JC007: Drilling the Mid-Atlantic Ridge
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About the cruise...
Drilling the Mid-Atlantic Ridge
RRS James Cook cruise JC007, 5 March 2007 – 17 April 2007
Mid-ocean ridges are a fascinating component of our planet's armour plating. Mid-ocean ridges are the place where new oceanic crust is born, with red-hot lava spewing out along the spreading axis as seafloor spreading progresses. However, the mechanisms by which this occurs are still not well understood by scientists - hardly surprising when you consider that mid-ocean ridges are located thousands of metres below the surface of the ocean.
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Scientists have discovered a large area thousands of square kilometres in extent in the middle of the Atlantic Ocean where the Earth's crust seems to be missing entirely. Instead, the mantle - the deep interior of the Earth, normally covered by crust many kilometres thick - is exposed on the seafloor, 3000m below the surface. It has been described as being like an open wound on the surface of the Earth. What scientists don't know is whether the ocean crust was first developed, and then ripped away by huge geological faults, or whether it never even developed in the first place. In March-April 2007, a team of scientists from Durham University, Cardiff University and NOCS will board the RRS James Cook to visit this special area of the Mid-Atlantic Ridge, which is called the Fifteen-Twenty Fracture Zone (FTFZ for short - the map on the left shows where this is located). The scientific team on board the ship is led by Prof. Roger Searle from University of Durham, Dr Chris MacLeod (University of Cardiff) and Dr Bramley Murton (NOCS). [more on the science behind this cruise...]
Left: Image of the Mid-Atlantic Ridge. You can see how the ridge is broken up into segments by fractures running roughly perpendicular to the ridge axis. The red dot shows the area where the team on board the ship will be working. Bathymetric image courtesy GEBCO. |
EquipmentThe first step on the expedition is to use TOBI (Towed Ocean Bottom Instrument - right) to build up images of the seafloor surface so that suitable drilling sites can be selected. TOBI images will enable the scientists to identify areas where peridotite is exposed at the seafloor, and give them information on the shape of the ridge, the distribution of faults and identify any areas where magma is erupting onto the seafloor. |
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When the team have selected suitable sites for drilling, they will use a robotic rock drill to take samples of the seabed. The drill is mounted on a metal tripod and is lowered onto the seafloor by a special cable. A camera on the tripod allows the scientsts to see exactly where they are placing the drill. The rotating drill bit is diamond-tipped to ensure that it's hard enough to cut through the rock, and produces cylindrical 'rods' of rock (cores, see below). An important feature of this particular drill is that the cores are specially marked to show which way is north so that the scientists know how they are oriented - very important when you're taking lots of samples across an area and you want to know how certain properties vary across space. By taking a series of these cores across the area and analysing them, the scientists will be able to determine how the seafloor spreading process varies across the region, and how the mantle came to be so close to the surface in this area.
[more about rock drilling]
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Above, from left: the BRIDGE drill being recovered after sampling the seabed; an image from the tripod camera, showing the rig sitting on bare rock, covered in the foreground by a dusting of white sediment. Note the leg of the drill rig, and a sea urchin for scale in the centre of the picture. The dark stripes are mineral-filled cracks in the seafloor; Core recovered from the BRIDGE rock drill. The rock is a gabbro – equivalent to the lavas erupted onto the seabed, but which instead crystallised slowly at a depth of a few kilometres below the seafloor.
The team may also carry out dredging - a primitive but still effective way of sampling the seafloor. Methods and equipment have changed little since the pioneering expeditions of HMS Challenger in 1872. A chain-link bag with large metal-jawed opening is lowered to the seabed on a cable, dragged along the bottom for some distance, and then brought to the surface. Although rough and ready, dredging is still a useful way of mapping the broad-scale distribution of rock types on the seafloor. Dredging will only be conducted during the cruise as a backup or in between deployments of the principal tools: the sidescan sonar and seabed rock drill.
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Above left: A rock dredge being deployed from a research ship.
Above right: Chris MacLeod plus full rock dredge. Note how the originally rectangular
mouth to the dredge has been bowed open as the rocks were forced in to the chain bag.
The RRS James Cook will set sail from port in Tenerife on 5 March 2007. This is the RRS James Cook's first scientfic mission....keep up to date with all the latest cruise news and developments by visiting the JC007 cruise dairy from 5 March.
