few meters of sediments on top of oceanic crust and not just chunks of rocks. A long, metal pipe is pushed into the oceanic sediments by a weight that is pulled down by gravity, that’s where the name comes from, gravity coring. Unfortunately for the gravity coring team, we have not been very lucky yet. The pipe bounces off the ocean floor, because the top layer consists of hard, volcanic conglomerate. It is very difficult for the pipe to break through this layer into the softer layers below. A similar problem happened with the heat flow probe, but luckily we are not even halfway through our cruise and there will be more chances for gravity coring and heat flow measurements.
Christmas rock. © Fabian Hampel
In the meantime the crew members and scientists are preparing for Christmas. Work goes on at the vessel 24/7, but there will still be a celebration. In the dining room there is Christmas decoration, and the food will be even more delicious than it normally is. We even made a Christmas greeting. There is also an activity, a German Christmas tradition called ‘Wichteln’ which involves presents. Obviously I am very excited about this mysterious tradition, and even though rock presents from the dredge are a very good alternative, nothing beats a package, wrapped in paper under the Christmas tree.
This blog post was published as The Archimedes-I Cruise: full of surprises at the cruise's website on 26 December 2018.