Since the first rocks of the pacific Ocean plates solidified from magma coming out of a rift 750 million years ago, shouldn't they be much older than the maximum age of oceanic crust (around 200 million years)?
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3$\begingroup$ Basically, because the ocean is bordered by subduction zones where the older crust is being swallowed. $\endgroup$– Jean-Marie PrivalCommented Mar 23 at 19:27
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Expanding on @Jean-Marie Prival's comment:
Basically, on geological timescales the ocean has a convener belt of oceanic crust underneath it, where new crust forms and old crust is destroyed. This means the ocean can be much older than the age of crust underneath it.
Oceanic plates are driven by convection currents in the mantle which is formed of molten rock, with the top-most part of the mantle (the asthenosphere) only being partially melted.
These convection currents create areas up uplift where magma (molten rock) pushes through to the crust and solidifies forming a mid-oceanic ridge and so new oceanic crust.
Credit Wikimedia User:Surachit Licence: CC BY-SA 3.0 DEED
This crust is moved along by the convection currents. The ridge is there because of the upwards pressure of the convection current. When the rock moves away this upwards pressure is no longer present so the surface lowers. Eventually the crust reaches a subduction zone and is reabsorbed by the mantle. If you traced some freshly-made rock you can find that the solid-rock's lifetime can be significantly shorter than the ocean's lifetime above it.
Subduction zones exist, because oceanic crust, which is mainly made up of low-silica basalts, is denser than continental crust, which is mainly made up of high-silica granites. This means, when the two meet, the denser oceanic crust goes underneath and the less-dense continental crust floats on top. The weight of the continental crust, pushes the oceanic crust further down where there is more heat so it begins to melt.
As an aside, the earth's magnetic field flips periodically. The polarity of the magnetic field is recorded in the rocks as they solidify. This means this polarity signal forms a mirror image on either side of the mid-oceanic ridge. This is one of the initial pieces of information that was used to show that oceanic plates move and also one of the ways the speed of the movement was calculated.
