<p>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.</p>

<p>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.   </p>

<p>Granite forms under pressure below the Earth's surface in conditions where the magma gradually cools and <a href="http://nautil.us/blog/how-water-paradoxically-creates-the-land-we-walk-on" rel="nofollow noreferrer">it requires the presence of water to form</a>.    Once formed, Granite is more durable than the denser crustal rock and fairly early in the Earth's history, <a href="http://earthsky.org/earth/when-earths-continents-rose-above-its-oceans" rel="nofollow noreferrer">about 3 billion years ago by this article</a>, this granite formed into continental crusts.</p>

<p>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).</p>

<p>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.</p>

<p>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.  </p>

<p><a href="https://i.sstatic.net/2UPLv.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2UPLv.png" alt="enter image description here"></a></p>

<p><a href="https://geo.libretexts.org/Textmaps/Book%3A_Physical_Geology_(Earle)/18%3A_Geology_of_the_Oceans/18.1%3A_The_Topography_of_the_Sea_Floor" rel="nofollow noreferrer">Source of picture</a></p>

<p>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.</p>

<p>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.  </p>

<p>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.   </p>

<p>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.</p>

<p>So ultimately I think the question boils down to these key factors.</p>

<p>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.</p>

<p>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.</p>

CC BY-SA 4.0
2018-07-26T01:23:05.907Z
@Universal_learner It's certainly reasonable to believe that the Himalayas in higher gravity wouldn't grow as tall, even if momentum of the colliding plates would balance out the higher gravity, they'd be just as big in volume, but more spread out.   Small point though, mountain ranges have little to do with ocean depth.   There's too few of them.  They have some effect, but it's probably a small percentage.  If asked about lower mountains I probably wouldn't have made this post.  Shallower oceans is more tricky, if continents form, the buoyancy of granite plates still exists in 1.3 gravity.
userLTK
CC BY-SA 4.0
2018-07-26T01:27:01.333Z
Think of an iceberg in the ocean.  90% is under water, 10% is above.   Increase gravity, the iceberg floats exactly the same in the ocean.  1.35 Earth gravity or 2 earth gravity or even 3 won't make much difference..  High enough gravity and the iceberg will begin to break down under stress, but that would require many times the Earth's gravity not 1.35 times.   To what degree something floats (and continents float), essentially doesn't change if the gravity is increased by 35%.
userLTK
CC BY-SA 4.0
2018-07-26T16:13:27.323Z
the increased pressure could force more light material out of the mantle making for more continental crust.
John
CC BY-SA 4.0
2018-07-25T04:29:22.990Z
2018-07-25T04:29:22.990Z
userLTK
<p>I have a question that I thought of when discussing a Worldbuilding scenario <a href="https://worldbuilding.stackexchange.com/questions/118953/how-would-coral-reefs-thrive-on-this-super-earth-world?noredirect=1#comment366364_118953">here</a>. 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. </p>

<p>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.</p>

<p>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 <strong>magnitude</strong> 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).</p>

<p><strong>Earth Science Questions:</strong> 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?</p>

<p>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 <em>crust</em> uniformity?</p>

CC BY-SA 4.0
2018-07-23T18:38:16.007Z
Indeed, this is for worldbuilding. Question: More gravity in the same size of the Earth? or the Earth would be bigger as well keeping the same composition? A planet with the same age as Earth? (a youger planet has a thinner crust). Same atmosphere as Earth? Because the erosion of atmosphere flatters mountains, but a planet with more gravity can keep a bigger and denser atmosphere.
Santiago
CC BY-SA 4.0
2018-07-23T18:56:25.080Z
@Santiago: For the purposes of the question on this site, and not to dive into a bunch of hypotheticals that are more suited for worldbuilding, I'll assume everything is as similar to earth as possible, save for the gravity. So let's increase the size of the earth and keep the other variables you mentioned the same.
user3390629
<plate-tectonics><gravity><planetary-formation>
Plate Tectonics vs Gravity for Creating Highs and Lows
user3390629
# Plate Tectonics vs Gravity for Creating Highs and Lows

(Asked by: username_0 on 2018-07-23)

I have a question that I thought of when discussing a Worldbuilding scenario [here](https://worldbuilding.stackexchange.com/questions/118953/how-would-coral-reefs-thrive-on-this-super-earth-world?noredirect=1#comment366364_118953). 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?

**Tags:** plate-tectonics, gravity, planetary-formation

> **Comments:**
> **username_1**: Indeed, this is for worldbuilding. Question: More gravity in the same size of the Earth? or the Earth would be bigger as well keeping the same composition? A planet with the same age as Earth? (a youger planet has a thinner crust). Same atmosphere as Earth? Because the erosion of atmosphere flatters mountains, but a planet with more gravity can keep a bigger and denser atmosphere.  
> **username_0**: @username_1: For the purposes of the question on this site, and not to dive into a bunch of hypotheticals that are more suited for worldbuilding, I'll assume everything is as similar to earth as possible, save for the gravity. So let's increase the size of the earth and keep the other variables you mentioned the same.  

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## 1 answers

### Answer by username_2 on 2018-07-25. Score: 4 | Accepted answer

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](http://nautil.us/blog/how-water-paradoxically-creates-the-land-we-walk-on). 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](http://earthsky.org/earth/when-earths-continents-rose-above-its-oceans), 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.

[![enter image description here](https://i.sstatic.net/2UPLv.png)](https://i.sstatic.net/2UPLv.png)

[Source of picture](https://geo.libretexts.org/Textmaps/Book%3A_Physical_Geology_(Earle)/18%3A_Geology_of_the_Oceans/18.1%3A_The_Topography_of_the_Sea_Floor)

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.

> **Comments:**
> **username_2**: @Universal_learner It's certainly reasonable to believe that the Himalayas in higher gravity wouldn't grow as tall, even if momentum of the colliding plates would balance out the higher gravity, they'd be just as big in volume, but more spread out.   Small point though, mountain ranges have little to do with ocean depth.   There's too few of them.  They have some effect, but it's probably a small percentage.  If asked about lower mountains I probably wouldn't have made this post.  Shallower oceans is more tricky, if continents form, the buoyancy of granite plates still exists in 1.3 gravity.  
> **username_2**: Think of an iceberg in the ocean.  90% is under water, 10% is above.   Increase gravity, the iceberg floats exactly the same in the ocean.  1.35 Earth gravity or 2 earth gravity or even 3 won't make much difference..  High enough gravity and the iceberg will begin to break down under stress, but that would require many times the Earth's gravity not 1.35 times.   To what degree something floats (and continents float), essentially doesn't change if the gravity is increased by 35%.  
> **username_3**: the increased pressure could force more light material out of the mantle making for more continental crust.  

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