As I’ve said, I like the science in my own fiction to be al dente, so just how overcooked are the ideas behind the Tectocalypse? This page will give you some idea.
First, water. With a sea-level 400 metres (1200 feet) higher than today, the world of Thalassa needs a lot of water. Where does it all come from? We’ll see that the 400 m figure is a bit iffy in itself, for what turn out to be good and very helpful reasons, but it gives me the kind of new coastlines (and more importantly – flooded land) that I wanted, as the image below shows.
That image shows that the US states of Mississippi, Louisiana, and Alabama (among others) are flooded. Those states have a combined land area of 395,690 square km according to Wikipedia, so 400 m on top of that is: 158,636,000 cubic metres of water. Ouch. That’s a lot. This calculation assumes that the states concerned are sea-level, which they aren’t; they have a mean elevation of about 100 m already, so maybe we only need 118,977,000 cubic metres of water. Phew! That’s a relief, eh?
If all the water in the ice-caps of Greenland and Antarctica was to melt, we get perhaps 33,000,000,000 cubic metres (33,000,000 square kilometres, link here), and that doesn’t include snow cover and the Arctic sea-ice, so flooding those US states is doable (sorry!). Except, the melted ice caps would add to water levels world-wide, not just in the US, and so the total sea-level rise would only be 216 feet (less than 100 m, link here): we’re still a thousand feet short of Thalassa.
Before you start cheering, this rise would be completely catastrophic for most cities, so the liberal metropolitan elites would suffer (why do I hear some of you cheering?) and also most farmland, which is kind of useful for, y’know, food, so while Thalassa it ain’t, it’s no picnic either. (Literally.)
How to get from the 216 feet to the 1200 feet needed for Thalassa? Some extra mechanisms are called for.
First, as water warms, it expands on its way to being a gas. So in a warmer world, the sea-level rise will be higher because the water will expand (link here). But still not enough. Hmmm. OK, weight! Water is heavy, as anyone knows who has ever had to schlepp a full bucket of water only 12 inches (30 cm) deep (and is there a better word than schlepp here?). Imagine a bucketful of water 216 feet deep. It’s heavy. Very heavy. The weight of all that water pushes the land down, squidging the lighter continental crust into the mantle below. This is isostatic depression, the opposite of isostatic rebound, which is when the rock rebounds and starts to rise again after the ice has melted (see also here). Isostatic rebound plays a big role in the Tectocalypse – it’s the champagne cork out of the bottle marking the beginning of the Thalassa party – but I’ll come to that in time. Isostatic depression would push the flooded land deeper, adding further to the rise in sea-level.
So, to recap: all the ice melts, the water expands, the land below sinks (this is why the 400 m sea-level rise is iffy: the water’s going up as the land is going down), and… It’s still not enough! Grrrrrr! Pesky real world limitations!
Enter, stage left, isostatic rebound! Freed from the ice, Antarctica and Greenland will rise up. Where I am now in England is sinking slowly as northern Britain recovers from the loss of the glaciers at the end of the last ice-age, and that was 10,000 years ago. But what if the rebound happened fast: imaginably but unrealistically and implausibly fast? Then you get the continental shelves rising to become dry land, pushing the seas back. And where do those seas go? Why, they flood the land and add to global sea-levels of course!
We’re almost done. The final piece of the Tectocalypse jigsaw is vulcanism. One phrase says it all: the Deccan Traps. Massive lava flows. If these happened underwater – if they could happen underwater – new land would be built, just like Iceland’s Surtsey and Hunga Tonga. This new land pushes up from the sea-floor and displaces yet more water onto what were the lowlands. New continents arise, burning black rocks breaking the surface, and the Old Earth drowns.
So, massively unrealistic in timescale, magnitude, and the fact that the mechanisms act in combination, but the mechanisms do exist to create Thalassa. Ish. (Did I mention this was fiction?)