File:North america basement rocks.png
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North American cratons and basement rocks.
As radiometric ages were determined for the shield-like rocks on the continents which were either exposed at the surface, underlay the flat rocks, or were within the interior of the mountain belts, patterns of age intervals were determined. These old rocks have been called the "basement". This has been interpreted to mean that the continents had somehow grown by development of rock assemblages with these characteristic age parameters. The expanding continental nuclei are part of the "craton" which consist of both exposed and buried basement rocks.
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This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an agency of the United States Department of the Interior. For more information, see the official USGS copyright policy. български | català | Deutsch | English | español | eesti | suomi | français | galego | italiano | 한국어 | македонски | മലയാളം | Nederlands | polski | português | русский | Türkçe | 中文（香港） | 中文（简体） | +/− |
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http://rst.gsfc.nasa.gov/Sect2/14strprv.jpg or http://instruct.uwo.ca/earth-sci/300b-001/14strprv.gif
English Wikipedia, original upload 22 April 2005 by Berkut
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Originally uploaded by User:SEWilco as Image:North america basement rocks.jpg. Replaced when this better version found.
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