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Following up on this question, where the answers made me wonder about another aspect of my world.

My planet is a super-earth orbiting really close to a Red Dwarf star, resulting in enormous tides. The difference between high and low tide is around 30 meters, and the difference between spring high and low tide being roughly 60 meters. They occur on a similar schedule as Earth's.

What would landmasses look like on this planet? Would the strength of the tides erode any landmasses before they even formed, or would something like strong vulcanism allow continents to form? What about shallow seas? Would there even be any continental shelf, or would all oceans just drop straight to the abyssal zone? What rock would be able to withstand such strong tides and thus support the growth of continents?

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  • $\begingroup$ When you say that the tidal range is between 60 and 120 meters, what do you mean by that? For example, if you were speaking about Earth, what would be the corresponding numbers? (Tidal range varies from place to place, with a minimum of about zero and a maximum of 15 to 16 meters. In open ocean, the tidal range is between zero and about 0.6 meters.) $\endgroup$
    – AlexP
    Commented Nov 21, 2020 at 16:52
  • $\begingroup$ I haven't done the math for how large tides are locally, because I don't know how to determine the exact numbers for that and it may be a bit too complex for my taste. My tide calculator for the whole planet determined that standard high tide is +25 meters and standard low tide is -25 meters. Spring high tide is +75, spring low tide -75 meters, neap high is +26, neap low is -26 meters. I believe this only applies to the open ocean, however. $\endgroup$
    – D. Daniels
    Commented Nov 21, 2020 at 17:23
  • $\begingroup$ Look at the numbers I gave you for Earth. The tidal range in open ocean is never more than 0.6 meters: and the maximum tidal range on the coast is 15 meters, 25 times larger. $\endgroup$
    – AlexP
    Commented Nov 21, 2020 at 17:43
  • $\begingroup$ Note: You need your planet to be very geologically active (not a problem given the tides) or your land will soon end up eroded away. $\endgroup$ Commented Nov 22, 2020 at 23:25

2 Answers 2

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Convex outlines for your continents.

Each incomming tide will dump a significant amount of silt on the shore, this will tend to turn bays into straight lines by filling them up. The outgoing tide will in turn remove material, targeting peninsulas and outcrops and grinding them away. The net effect will be a tendency for any spot on the coast the be essentially straight. This will result in convex continents with smooth corners.

Differences in composition wearing at different rates will occasionally produce coastal features of interest, but these will be very quickly smoothed out by the tides.

I see no reason why this would change the continental shelf, so I'd assume that remains unchanged.

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So.

I presume this world is tidally locked, as it is so close to its parent star. If that is the case, there will be a ring around the day night terminator from pole to pole that is just about encircling the prime meridian. This is where you have a large island-continent. As per Kip Thorne's explanation in the science of Interstellar, the tidally locked planet will rock back and forth generating tides. But these rocking motions would be nullified neither on the side closer to or farther from the star, imagining the planet is an (extremely exaggerated) egg with its poles pointing towards and away.

This results in your planet only having one super-stretched continent all around the "prime meridian". The continental shelf, stronger-than-normal rock, etc. remains unchanged, because due to the way the tides play out, near the midpoint of the planet, everything is as normal.

EDIT: Shallow seas, yes. Everywhere except the continent-island will have extreme tidal forces either towards or away from the star, so it will definitely result in shallow water bodies. RE-EDIT: Especially if there is up to a 60 meter difference. That would reduce the sea depth to lower than normal on a regular (constant) basis due to so many waves and such.

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