In "Dagon", the protagonist, falld asleep drifting on the ocean in a small boat. He wakes up one morning to discover he's been grounded on what appears to be a vast region of mire that used to be the ocean floor:
Through some unprecedented volcanic upheaval, a portion of the ocean floor must have been thrown to the surface, exposing regions which for innumerable millions of years had lain hidden under unfathomable watery depths. So great was the extent of the new land which had risen beneath me, that I could not detect the faintest noise of the surging ocean, strain my ears as I might. Nor were there any sea-fowl to prey upon the dead things. [...] The region was putrid with the carcasses of decaying fish, and of other less describable things which I saw protruding from the nasty mud of the unending plain.
This seems to indicate that some enormous geological activity of vague nature resulted in part of the sea floor itself being thrust up to the surface in what can only be assumed to be the sudden formation of a large new island. This activity is implied to have happened without anyone knowing about it (until now).
I know very little about oceanic geology, and the plausibility of this whole thing confuses me. Is it possible for the ocean floor itself to be pushed up above the water? Is it possible for this, or any other process of land formation, to occur so fast as to leave marine life stranded on the now-surface? Lastly, is it possible for such geologic activity to occur unnoticed by humans?