Subducting oceanic plates drag down with them a great deal of water. When that flashes to steam as it hits magma and the mantle, tremendous pressures are generated. Many volcanoes (though not all) are in this regard "steam engines", where much of the driving pressure is a consequence of subducted ocean water, and possible some innate crustal water, interacting with geothermal heat. I'm supposing this is why steam is a major factor in many volcanic eruptions.
Reminds me of the Vajont Dam disaster, in which filling of a reservoir behind a newly constructed dam lead to catastrophic collapse of a mountainside in the Italian Alps, where layered sedimentary limestone separated by thin layers of clay became unstable as the clay was wetted and lubricated by the reservoir.
Shortly before 11 pm on 9 October 1963, a two-kilometer-long landslide triggered a 250m (850 ft) megatsunami. That killed a number of engineers who'd gathered on top of the dam to observe its filling, as well as around 2,000 inhabitants of villages adjacent to and downstream of the reservoir, most notably Longarone which was virtually scrubbed from the map.
The dam itself was virtually undamaged by the incident, save the topmost metre or so of nonstructural masonry.
There's an explainer here: <https://www.nps.gov/subjects/geology/plate-tectonics-subduct...>
As to the role in tectonics and plate movement, I'm less aware.