THE OCEANIC Ridge system is a near-continuous underwater range, more than 60,000km long, that winds, like a tennis-ball seam, around the Earth on the ocean floor. Up to thousands of kilometres wide, it travels through every ocean basin.
In 1855 the US Navy’s Matthew Fontaine Maury proposed its existence as a shallow “middle ground” in his then-new Atlantic Ocean chart. It was originally called the Mid-Oceanic Ridge, but later discovered to have some of its sections, such as the East Pacific Rise, not in the middle of an ocean. It was not until the 1950s, when systematic ocean floor surveys began, that the Oceanic Ridge’s existence was proved.
These submarine mountain ridges rise from about 5km deep, on the ocean floor, to a fairly uniform depth of about 2.6km below the surface. At various locations along the crests of these mountains are ‘spreading centres’, regions where new ocean crust is created. The crust is literally spread apart by new crust pushed up from below. The spreading rate varies from 10mm to 160mm/year.
The geography (the dip at the top) around the actual spreading centre is related to how fast the crust pulls apart. If the rate is high, as it is at the East Pacific Rise, the actual spreading centre is in a narrow and sharp dip. But a slower spreading site, such as the Mid-Atlantic Ridge, tends to sit inside a larger rift valley that can spread as wide as 20km.
The source of the new crust could be vast underground chambers of hot molten magma. In the East Pacific Rise these chambers seem to be about 2km below the ocean floor, about 1–4km across and about 2–6km thick. Across the whole planet, about 4cu.km of brand new ocean crust is formed each year.
It seems that about 20 per cent of all heat rising from the core of the Earth comes out through these spreading centres that sit on top of world’s longest (and unseen) mountain range.