<p>Expanding on @Jean-Marie Prival's comment:</p>
<p>Basically, on geological timescales the ocean has a convener belt of <a href="https://en.wikipedia.org/wiki/Oceanic_crust" rel="nofollow noreferrer">oceanic crust</a> 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.</p>
<p>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 <a href="https://en.wikipedia.org/wiki/Asthenosphere" rel="nofollow noreferrer">asthenosphere</a>) only being partially melted.</p>
<p>These convection currents create areas up uplift where magma (molten rock) pushes through to the crust and solidifies forming a <a href="https://en.wikipedia.org/wiki/Mid-ocean_ridge" rel="nofollow noreferrer">mid-oceanic ridge</a> and so new oceanic crust.</p>
<p><a href="https://i.sstatic.net/7u5QO.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/7u5QO.png" alt="Simplified schematic of convection currents in the mantle driving plate tectonics with areas of uplift where new oceanic crust is formed and areas of subduction where it is melted." /></a></p>
<p><em>Credit <a href="https://commons.wikimedia.org/wiki/File:Oceanic_spreading.svg" rel="nofollow noreferrer">Wikimedia User:Surachit</a> Licence: <a href="https://creativecommons.org/licenses/by-sa/3.0/deed.en" rel="nofollow noreferrer">CC BY-SA 3.0 DEED</a></em></p>
<p>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 <a href="https://en.wikipedia.org/wiki/Subduction" rel="nofollow noreferrer">subduction zone</a> 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.</p>
<p>Subduction zones exist, because oceanic crust, which is mainly made up of low-silica <a href="https://en.wikipedia.org/wiki/Basalt" rel="nofollow noreferrer">basalts</a>, is denser than <a href="https://en.wikipedia.org/wiki/Continental_crust" rel="nofollow noreferrer">continental crust</a>, which is mainly made up of high-silica <a href="https://en.wikipedia.org/wiki/Granite" rel="nofollow noreferrer">granites</a>. 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.</p>
<p>As an aside, the earth's magnetic field flips periodically. The <a href="https://flexbooks.ck12.org/cbook/ck-12-middle-school-earth-science-flexbook-2.0/section/5.5/primary/lesson/magnetic-evidence-for-seafloor-spreading-ms-es/" rel="nofollow noreferrer">polarity of the magnetic field is recorded in the rocks</a> 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.</p>

CC BY-SA 4.0
2024-03-26T09:50:34.107Z
2024-04-03T10:56:58.147Z
2024-04-03T10:56:58.147Z
Mehmet Karatay
<p>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)?</p>

CC BY-SA 4.0
2024-03-23T19:27:09.667Z
Basically, because the ocean is bordered by subduction zones where the older crust is being swallowed.
Jean-Marie Prival
<plate-tectonics><oceanic-crust>
If the Pacific Ocean is 750 million years old, Why can't oceanic crust be older than 200 million years?
Antozoa
# If the Pacific Ocean is 750 million years old, Why can't oceanic crust be older than 200 million years?

(Asked by: username_0 on 2024-03-23)

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)?

**Tags:** plate-tectonics, oceanic-crust

> **Comments:**
> **username_1**: Basically, because the ocean is bordered by subduction zones where the older crust is being swallowed.  

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## 1 answers

### Answer by username_2 on 2024-03-26. Score: 6

Expanding on @username_1's comment:

Basically, on geological timescales the ocean has a convener belt of [oceanic crust](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Mid-ocean_ridge) and so new oceanic crust.

[![Simplified schematic of convection currents in the mantle driving plate tectonics with areas of uplift where new oceanic crust is formed and areas of subduction where it is melted.](https://i.sstatic.net/7u5QO.png)](https://i.sstatic.net/7u5QO.png)

*Credit [Wikimedia User:Surachit](https://commons.wikimedia.org/wiki/File:Oceanic_spreading.svg) Licence: [CC BY-SA 3.0 DEED](https://creativecommons.org/licenses/by-sa/3.0/deed.en)*

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](https://en.wikipedia.org/wiki/Subduction) 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](https://en.wikipedia.org/wiki/Basalt), is denser than [continental crust](https://en.wikipedia.org/wiki/Continental_crust), which is mainly made up of high-silica [granites](https://en.wikipedia.org/wiki/Granite). 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](https://flexbooks.ck12.org/cbook/ck-12-middle-school-earth-science-flexbook-2.0/section/5.5/primary/lesson/magnetic-evidence-for-seafloor-spreading-ms-es/) 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.


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