[Somes J (OP)]

Hi. I wasn't sure whether this better belonged here or in General Science, it seemed appropriate here.

1) If somebody terraformed a planet like Venus, over the long term, would plate tectonics develop now that a hydrosphere is present (assume the terraforming is stable over million year timescales)? If you came back hundreds of millions or billions of years later (assuming the terraforming is stable for that long) what would the geology of such a world look like?

2) If you had a world that was substantially drier than Earth, with oceans say ~1-2 km deep, would it have plate tectonics? This would expose the tops of the midocean ridges and I understand this is an important zone for hydration of the oceanic crust that makes plate tectonics possible. What would the geology of such a world look like?

In case you're wondering at these odd questions, I'm an aspiring science fiction writer and these questions relate to my fiction.

Thanks.

[JimBob]

1.) There must be a crust over a simi-solid substance for a planet to have plate tectonics. As the mechanism is not yet fully explained to everyone's satisfaction, but most agreeing that it is due to heat from the very core, you could do a variation on the cause but not the mechanics.

However, terraforming is not a reason for tectonics.

2.) Water is probably not absolutely needed for crustal movement. But I could be wrong. It most likely is needed for vulcanism.

I need to think about the last question.

[LeeE]

It most likely is needed for vulcanism.

I need to think about the last question.

I think I can see why water may play a large role in vulcanism linked to subduction zones but if the origins of hot-spots are much deeper down, as generally believed, then I would have thought that water played little part in vulcanism due to them, at least until the magma was so close to the surface that any water present might only change the characteristics of what was already an inevitable eruption.

[JimBob]

Vadose water is a significant part of the Yellowstone system and any other explosive eruptive system. The out-gasing of magmatic bodies is not enough to cause volcanic eruptions of much consequence. Basaltic eruptions are not explosive 99% of the time. E.G. Hawaii.  So if an eruption of any magnitude is to occur the addition of water to the system must occur for the build up of gasses that provide the explosive force necessary for them to erupt. 

(Vados, right or wrong, - I can't spell and the AGI glossary is in another room. I am also lazy. OK I looked it up)

[frethack]

As for water needed for plate tectonics, apparently the answer is yes (at least according to my structural geology professor).  Water helps to break down the silica/oxygen bonds throughout the mantle, and especially in the asthenosphere, which increases the mantles elasticity exponentially and allows it to flow.

Ill have to try to dig up more on this and see if there is an exact mechanism.  Ill also visit my professors office hours next week and post more.

I dont think that just adding water would kick start tectonics, though.  You would need an abundance of radioactive elements to sustain heat and create a liquid core, and you would have the daunting task of resupplying the planet with water.  Venus is very similar in size to the earth, but her core has cooled at a much faster rate.  This leaves her without a magnetic field and allows solar radiation to break down water vapor and the resultant hydrogen to be blown away by the solar wind.  Hence...very little water.  A magnetic field would be absolutely necessary to sustain life.  Unless of course you lived underground (I always imagined that our first settlements would be subterranean)

[LeeE]

...A magnetic field would be absolutely necessary to sustain life...

For DNA type life, where the DNA analogue was similarly fragile as our own I think this is probably true, but there's no reason why a less fragile and more radiation resistant analogue could not occur.  On the down side though, such a more radiation resistant DNA analogue would also result in a slower rate of evolution.

[frethack]

...A magnetic field would be absolutely necessary to sustain life...

For DNA type life, where the DNA analogue was similarly fragile as our own I think this is probably true, but there's no reason why a less fragile and more radiation resistant analogue could not occur.  On the down side though, such a more radiation resistant DNA analogue would also result in a slower rate of evolution.

This is true.  I was mainly referring to the planets ability to retain atmospheric water vapor, and hence, liquid water.  It would be unfortunate to terraform a planet and replace its lost water only to lose it again.

[LeeE]

Good point - I'd forgotten about about retaining an atmosphere (even though I've raised exactly that issue in other threads here - Doh!)