<P> As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus . The amount of melt produced depends only on the temperature of the mantle as it rises . Hence most oceanic crust is the same thickness (7 ± 1 km). Very slow spreading ridges (<1 cm yr half - rate) produce thinner crust (4--5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust . An example of this is the Gakkel Ridge under the Arctic Ocean . Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust . An example of this is Iceland which has crust of thickness ~ 20 km . </P> <P> The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it . The oceanic lithosphere subducts at what are known as convergent boundaries . These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another . In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense . The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old . The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle . </P> <P> The oldest large scale oceanic crust is in the west Pacific and north - west Atlantic - both are about up to 180 - 200 million years old . However, parts of the eastern Mediterranean Sea are remnants of the much older Tethys ocean, at about 270 and up to 340 million years old . </P> <P> The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the basalt . A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge . New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point . When the magma cools to form rock, its magnetic polarity is aligned with the then - current positions of the magnetic poles of the Earth . New magma then forces the older cooled magma away from the ridge . This process results in parallel sections of oceanic crust of alternating magnetic polarity . </P>

Where can the oldest oceanic crust be found