On Earth, the presence of tides due to the Sun and Moon influence certain currents and other movements of water. This in turn leads to different forms of erosion, which can transform a landscape.

I'm working on a moon that's tidally locked to a planet - pretty much identical to the Earth-Moon system, except that the moon is slightly more massive, and has an atmosphere, water, and life (thanks in part to a magnetic field).

How will the tidal locking - meaning a lack of tides - influence the geography of this moon? Will erosion rates be different? Will that land be shaped differently?


4 Answers 4


I believe in this case Sun Tides will become dominant.

Even though the Sun is 391 times as far away from the Earth as the Moon, its force on the Earth is about 175 times as large. Yet its tidal effect is smaller than that of the Moon because tides are caused by the difference in gravity field across the Earth. The Earth's diameter is such a small fraction of the Sun-Earth distance that the gravity field changes by only a factor of 1.00017 across the Earth. The actual force differential across the Earth is 0.00017 x 174.5 = 0.03 times the Moon's force, compared to 0.068 difference across the Earth for the Moon's force. The actual tidal influence then is then 44% of that of the Moon.

So while tides will be much less than a non-tidally locked system, it's only by a factor of perhaps 2/3rds. That's enough to still leave significant tides, so I expect you won't see a significant difference in the factors you mention as important - geography, land shape, erosion.


The tidal power effects are felt most greatly on larger bodies of water. Although waves are partly influenced by tides, most of their ferocity, and therefore costal erosion, is caused by winds so waves would remain largely unaffected by a tideless environment. Tides don't affect so much ferocity of waves but how far "inland" they travel.

The tides of Lake Superior vary by no more than two inches. On the other side of the spectrum tides on the Bay of Fundy can vary as much as 50 feet. So some parts that in a tidal planet would be wet part of the time would be left high and dry.

The greatest challenge to a tideless planet would not be erosion but the ecological challenges that a diminished churning of the oceans would present. There would be areas of high nutrient concentration bordering on toxic and dead areas almost completely devoid of nutrients.

Hope this helps.


I don't think you'll get a magnetic field without faster rotation, and month-long sidereal day is not up to it.

Without tides from the primary you'll still have solar tides and tides from other moons (as with Pluto/Charon + 3 "regular" moons) and any libration if the orbit is not exactly circular and exactly over the equator.

You get erosion from rain on the mountains weathering the rocks, and wind blowing sand. It doesn't need coastal erosion only. I think the overall quantity of weathering will vary more due to the kinds of rocks and weather patters, so you can tune for proper feedback even without tides.

The overall plate tectonics, mantle convention, and geologic processes don't have any influence from monthly tides.

  • $\begingroup$ I understand that erosion can come from other sources. I'm asking what would change without the influence of the relatively strong tides from an orbiting body. $\endgroup$
    – HDE 226868
    Commented Nov 24, 2015 at 1:31
  • $\begingroup$ Ok, I Updated the answer. $\endgroup$
    – JDługosz
    Commented Nov 24, 2015 at 3:55

I do not think tides have any considerable role in forming the landscape. An influence of storms and weather patterns in general is much stronger.

For example, consider Black Sea which had no visible tides for few last Mys. You cannot tell the difference from any other seashore in a similar climatic area.


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