why is it said that gravity will only allow mountains to reach a certain height ?

I've never heard that but I would guess that plate tectonic motion is driven by interaction between the plate's weight (distribution) and the shifting of magma underneath (these are just guesses - I haven't read much on plate tectonics). But since mountains are supposedly formed with plates collide, the force of the collisions would be logically limited by gravity, I think.

The ultimate theoretical height of a mountain depends upon its mass and constituents. If you had a pure lead mountain, it would sink if it got very big and this would be long before it was a very tall mountain. If you had a mountain of Styrofoam you might be able to climb it to reach orbital altitude if it was strong enough to bear its own weight. Where did you hear about the "certain Height" rule. SM

I believe it has been found that the smaller the force of gravity on a planet, the higher will be the mountains and the deeper will be the valleys. http://www.ph.bham.ac.uk/schools/mars.shtml

The ultimate theoretical height of a mountain depends upon its mass and constituents. If you had a pure lead mountain, it would sink if it got very big and this would be long before it was a very tall mountain. If you had a mountain of Styrofoam you might be able to climb it to reach orbital altitude if it was strong enough to bear its own weight. Where did you hear about the "certain Height" rule. SM

 

 

dr brian cox says gravity determines the size allowed to reach

 

I've never heard that but I would guess that plate tectonic motion is driven by interaction between the plate's weight (distribution) and the shifting of magma underneath (these are just guesses - I haven't read much on plate tectonics). But since mountains are supposedly formed with plates collide, the force of the collisions would be logically limited by gravity, I think.

 

what happens if mass is greater than gravity and the plates are stronger

 

I believe it has been found that the smaller the force of gravity on a planet, the higher will be the mountains and the deeper will be the valleys. http://www.ph.bham.a...ools/mars.shtml

 

why are the mountains on the moon restricted or are they formed from comets or asteroids

Moon "mountains" have been generally formed from asteroid impacts and are really the rims of craters. Even so they are much higher than mountains on earth which has a lot to do with the moon's low gravity.

Moon "mountains" have been generally formed from asteroid impacts and are really the rims of craters. Even so they are much higher than mountains on earth which has a lot to do with the moon's low gravity.

 

is this due to speed of impact and volume of mass?