Plate tectonics on Water worlds

[Moontanman]

On Earth it is thought in some circles that plate tectonics occurs on Earth due to the interaction of water making subduction easier if not the key to making it happen. This results in various minerals like granite which make up large portions of the Earth's continents and cannot occur without the interaction of magma and water. What role does being exposed to an atmosphere have in this? Would a world with no dry land have plate tectonics and or rocks like granite? 

[pavelcherepan]

On 7/11/2018 at 8:42 AM, Moontanman said:

On Earth it is thought in some circles that plate tectonics occurs on Earth due to the interaction of water making subduction easier if not the key to making it happen. This results in various minerals like granite which make up large portions of the Earth's continents and cannot occur without the interaction of magma and water. What role does being exposed to an atmosphere have in this? Would a world with no dry land have plate tectonics and or rocks like granite? 

Overall, there is no reason not to have plate tectonics on a water world. Tectonics is assisted by presence of water, but is driven mostly by variations in density of material and mantle material convection. At the same time there are no definite signs of plate tectonics in any other rocky bodies of the solar system and there is no dominant theory of it's origins on Earth (the based on current knowledge the planet did exist for almost a billion years without it). 

As far as granite is concerned, I don't think presence of water is truly required for it to occur. Water can help it in subduction zones as it considerably reduces the melting temperature, but it's not essential. 

 

[studiot]

Several questions arise about this idea, not least being

Where does all the water come from?

Worlds with tectonic activity (as we know it) get that way because there is sufficient trapped heat in the core, created as the heavier material 'sinks' to the centre during the planet's formation and early history.
The planet then spends billions of years trying to slough off this heat, creating the driving conditions for tectonics in the process.

This will not happen if the planet is just not big enough - and of course those are the most likely to be totally submerged ones.

So the end result is, as always, a balance of competing factors.

[Moontanman]

11 minutes ago, studiot said:

Several questions arise about this idea, not least being

Where does all the water come from?

Worlds with tectonic activity (as we know it) get that way because there is sufficient trapped heat in the core, created as the heavier material 'sinks' to the centre during the planet's formation and early history.
The planet then spends billions of years trying to slough off this heat, creating the driving conditions for tectonics in the process.

This will not happen if the planet is just not big enough - and of course those are the most likely to be totally submerged ones.

So the end result is, as always, a balance of competing factors.

Let's assume a planet bigger than earth but somewhat less dense, more SiO2 that earth for sure but big enough to drive plate tectonics for several billion years...  The ocean are deep but not deep enough to allow for the formation of pressure ice at the bottom of the ocean... 

Edited July 15, 2018 by Moontanman

[studiot]

On 11/07/2018 at 1:42 AM, Moontanman said:

What role does being exposed to an atmosphere have in this? Would a world with no dry land have plate tectonics and or rocks like granite? 

Granite is an essentially intrusive rock, so is not exposed anyway.

Hard to tell what role the atmosphere might have.
It carries the dust clouds from extrusive rocks.

Quote

A common example of an intrusive igneous rock is granite. Extrusive igneous rocks come from lava. Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly. Solidifying rocks at the surface cool too quickly for large crystals to form.

Igneous Rocks - ScienceViews.com

scienceviews.com/geology/igneous.html

But both types need the internal magmas to happen.