I've had some fun time looking at some plate-tectonics sites, looking at how the Earth's land surface looked over the last billion years or so.

But there appears to be no evidence of plate tectonics elsewhere in the Solar System. Mars has some almost plate tectonics, but that may be because it is smaller, meaning less internal heat to drive it. Venus has the same mass and density as the Earth, but no sign of plate tectonics.

Could Earth's liquid-water supply have affected rocks' mineralogy in a way that would make plate tectonics possible? That would account for the difference between Earth and Venus, since Venus's liquid water resides in its high cloud layers.

This is an interesting discussion of possible reasons why Earth has extensive plate tectonics, and Venus has so little.

Its conclusion seems to be that plate tectonics needs thin ocean crust--which Venus doesn't have. One could then ask the question, why doesn't it? But of course the Earth continents are constantly growing--in time, they too could completely cover the sphere, and the movement stop.

That article claims that the "Big Whack" that formed the Moon also stripped the Earth of much of its original crust, making the surviving crust relatively thin, which then allows plate tectonics to take place.

Venus, however, had kept most of its original crust, which is too thick to indulge in plate tectonics. Thus, it has some big shield volcanoes instead, as Mars also does. Imagine what would happen if the Earth's Pacific plate was not drifting over the mantle; the mantle plume that has been making Hawaiian and Emperor volcanoes would instead make one very big volcano.

Then anyone wealthy enough to have a spacesuit and a hightly maneuverable high-altitude balloon would be able to enjoy some of the best skiing in the solar system. [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
If E = MC², why do I have less energy the more mass my body acquires?
That is all.

--Azpod... Formerly known as James Justin

It might be fun to estimate the size of this hypothetical super-Hawaii.

The "Big Island" has a height of about 30,000 ft (9 km) above the nearby ocean floor, making it the highest mountain on Earth by that measure. Mt. Everest is highest above sea level (29,000 ft or 9 km), but it is surrounded by the high Tibetan Plateau.

The Big Island's volume is about 100,000 km^3; by comparison, the total volume of lava that went into the Hawaiian-Emperor seamount chain has been estimated at around 10 times this. Scaling the Big Island to that volume means multiplying its dimensions by 2, yielding an above-seafloor height of 20 km. However, the ocean depth near Hawaii is 16,000 ft (5 km). This yields a height of 15 km above sea level, greater than Mt Everest's sea-level height.

Interestingly, the Martian volcano Olympus Mons has a height of 25 km above the surrounding Tharsis bulge; its total bulk is close to that of the Hawaiian-Emperor chain.

Venus has some big volcanoes, but the highest one, Maat Mons, has a height of only 8 km, about that of the Big Island. For some reason, Venus has not produced any super-Hawaiis the way that Mars has, despite its lack of noticeable plate tectonics.

Maybe it's because we have found evidence(in one form or another) of H<subscript>2</subscript>O on Mars, whereas, we haven't in the case of Venus.

__________________
"You can't convince a believer of anything; for their belief is not based on evidence, it's based on a deep seated need to believe." [Carl Sagan]

Well, at some point, the crust probably was thin, even on Venus and the early Earth.

Perhaps there never was an moon-forming impact, and the Earth just evolved, like it does today. Why didn't Venus? Maybe it did, but continental growth chewed up the thin basins somehow--ah yes, there is your original question. Maybe water is a lubricant that allows the continents to subduct the crust. I think I've seen that theory before somewhere...I'll check.

Ah, Grapes, I think I know what you're talking about. I remember some years back that the Army Corp of Engineers had poured water down holes dug in the Rockies which, they found, caused mini-quakes to happen in that area of the Rockies.

__________________
"You can't convince a believer of anything; for their belief is not based on evidence, it's based on a deep seated need to believe." [Carl Sagan]

Alternatively, water can change the physical and chemical properties of rock to give it properties more convenient for plate tectonics; water would get into sedimentary rocks as a result of their formation, and ocean-floor sedimentary rocks may get subducted, dragging water-containing rocks into the Earth's interior.

I recall seeing something like this as a hypothesis for the origin of granite, but I don't recall the details.

However, this water hypothesis would not explain the original plate tectonics very well, because it would have had to start without water-containing sedimentary rocks.

Mt. Everest is about as high as any mountain can be on Earth - or Venus. At the sea level it produces 3300 atm of pressure - almost enough to cause rock to flow. Any higher, and the rock WILL flow, settling the mountain. (Below sea level the pressure from above is balanced by the pressure from the hydrosphere.)

By the same reason, your hypothetical Super-Hawaii would never grow to 15 km. Once it reached about 9 km, it would only expand outward, not up. And with Mars' gravity being 38% of Earth, 25 km also seems to be the height limit.

Note: A mountain made of solid iron could be talller