In George R.R. Martin's A Song of Ice and Fire novels, the land of Valyria was transformed from peninsula to archipelago by a volcanic cataclysm followed by earthquakes, tidal waves, a miasma of smoke and gas, and storms of obsidian shards. Excepting the obsidian storms, cool as they are, is this a plausible scenario? Could chained tectonic disasters of an intensity which could plausibly be imagined on Earth rend a contiguous landmass of several hundred kilometers to a side asunder? Could the ocean fill the rents, creating many islands and a new sea? Could all this happen in the space of a couple days?
For the purposes of this question, let's choose a specific landmass on Earth and test the plausibility of this scenario against it's (ever so slightly terraformed) geography. In the novels, Valyria is depicted as being peninsular, near-equatorial, and kinda Greece-shaped.
Valyria before the Doom:
Valyria after the doom:
So let's choose a peninsular, near-equatorial (sorta), Greece-shaped landmass. Perhaps Greece?
Let's assume that our "Greece" has astonishingly active tectonics and vulcanism. Lots and lots of vulcanism. At least 14 active volcanoes on the mainland, ideally more. With a tectonics & vulcanism budget limited only by the number of volcanoes and fault lines that can feasibly fit in "Greece" (with at least nominal regard to what is plausibly believable of nature), can we destroy our peninsula and make it into even more islands? Other tectonic lines of the planet can be bent, erased, or added to facilitate our project, so too for any other geographical feature that needs to be altered. Lastly, what could trigger that series of eruptions, what sort of tectonic action needs to occur for everything to go boom at once?