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I read, periodically, that there is more water trapped in the mantle than there is in the oceans - possibly a lot more.

If so, why don't any geologists believe that Earth's surface and near-surface waters were brought up via plate tectonics?

Why do they always claim it is a great mystery concerning the origins of Earth's oceans?

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    $\begingroup$ Ok, so how did get it into the mantle then...? Who is they? [citation needed] $\endgroup$
    – AtmosphericPrisonEscape
    Commented Sep 28, 2023 at 22:36
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    $\begingroup$ "I read, periodically". Where do you read? Cite your sources. $\endgroup$
    – Nilay Ghosh
    Commented Sep 29, 2023 at 0:21
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    $\begingroup$ Eg, Dehydration melting at the top of the lower mantle Schmandt et al. combined seismological observations beneath North America with geodynamical modeling and high-pressure and -temperature melting experiments. They conclude that the mantle transition zone—410 to 660 km below Earth's surface—acts as a large reservoir of water. $\endgroup$
    – PM 2Ring
    Commented Sep 29, 2023 at 14:19
  • $\begingroup$ If this was a question about how water arrived on the Earth, it would be an astronomy (astrophysics/cosmology) question. But it's not framed that way – instead, it's about a geological issue, which is off-topic here. I'm voting to close. $\endgroup$
    – Chappo Hasn't Forgotten
    Commented Oct 17, 2023 at 7:11

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If the question is actually "why don't any geologists believe that Earth's surface and near-surface waters were brought up from the Earth's interior?", then the premise is false since many geologists do believe that is possible, e.g. see

If you are specifically referring to plate tectonics, then as discussed in "Was Earth completely covered in Oceans prior to the onset of plate tectonics?", the answer may depend on how "plate tectonics" is defined. Earth scientists have have done research related to the hypothesis "that Earth was formed with entire oceans of water in its interior, and has been continuously supplying water to the surface via plate tectonics ever since".

There certainly exists a deep water cycle exchanging water between the oceans and the mantle, so your question is just about what the starting conditions were: all the water in the mantle, all the water on the surface, or water in both. We don't know the precise answer, but that doesn't make it a "great mystery", just an area of research.

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