Plate Tectonics vs Gravity for Creating Highs and Lows

Question

I have a question that I thought of when discussing a Worldbuilding scenario here. In that question, a super earth with gravity 1.35 times that of earth's is discussed, and is assumed to have shallow oceans. It is the assumed cause of these shallow oceans that got me thinking.

The question supposes that the shallow oceans are a result of a crust more uniform than that of earth, i.e without significant variations in depth, and that said uniformity is caused by gravity evening things out somehow. However, the imaginary world is also said to have more volcanic activity than earth, which I interpret to mean at least some tectonic activity.

My first thought was that there is no reason for gravity, at the scale we are discussing, to influence the uniformity of a planet's crust after its formation, and that plate tectonics would play a much larger role. I suppose the crust would be a bit denser depending on its original composition, and perhaps the magnitude of gravity influences the tectonics themselves more significantly than I realize. (Obviously with no gravity, there are no plate tectonics, and in fact no planet, so that's not what I'm after).

Earth Science Questions: What effects, if any, does gravity have on the mountains and depths of earth's crust? Assuming earth-like dynamics at play, would plate tectonics have more of an influence on the formation of a planet's crust's highs and lows than gravity? If not, why?

Bonus (World-Building) Question: Is there a way to construct a planet's crust and core such that volcanic activity is possible, yet gravity has the primary influence over crust uniformity?

Answer 1

I think this question is less intuitively obvious than it appears. and I like @Universal_learner 's answer. I thought I'd give a different approach to the question.

Earth is the only object in the solar system with significant amounts of granite and granite is very important to your question because of it's durability and it's buoyancy.

Granite forms under pressure below the Earth's surface in conditions where the magma gradually cools and it requires the presence of water to form. Once formed, Granite is more durable than the denser crustal rock and fairly early in the Earth's history, about 3 billion years ago by this article, this granite formed into continental crusts.

Because Granite is lighter than other crustal rock, it floats like a cork in the Earth's crust, about 2/3rds below the surface or ocean, and one third, extending above sea level. (Continental crust extends beyond the land/ocean boundary).

Higher gravity would likely flatten out the granite to a degree in terms of mountains and valleys but continental crust is in balance with the heavier basalt crust, so I don't see how gravity would effect the continents much. The rate of erosion from land to sea might increase, but the buoyancy of the granite wouldn't be affected by gravity.

The effect on the depth of the oceans is unclear. Outside of the continental crust that extends below the oceans, and the ridges and trenches, ocean depth is reasonably consistent.

Source of picture

If on Earth, for example, if you were to drain the oceans, with the weight removed the ocean floor would rise and the continents would sink somewhat, much like they do when a mile of glacial ice forms over and then recedes from the continents and the land shifts up and down, but the altitude variation on Earth, average continental height to ocean floor wouldn't change all that much. A few hundred feet perhaps out of 2-3 miles in variation.

The point is, the ratio of granite continental crust, to surface water would be the primary factor in ocean depth. Those ratios would be more important than gravity. Other factors to consider would be weathering and how active the planet's tectonic movement is.

The highest peaks on the Earth are formed by the comparatively rapid movement of two tectonic plates crashing into each other and their height is limited by two factors, gravity and weathering, and I'm not even sure gravity is the bigger of the two factors. Olympus Mons on Mars is as tall as it is, in part, because Mars has no weathering and in part because Mars has no tectonic movement, so the volcano under Olympus mons keeps adding to the same spot. That doesn't happen on Earth because our crusts move. Olympus mons is also very flat because it was formed by lava, not by two crusts pressing against each other, which can form more jagged peaks.

Take two granite continental plates slowly pushing into each other at a fixed speed. How high the mountain range grows would be a factor of their momentum pushing into each other. Gravity would play a role in how high the mountains could grow before they spread out/landslide across the surface, but the volume of rock being pushed upwards would be a property of the velocity and mass of the two moving plates. Gravity would add just as much momentum to the collision as it would discourage building the mountains higher.

So ultimately I think the question boils down to these key factors.

Ratio of continental crust granite to ocean floor. How much water is on the planet. It's not hard to imagine a dry planet with almost no oceans to speak of or a water world with no land at all. how active the tectonic movement is how much weathering is done by the atmosphere on the planet and . . . gravity is a factor too. But to ask the question considering gravity alone, I think there are too many variables to answer. From a worldbuilding standpoint, you'd have to consider other things as well.

Finally, a planet with 1.35 times Earth's gravity, if we assume similar density, equal density to gravity the ratio is 1 to 1, so such a planet would have 1.35^3 or about 2.5 times the mass of the Earth and 1.8 times the surface area. It would also cool more slowly, having more heat of formation and would, everything else being equal, probably be more volcanic. How the extra gravity, extra heat and additional matter would affect continent formation and plate tectonics is above my pay grade. You might need some super-computer modeling to get a more accurate answer.

Answer 2

What effects, if any, does gravity have on the mountains and depths of earth's crust? Assuming earth-like dynamics at play, would plate tectonics have more of an influence on the formation of a planet's crust's highs and lows than gravity? If not, why?

Consequences at volcanoes and maybe see-mounts altitude would be:

More gravity, before they collapse->less altitude.

We can measure at Earth the slope where a dune or a volcano collapses, but that's true for 9,8G; physics says the slope would be lower at a planet with 12G.

That's the reason -inversed- why Olympus Mons at Mars reaches 20.000 metters, not seen at Earth.

That doesn't mean the planet would have less altitude at his mountain systems, my guess is orogenies would create systems a bit higger than at Earth are. But individual forms as summits, hillsides,..., resulting from the denundation of the relief created by the orogeny, would be less spectacular. Volcanoes as Teide (8.000m from seabed) would have collapsed before reaching that altitude too.

Is there a way to construct a planet's crust and core such that volcanic activity is possible, yet gravity has the primary influence over crust uniformity?

I guess you are talking about a rocky Earth kindly planet. Crust formation and his thick is consequency of the proccess of cooling of the planet. It is an insulating layer. If your planet is rocky it will have Crust. His thick is mainly related with the stage of the planet and the starting heat/mass had.

You need to take in consideration at certain depp (certain presure) Crust melts and pass to form a part of Mantle, so the heavy planet may have a non specialy depp Crust. In fact, if the gravity is higger, you would expect to find the discontinuity between Crust and Mantle at lower deep. That doesn't mean at the heavy planet Continental Crust would not occupy a bigger area on the surface of the planet, making the oceans deeper by the way.

Crust is not uniform. There is Continental Crust and Oceanic Crust, with chemical and genetical big differences. But I think you are asking about the uniformity of topography, wich is argued previously and bellow.

Unfortunately (I guess for you) if that planet has a bit more gravity the mounts of the videogame, acording to physic laws, should be a bit less spectacular than at Earth are. There would be more landslides at most of geomorphological forms that had a slope. The components of the forms are the same at both planets, but the gravity makes lower the minimal slope when the form collapse.

Volcanic activity and tectonics are related with the total volume of mantle kinding rocks the planet has and the power of the heat the Core shares with the Mantle. Gravity migth be balanced as more 'mantle mass/core heat-mass', more gravity, so your planet could have a bit more of igneous activity, limitated a bit by gravity (volcanic activity, not the internal igneous activity).

So my answer is yes, you can design a planet wich has a little more gravity/mass, but very similar to Earth with Core, Mantle and Crust; volcanoes, black smokers at sea bed....

Would the planet have shallower oceans?

The deep of the ocean is consequence of how much developed the Continental Crust is and how much water the planet has. Sea-bed level can be considered the first surface level of the planet, then it will be deeper if the Continental Crust has emerged at long areas of the surface at the continents, and the bulk of water is relatively high. I am not saying gravity would have not any influence (and I am not geophysicist), but my guess is if A planet has 1.3 times B planet's gravity and the main bulk relations of different components are the same then A planet would have 1.3 times the altitude difference between continental_masslands_altitude / average_seabed_altitude, and the deep of his seabed will be at least 1.3 times if there is also 1.3 times the bulk of total water B planet has and they are at the same geological stage.

So my answer here is no, unless the heavy planet is at an earlier geological stage with no so much Continental Crust developed or has less total amount of water.