JC007: Drilling the Mid-Atlantic Ridge


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 suprising when you consider that mid-ocean ridges are located thousands of metres below the surface of the ocean.

We know that mid-ocean ridges form when the earth's plates are pulled apart as part of the plate tectonic process. However, the rate of spreading is not the same all along the spreading ridge, causing the ridge to split into segments - this creates the jagged offset pattern you can see on maps of the seafloor, like the one shown on the left. Some parts of mid-ocean ridges are spreading relatively quickly, others are spreading very slowly. This difference in spreading rates results in sections of the mid-ocean ridge with very different physical characteristics.

Within each ridge segment, particularly where the overall spreading rate is slow, the rate at which molten magma rises to the surface can vary. Scientists have assumed for a long time that the centre of the segment is the place where the magma rises most quickly and intensely, meaning that the centre of the segment is therefore hotter the edges of the ridge segment. The ocean crust in the centre of the segments is often also stretched more thinly, contributing to the higher temperatures in this area. In contrast, the edges of the ridge segments are thicker and cooler.

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.

This variation in temperature affects the way in which the ridge segment deforms in response to pressure - hot areas are more "squidgy" and can deform more easily than the edges of segments which are colder and more brittle. This means that at the edges of segments, the spreading motion is accommodated by cracking or tectonic faulting of the crust, instead of a smooth spreading mechanism dominated by the injection of magma into the central ridge axis.

This section of the Mid-Atlantic Ridge is special because it is the best-known example of a 'magma-starved' ridge - in other words, an area where spreading takes place by tectonic (cracking) processes instead of by magma injection. The ocean crust in this area is exceptionally thin, meaning that a rock called peridotite is exposed in the central valley of the ridge. Usually, this rock is buried beneath kilometres of ocean crust, so by taking samples of this rock the scientific team can gather important information about the structure of the ridge in this area and the mechanisms by which seafloor spreading takes place. A key question to answer is whether the oceanic crust has been removed by tectonic processes, or whether it was never there in the first place.

in addition to going to sea to study how the oceanic basement is formed, geologists also study on-land fragments of ocean lithosphere, known as ophiolites. Bram and Chris have extensive experience of working on such rocks, particularly the Troodos ophiolite of Cyprus. The Lizard in Cornwall (UK) is another example of an ophiolite, where plate tectonic processes have resulted in ancient oceanic crust being exposed on land.

Above: Chris on fieldwork in the ophiolite of the Sultanate of Oman, actually standing on rocks
which once formed part of the mantle. The crust-mantle boundary - the 'Moho' - lies below the
mountain top on the extreme right of the picture.

More on...

JC007 cruise diary (from 5 March 2007)
Other Classroom@Sea cruises
Plate tectonics and seafloor spreading
Seafloor surveying
National Oceanography Centre, Southampton
Dept. of Earth Sciences, University of Durham
School of Ocean, Earth and Planetary Sciences, University of Cardiff

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February 2007