CONTINENTAL SHEILDS
The centres of the earth's continents ,known as sheilds ,were established between 2500 and 500 millions years ago ,some certain rocks over three billion years old . They were formed by a series of turbulent events ,plate movements , earthquakes ,and volcanic eruptions . Since the pre-cambrian period ,over 570 million year ago ,they have experienced little tectonic activity ,and today ,these flat ,low-lying slabs of solidified molten rock from the stable centres of the continents . They are bounded or covered by successive belts of younger sedimentary rock.
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