Monday, 22 July 2013

Heading for the Mid-Atlantic Ridge

Monday July 22nd, 12pm, 49N, 25W

At noon every day, the reassuring tones of the Commodore echo across the ship as he updates us on the progress of our voyage to the United States.  Queen Mary 2 lets out two deep toned whistles, one from the original Queen Mary (now moored in Longbeach, California) and another, newer one manufactured by the same company.  The ships bell is struck eight times (‘8 bells’) at noon in the Grand Lobby.  As I glance up from my laptop, there is a school of dolphins leaping from the waves off our bow.

At noon yesterday we were at 49.9N, 10.5W, about 100 miles to the south of Mizzen Head on the Republic of Ireland.  We were in relatively shallow waters, just 110m beneath the keel as we sailed over the Eurasian continental shelf.  Today, 24 hours later we have left the Celtic sea for the Atlantic Ocean, sailing for the Maxwell Fracture Zone, an undersea mountain ridge forming a part of the Mid-Atlantic ridge.  As we pass the ridge, depths shallow to around 1 km, before plummeting to 3.5 km on the far side of the ridge.  The ridge itself is a spectacular example of the forces shaping our evolving planet, as the North American and Eurasian tectonic plates move away from each other, and magma from the Earth’s mantle rises to rapidly cool as it contacts the ocean depths.  The ridge evolves slowly, as the two plates separate and ‘fresh’ continental plate is formed.

One interesting feature of the crust beneath the oceans is the magnetic striping, infused by the direction of the Earth’s magnetic field as the magma cooled and solidified to rock.  The direction of the magnetic field, the polarity, reverses direction over long timescales, signs that the direction of the Earth’s magnetic field has flipped several times over the geological timescales.  Magnetic north switched from north to south, and during the intermediate stages of reversal our protective magnetic field, which prevents continuous bombardment by accelerated charged particles from the Sun, would have weakened and vanished.  Those events are recorded in the striping of the sea floor either side of the Atlantic ridge.

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