Day 20: Saturday 24 March
Sampling area: TOBI survey
Far below the floor of the Atlantic Ocean, deep within the Earth’s crust, a spark of pale-blue light flickers into life, then fades slowly away. Then another. And another. Until they form a glimmering constellation. Each minute flash, the result of individual crystals that are straining and cracking under enormous stress. A swarm of small earthquakes follows and are the first harbingers of the cataclysmic events that will conclude with the exposure of the Earth’s mantle on the abyssal ocean floor.
Still hot, after its ascent from hundreds of kilometres deep within the interior of the planet, the mantle rock that is now cracking and breaking has recently been depressurised, squeezed and melted. In its ductile state, it cannot support the stresses required to cause earthquakes. But now it is nearer the surface and has cooled down, it has become brittle. The stresses imposed on this rock are the result of the huge tectonic plates of Africa and America moving away from one another at the Mid-Atlantic Ridge. Although slow, only a few centimetres a year, this divergence builds up enormous stresses, resulting in the snapping and cracking of the crust and upwelling of the deep, hot mantle.
On a normal mid-ocean ridge, this upwelling causes the mantle to melt and form new oceanic crust. But here, between 15.5°N and 13°N, the melt is in short supply. As a result, the gap caused by the plate divergence is not being filled by molten rock. Instead, the mantle itself must fulfil this role. Over the next few thousand years, earthquakes will split the oceanic crust right through. The resulting crack will then begin to slip faster and faster, eventually taking up the divergent plate motion. Beneath the ocean, as the crack breaks through the crust, seawater penetrates downwards, finally infiltrating into the mantle. Unlike the crust, which is made of volcanic rock, the mantle is predominately formed from crystals of olivine. In its dry state, olivine forms extremely strong rock (called peridotite), much stronger than the crust. But when seawater reacts with it, a new mineral is formed. This is serpentine, an extremely weak material that slips and yields easily. It is along this serpentinised slip-plane that the mantle now begins to emerge.
What causes the mantle to yield less melt than usual, and how the overlying crust can be torn away, remains a mystery. Is it because the mantle is cold and unable to melt as much, or is it ‘infertile’, having already partially melted many eons ago? Does the mantle become deeply serpentinised, and is that in part responsible for its exhumation as the crust tears away? What secrets of its history, when buried deep within the Earth, does the exposed mantle rock yield?
These are the questions that we hope to answer during our voyage and after, having spent long hours in the laboratory analysing our samples. The images we are getting from our unique sonar, black and white pictures of the ocean floor that resemble those from Venus or the Moon, will tell us much about how the crust is torn away and the mantle exhumed. The chemistry of the rocks, both the peridotites and the lavas, will also tell us about the deformation and melting history of the mantle. From this evidence we hope to unravel the mystery of this rent in the Earth’s crust and thereby get closer to understanding the workings of our planet....