Movements of the 'Tectonic Plates' are believed to have formed almost all of the Earth's current-day crust - relying on a process called 'Subduction'.

In that process, the oceanic lithosphere of a tectonic plate collides with the less dense lithosphere of a second plate. Under pressure, the heavier plate slides under the second plate and sinks down into the mantle. See Wikipedia

But what initiates the process is currently unknown.

One theory is that fluid water lubricates the rock surfaces, creating positive feedback mechanisms which encourage further subduction.

The initiation of subduction remains one the unresolved challenges of plate tectonics.

See The Initiation of Subduction: Criticality by Addition of Water? Science • Vol 294, Issue 5542 • pp. 578-580 (a full copy of which may be found here )

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