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Suppose you drained the ocean without any repercussions caused by the transport and allocation of that much water. Looking at land like the ring of fire or deep sea thermal sites, would the crust remain thin enough for heavy volcanic activity? Or would it thicken up and cause that activity to cease?

For some context, there was an impact on this planet in its deep geological history that somewhat changed the planetary shape and rotation, and over time, the ocean was periodically locked in glacial ice (or was thrown into space on impact, idc). Much of the modern world, for lack of liquid water, is open landscape. I want to know if there will be large patches of volcanic land in these areas far rivalling Iceland and the like due to plate thickness. I'm really hoping for vast nation-sized Mordor locations but they aren't necessary. Miniscule activity sites ala Yellowstone are sufficient. I am willing to bet that the impact may have also ruptured a portion of the crust outward from its crater. It wasn't a terribly traumatic impact (as in the planet probably had an atmosphere ported in later but it's still in one piece), the important part is that it have the planet a nigh undetectable hump kind of like Earth. I am also willing to skip the impact, have the oceans disappear/hump appear some other way, they're not important details.

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You don't want "thrown into space." Displacing more than a couple cubic km of matter is going to wind up producing so much heat that your planet's surface is molten. Probably to a depth of several km. The cooling time from such an event is enormous, certainly 100's of thousands of years, possibly millions of years. And don't forget that an ocean only 100 km by 100 km and 1 km deep (the general scale of the Great Lakes of North America) is 10,000 cubic km. To throw that into space would pretty much destroy your planet.

Ice age effects on the crust are substantial. The part under the ice gets pushed down, the part that was under the ocean rises up. Then when the ice age goes away, things start to push back.

Here is an example: enter image description here

The pressure of the ice can, depending on the details, either suppress or ehance volcanic activity. So, presuming there was volcanic activity before, the active and dormant areas could swap when it changed from ice to warm or warm to ice. It is unlikely to stop the volcanic activity entirely.

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  • $\begingroup$ First part not entirely true. Chiqxulub dispensed billions of lbs of rock into space. I'm certain that's a drop in the ocean but water isn't as dense. I imagine an ocean impact could cause a lot more water relocation. That said the event happened 400mya years back so that may help. I mentioned deep geological history. $\endgroup$
    – Quinn
    Apr 8, 2023 at 17:07
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    $\begingroup$ @Quinn: (1) Rocks are not really so much heavier than water. For example, the relative density of basalt is only about 3. (2) You must realize that a billion pounds of water is a modest sized lake; taking or adding a hundred billion pounds of water to the ocean would have no noticeable effect on the sea level. For example, Lake Constance (in between Austria, Germany, and Switzerland) contains about one hundred billion pounds of water. $\endgroup$
    – AlexP
    Apr 9, 2023 at 0:12
  • $\begingroup$ I said water isn't as dense and weight has nothing to do with it. That really isn't the point of my question though, Please don't get wrapped up in a minor detail - I really couldn't be concerned with how much water is or isn't ejected from the impact. Context exists for supplementary reasons. $\endgroup$
    – Quinn
    Apr 9, 2023 at 6:33
  • $\begingroup$ I suspect that if the water was to disappear (by whatever magical means) that Tectonic activity would be affected. Over a long period the missing weight of the oceans would allow the oceanic crust to be pushed up. In addition the lack of water might have an adverse effect in subduction zones changing the nature of the rocks and the volcanic activity in the mountain ranges above. So maybe a different sort of tectonic activity with less subduction and more collisions between plates. $\endgroup$
    – Slarty
    Apr 9, 2023 at 12:15
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The volcanic activity is not due to the thickness of the crust. We have plate tectonics on earth where the continents are lighter than the ocean crust. Thus, where the mantle heat rises up, the ocean crust breaks and splits apart pushing the crust against other plates. Since the ocean crust is heavier than the continental crust, it goes down under the continental crust. Volcanoes happen where the ocean crust reaches the depth to change the composition.

Now, one key factor in volcanoes is that the ocean crust has absorbed a lot of water while being under the sea. That water is key. The current thinking is that when the ocean crust reaches about 100-150 km depth, it gives off that water. That water then rises up through the mantle and starts the melting of the continental crust which generates the volcanoes.

In other words, if you got rid of all the water, you will get rid of the volcanoes that are now on the "ring of fire". This is for all the explosive style volcanoes.

You will still have the basaltic volcanoes as those are expelling mantle lava. Think Hawaii style volcanoes - large, dome shaped mountains with lava that runs for miles. These happen in spots. Or the ones where the plates are spreading apart - long ridges.

So, if you got rid of the oceans, you will get rid of all those large cone shaped volcanoes and leave behind domes and long ridges of volcanoes.

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