Ocean ridge discovery
From the late 1930s, new techniques have opened up in submarine geology. Gravity measurements and geotectonic imagery have allowed accurate mapping of the sea surface and the bottom structure of the oceans.
The ocean floor, far from being smooth and flat, is marked by huge mountain ranges – the mid-ocean ridge that form part of a global network which extends for more than 80,000 km. In places such as Iceland, Ascension and the Galapagos Islands, the ridges rise above sea level. The ocean floor is also cut by deep trenches which mark subduction zones and are punctuated by isolated seamounts.
Because a mid-ocean ridge is submerged at very deep depths in the ocean, its existence was not even known until the 1950s, when it was discovered by Vema, a ship of the Lamont-Doherty Geological Observatory of Columbia University, that traversed the Atlantic Ocean and recorded data about the ocean floor from the ocean surface. The mountain range was named the Mid-Atlantic Ridge. At first, it was thought to be a phenomenon specific to the Atlantic Ocean, because nothing like such a massively-long undersea mountain chain had ever been discovered before.
However, as surveys of the ocean floor continued to be conducted around the world, it was discovered that every ocean contained parts of the mid-ocean ridge.
The discovery of what mid-ocean ridge systems represented, the sites of crust formation, or constructive plate margins, was a major breakthrough in earth sciences. Basaltic volcanism, upwelling of magma consisting mainly of basalt characterises ocean ridge.
Convective movements within the mantle force the overlying lithosphere move apart, allowing hot magma to reach the sea floor. At ridge crests, a zone of rifting separates regions of sea floor which are moving apart at 2-15 centimetres per year. Because the oceanic crust cannot withstand sufficient stress to allow for variations in spreading rate and changes in convection pattern, ocean ridge consist of straight sections offset by transform faults, along which different sections of a plate slide past each other.
This phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere.
One of the key pieces of information came from paleomagnetic studies along the Mid-Atlantic Ridge. It was found that only half the rocks on each side of the ridge-axis near Iceland showed normal magnetic polarity; the remainder had a reversed polarity (a magnetic needle would point south).
The pattern of normal and reversed polarity was manifested in a magnetic striping of the oceanic crust, mirrored on each side of the ridge crest. The alternating pattern of normal and reversed polarity rocks is produced as successive belts of lava are extruded at the site of a divergent plate margin. At the mid-ocean ridge and associated rift zones, the new sea floor is generated then carried away from the ridge-axis by lateral mantle motions.
When individual stripes were dated, it was found that the rocks became older with increasing distance from the crest. In other words, the sea floor was spreading apart. Such spreading characterises all ocean ridge where lithospheric plate divergence occurs.
During the past 80 million years, the Atlantic has spread at a rate of 2 centimetres per year. About 4 cubic km of new crust is produced at mid-ocean ridge every year.