A few days ago I expressed skepticicm about Michael Heads' claim, advanced in defence of the idea that the plants and animals of Hawaii did not get there through recent long distance dispersal but instead evolved in place for tens of millions of years, of the Pacific having undergone thousands of metres of subsidence over that time. Intuitively I was confident that that would have been impossible because most of the continents would have had to be under water.
I now stand corrected. I made a back of the envelope calculation using admittedly very conservative parameters, and it turns out that there is more continent than I thought:
The Pacific Ocean is 165.25 million km2 = 1.6525 x 10^14 m2 large. Assuming that a quarter of that area needed to have a 2,000 metres higher sea floor, the water displacement would be 2,000 m x 1.6525 x 10^14 m2 = 8.2625 x 10^16 m3. Assuming further that this needs to be distributed evenly across the surface of the planet, we would have the sea level higher by 8.2625 x 10^16 m3 / 5.1 x 10^14 m2 = 162 m.
Punching that number into floodmap.net, we see that by far most of the land is still above the sea:
Remember, of course, that I am leaning over backwards here. I have no idea if 25% of the Pacific and 2,000 metres would be enough or if Heads' claim involves larger numbers; that might change something (if you assume all of the Pacific, or half the Pacific and 4,000 metres, there is very little land left). Further, the Pacific was larger 65 million years ago than it is now, with the continents having drifted further apart. Of course the water would not be equally distributed either because the land displaces it too, so it would raise the oceans by more, but because most of the world is ocean that wouldn't matter so much. Perhaps more importantly though, if the floor is higher under the Pacific it has to be lower elsewhere, as there is a limited amount of Earth, and that would likely mean lower land masses at least somewhere.
For various other reasons I remain unconvinced of the vicariance scenario for Hawaii, but the subsidence of the Pacific is not as implausible as I naively assumed at the beginning, at least when using the above conservative estimates.