How to Cause a Hydrosphere Apocalypse III: Erosion and a Shallow Sea

Question

This is a bit of an addendum to a previous question, shown here. (How to Cause a Hydrosphere Apocalypse Part II The Great Flood). I like the rain of ice from a dying moon, answer and I have decided to use the world in part two as a springboard for a setting in a novel.

But I have run into a tad bit of a snag.

The wiki for the film Waterworld, (a major bit of inspiration) says that the global flood added 25,000 ft to the global sea level. Now, this presents a couple problem for the story I wish to tell.

See the locals are air breathers capable of holding their breath for 30 minutes. I want them to be able to reach the sea bed, (at least for the shallower parts) and scavenge for parts and one set piece I really wanted to use is the ruins of a flooded city with the tops of sky scrapers poking out of the waves, inspired by this image by Slava Vidyapin.

Now the shallowest sea bed being 4.7 miles down presents an understandable problem to all those requirements.

But then it hit me. Erosion.

The flood that will swallow this particular world isn’t coming up from a rising sea, it’s coming from above in a rain of ice and water. And not just for 40 days and nights but for centuries even millennia. And usually such rains can wash away the land and thus maybe make the resulting sea that much shallower. But I wanted the opinions of those who know better than I.

So, Would a thousand years of rain and icy meteors lead to enough erosion to ensure a shallow sea water world?

Answer 1

Not in a way you want.

Land tectonic plate thickness is about 100km thick, sea tectonic plate thickness is much less, about 10 km. Even if we take just the hight difference on the surface, thats still 4km.

If you manage to erode this much matter, how would city ever stay available to scavangers of any sort? Even 1m erosion will make almost all city ruins unaccessible without modern machinery. 100m erosion will make it non-existant but detectable on chemical level. 10km erosion will make it only detectable on isotope level. Doesnt matter, if it is erosion by weather or meteorites or whatever, same rule applies.

If you want water world with accessible city ruins your options are:

• Flat planet with large ice caps. When they melt, most land is under water, softly, without destruction. Centuries - people will adapt somewhat - floating bases.

• Flat planet and meteorite shower. Very small meteorites, none of them reaching the ground, large water content. Even then dust from them will still cover the ruins. And it will take millions of years - people will adapt fully in time. If any faster - atmosphere will overheat.

• Some strange tectonic activity, so that all tectonic plates melt and reform at about equal thickness. So that land and sea surfaces evens out. Likely includes extreme volcanism and earthquakes, making ruins barely accessible. Will still take millions of years. People will fully adapt in time. If any faster - requires extreme energy that doesnt exist in a planet normally. Like a black hole or a large payload of antimatter.

• Local catastrophe. Be it a large nuke or volcano, if flat planet consist of one deep sea, it could be 'filled up' with local material. It will destroy most of the ruins, but some will remain.

Anyway, consider the amount of heat your idea involves.

P.S. city ruins are useful only for a century. After that it is an ore deposit.

Answer 2

Rain from below.

Continents sit on thick lumps of continental crust. These are the floats that keep continents up - without them, the continents would go down.

Now, there's obviously no way on the Earth we know to say that these will all wear away all over the planet any time soon. But maybe for your planet, we can propose there is a continental crust sort of like ours, but it overlies a layer of "liquid core" with no separate "mantle" between. That liquid core is cooling, turning solid at the center, like liquid cores on planets we know - but this particular one has gotten very, very thin. So thin that the underlying solid core (which rotates relative to the crust below, like ours does on Earth) has begun to rub against the continents in places, throwing rubble back and forth between the solid continents and the solid inner core. As the rubble crashes into the roots of the continents, it wears them away and mixes them with the remaining liquid core material, which is rapidly heating from the friction.

The result we're looking to get to is that all the continents are ground down from below. They sink within your time frame mostly because this is not Earth (ours are still rebounding from the Ice Age), but we'll say they're a little less stiff for some reason.

The net outcome is a planet with a thin shell of liquid core, surrounded by a crust that has been sanded smooth, with all the terrain above evenly distributed beneath its ocean. No water needed to be added, but now everything is underwater.