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Old 06-02-2009, 06:53 PM
Location: Houston, Texas
1,084 posts, read 1,339,140 times
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I'm watching one of the History Channel's shows on the evolution of earth and they mention that the Earth was once a granite planet. They said that because granite is lighter than basalt it literally floats on basalt. But they didn't take into account that this would only happen if the two were in their liquid form. Solid granite can not just rise up through solid basalt, no matter how "light" it is. And if the basalt is liquid, then the granite would melt. And besides, aren't liquid basalt and liquid granite the same thing: Magma?

I decided to look online for some more information. Maybe liquid granite and liquid basalt are not the same thing. I know that wikipedia isn't the best source of information but it's at least somewhat credible, at least as a starting point. And it raised a very interesting point. Granite was mentioned as an intrusive igneous rock, meaning that it solidifies underground. The granite I suppose pushes basalt out of the way (melting some of the basalt I would assume), then solidifies underground. Basalt is an extrusive igneous rock. It is solidifies at the surface. My understanding is that if the "granite" reached the surface it would solidify as basalt, not granite.

Doesn't this contradict the idea of a granite Earth? I mean at the beginning of the granite Earth period the Earth was covered by water which sat on top of basalt. There were some basaltic islands poking out of the water, but no great land masses. The only way for there to be large masses of granite would be for there to be even larger land masses of basalt to weather down. But the granite earth is supposed to explain why we have large land masses now. Granite couldn't just rise out of the water since it is created underground. The only thing that could make granite rise above the oceans would be plate tectonics pushing them up like they did with the Alps. But as far as I know, this isn't what they say occurred with the Granite Earth.

Any geologists here? Thoughts?
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Old 06-02-2009, 09:41 PM
Location: Westwood, MA
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I'm unfamiliar with this program, so I'll just try to answer your statements.

First off, the earth isn't a granite planet, if anything it's an iron planet. We've got some granite on the crust--mostly the continents--which are significantly lighter than the basaltic ocean crusts, so they do indeed float a little higher and thus tend to be above ocean level while basalts tend to form ocean crust. The bulk of the planet's mass is iron, though, so any theory that posits that the earth started out as granite is probably wronge (granite is a silicate (silicon and oxygen) and iron is iron--no processes accessible on earth can transmute elements). Perhaps there is another meaning by "the Earth was once a granite planet"?

Second, the idea that the solids can't rise through each other is just plain wrong. On human timescales, rocks seems soild and immovable, but on geologic timescales they do move and flow in a manner similar to liquids. This part solid/part liquid behavior is termed visocelasticity and the ratio of the time scale over which this relaxation (tau) divided by the timescale of observation (t) is defined as the Deborah number (which comes from the Bible of all places "The mountains flowed before the Lord", which, remarkably, they do). Granite rising through basalt is a small Deborah number process, but you walking on the mountain is a large Deborah number process. While you'll never be able to see flowing rock, the process is quite similar in silly putty but the relaxation time is shorter than your lifetime. If you through it against the wall (a fast process), it will behave like a solid, but if just let it sit for 15 minutes (a slow process) it will flow.

So the process is probably like what you're describing can't happen. Why do you think tectonics lifts certain plates and not others? It all comes down to the relative densities of granitic and basaltic rocks, the basalts sink and the granites rise.

This is a pretty informative webpage http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry27.html
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Old 06-03-2009, 11:49 AM
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I saw the same show. "Granite Planet" refers to that time when the granite was initially formed and, being lighter, floated to the surface and created the continents. Hence a "Granite Planet".

BTW, that was a fascinating show. "How the Earth was Made" was the title I think.
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