The Pacific Ocean basin has been progressively shrinking over geological time because its surrounding subduction zones (the 'Ring of Fire') consume oceanic crust faster than the East Pacific Rise can create new crust, in contrast to actively opening basins like the Atlantic and Red Sea, which are growing via seafloor spreading.
The Pacific Ocean is well-documented to be decreasing in total area over the past several million years, as subduction around its margins consumes oceanic lithosphere...
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