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When rised the continents above sea level

 
 
Reply Wed 6 Aug, 2014 12:18 pm
I have searched on Wikipedia and elsewhere for while now to answer the question, when the continents or the continent, Vaalbara possibly, came out of the water for the first time after the primal ocean was formed, without finding a conclusive answer. The oldest cratons that formed were the Canadian shield, the Kaapvaal and Pilbara Cratons around 3.5 billion years ago. But at this time they were surely still under water. On Wikipedia they say: "The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained almost wholly untouched by successive encroachments of the sea upon the continent." Unfortunately, they don't tell when this happened. Is there some theory, data, discussion or anything on this issue on the web, a book or somewhere?
 
rosborne979
 
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Reply Wed 6 Aug, 2014 08:13 pm
@Huldrich,
Earth was initially molten (or close to it) due to regular bombardment and accumulation of matter. At some point a large object struck the new planet and broke off a piece which became our Moon.

The Earth was too hot to condense water at that time. Eventually the temperatures dropped and water began to condense and the oceans began to fill. In that sense, none of the continents rose from the ocean. The oceans simply filled the basins and stopped rising before they covered everything.

Plate tectonics have changed the Earth's surface dramatically over the 4.3B year life of the planet, so I'm not sure if there are many rocks left from the original formations. But I think I have read that there are some very old rocks in Canada, and I think some in Australia as well.
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Setanta
 
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Reply Thu 7 Aug, 2014 01:54 am
As Roswell points out, the oldest known rocks are in Canada--this article form the Canadian Encyclopedia suggests that some of them may be more than four billion years old.

. . . and in Autralia. this CNN article tells of a crystal found in Australia which is well over four billion years old.
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Huldrich
 
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Reply Thu 7 Aug, 2014 03:52 am
I don't think that there were already basins at the time when the oceans were formed because the crust was too thin and more or less regularly composed of light continental crust. So initially the crust must have been covered with water.

If the earth was perfectly spherical, the ocean depth would be 2000 meters so there must be a lot of plate tectonics in order to make the crust irregular and elevate above sea level. The ocean basins were formed later when the crust was already solid. They form along ridges where heavier basalt is coming upwards from the mantle, becomes solid and pushes away laterally. This takes some time so besides some volcanic islands the earth was covered with water at some early stage. This is also why life formed in shallow waters and land plants appeared very late in earth history.

@Setanta: The zircons you refer to were formed in a watery environment.
Setanta
 
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Reply Thu 7 Aug, 2014 04:01 am
I simply provided a link, i didn't refer to the formation of anything. The basin of the Pacific Ocean was formed, as Rowell has pointed out, by the collision with a planetesimal which created the satellite we call the moon. If you already had your mind made up, why did you come here to ask this question?
Setanta
 
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Reply Thu 7 Aug, 2014 04:06 am
Nothing in the CNN article confirms you claim that the zircon (singular) was formed under water. Furthermore, the article disputes your claim about a late formation of the earth's crust.
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rosborne979
 
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Reply Thu 7 Aug, 2014 04:32 am
@Huldrich,
Maybe I shouldn't have used the term "basins". All I meant was that the low parts would fill first. If the earth was a flat sphere at the time then you would be correct, but I have never read any reports of this so I don't know if it's accurate or not. Where did you read that?
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Setanta
 
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Reply Thu 7 Aug, 2014 04:35 am
We have a competent, working geologist in the membership here, so perhaps he'll arrive to comment.
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Huldrich
 
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Reply Thu 7 Aug, 2014 04:35 am
@Setanta,
Hey calm down! My initial question was if anyone knows something about WHEN and not IF the first cratons came out of the water...
Setanta
 
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Reply Thu 7 Aug, 2014 04:37 am
@Huldrich,
I'm calm, buddy--you're not that important. The OP assumes that a continent or continents arose from the water.
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farmerman
 
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Reply Thu 7 Aug, 2014 05:48 am
@Huldrich,
Its a question that occupies lots of ongoing research even today. To start, Id see whether you could get an easyshare copy of Rodgers and Santosh' book Continents and Supercontinents, its one of the bset books on thi very subject. Remember before a specific time it would be virtually impossible to determine the answer to your question because the earth was moltedn. We use certain minerals and patterns on such things as Shield Rocks to "Speculate" about their position above a sea level datum (if indeed, such a datum even existed at the time we measured the zircon ages).
We mostly use the existence of free quartz, clastics, and metamorphosed sandstones to determine the existence of an emergent craton or cratonic shelf.

Good Reading
Huldrich
 
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Reply Thu 7 Aug, 2014 08:33 am
@farmerman,
Thanks farmerman! It's already very good to know that there is still no conclusive answer and that research is going on. This is what I though because otherwise I would have found something on the Internet or in books. So I will not waste my time further and search to answer this question. But I will nevertheless try to get a copy of this book...
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