Geoscience Reference
In-Depth Information
The North Atlantic Ridge was discovered in the mid-19th century by a ship attempting to
lay the first transatlantic cable. The ridges are broad, between 1,000 and 4,000 kilometres
wide, and rise slowly towards a central line of peaks, typically 2,500 metres above the deep
ocean floor but still a further 2,500 metres below the sea surface. The ridge is offset by
numerous transform faults perpendicular to its length, displacing the ridge crest by many
kilometres. The crest of the ridge often consists of a double line of peaks with a central rift
between them. In the first half of the 20th century, proponents of the theory of continental
drift such as Arthur Holmes suggested that the ridges might mark places where convection
in the mantle brought new crust to the surface, but it was magnetic surveys which finally
confirmed one of the most important discoveries in geology: sea floor spreading.
9. The global system of ocean ridges and the main transform fracture zones that cut
it. The hot-spots of Hawaii and Iceland are circled.
Magnetic stripes
In the 1950s the US navy needed detailed maps of the ocean floor to aid their submarines.
So research vessels began sailing to and fro making sonar measurements. Scientists were
given the chance to contribute other experiments, and so it was that a sensitive magneto-
meter was towed across the oceans, mapping out the magnetic field. The map showed a
series of highs and lows in the field strength that appeared like parallel stripes on either side
of the mid-ocean ridges. It was Fred Vine and Drum Matthews at Cambridge who were
able to confirm what was happening. As volcanic lava erupts and cools, it traps magnet-
